Merge master.kernel.org:/pub/scm/linux/kernel/git/herbert/crypto-2.6

[linux-drm-fsl-dcu.git] / drivers / char / tty_io.c

/*
 *  linux/drivers/char/tty_io.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 * 'tty_io.c' gives an orthogonal feeling to tty's, be they consoles
 * or rs-channels. It also implements echoing, cooked mode etc.
 *
 * Kill-line thanks to John T Kohl, who also corrected VMIN = VTIME = 0.
 *
 * Modified by Theodore Ts'o, 9/14/92, to dynamically allocate the
 * tty_struct and tty_queue structures.  Previously there was an array
 * of 256 tty_struct's which was statically allocated, and the
 * tty_queue structures were allocated at boot time.  Both are now
 * dynamically allocated only when the tty is open.
 *
 * Also restructured routines so that there is more of a separation
 * between the high-level tty routines (tty_io.c and tty_ioctl.c) and
 * the low-level tty routines (serial.c, pty.c, console.c).  This
 * makes for cleaner and more compact code.  -TYT, 9/17/92 
 *
 * Modified by Fred N. van Kempen, 01/29/93, to add line disciplines
 * which can be dynamically activated and de-activated by the line
 * discipline handling modules (like SLIP).
 *
 * NOTE: pay no attention to the line discipline code (yet); its
 * interface is still subject to change in this version...
 * -- TYT, 1/31/92
 *
 * Added functionality to the OPOST tty handling.  No delays, but all
 * other bits should be there.
 *      -- Nick Holloway <alfie@dcs.warwick.ac.uk>, 27th May 1993.
 *
 * Rewrote canonical mode and added more termios flags.
 *      -- julian@uhunix.uhcc.hawaii.edu (J. Cowley), 13Jan94
 *
 * Reorganized FASYNC support so mouse code can share it.
 *      -- ctm@ardi.com, 9Sep95
 *
 * New TIOCLINUX variants added.
 *      -- mj@k332.feld.cvut.cz, 19-Nov-95
 * 
 * Restrict vt switching via ioctl()
 *      -- grif@cs.ucr.edu, 5-Dec-95
 *
 * Move console and virtual terminal code to more appropriate files,
 * implement CONFIG_VT and generalize console device interface.
 *      -- Marko Kohtala <Marko.Kohtala@hut.fi>, March 97
 *
 * Rewrote init_dev and release_dev to eliminate races.
 *      -- Bill Hawes <whawes@star.net>, June 97
 *
 * Added devfs support.
 *      -- C. Scott Ananian <cananian@alumni.princeton.edu>, 13-Jan-1998
 *
 * Added support for a Unix98-style ptmx device.
 *      -- C. Scott Ananian <cananian@alumni.princeton.edu>, 14-Jan-1998
 *
 * Reduced memory usage for older ARM systems
 *      -- Russell King <rmk@arm.linux.org.uk>
 *
 * Move do_SAK() into process context.  Less stack use in devfs functions.
 * alloc_tty_struct() always uses kmalloc() -- Andrew Morton <andrewm@uow.edu.eu> 17Mar01
 */

#include <linux/types.h>
#include <linux/major.h>
#include <linux/errno.h>
#include <linux/signal.h>
#include <linux/fcntl.h>
#include <linux/sched.h>
#include <linux/interrupt.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/tty_flip.h>
#include <linux/devpts_fs.h>
#include <linux/file.h>
#include <linux/console.h>
#include <linux/timer.h>
#include <linux/ctype.h>
#include <linux/kd.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/proc_fs.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/smp_lock.h>
#include <linux/device.h>
#include <linux/idr.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/delay.h>

#include <asm/uaccess.h>
#include <asm/system.h>

#include <linux/kbd_kern.h>
#include <linux/vt_kern.h>
#include <linux/selection.h>

#include <linux/kmod.h>

#undef TTY_DEBUG_HANGUP

#define TTY_PARANOIA_CHECK 1
#define CHECK_TTY_COUNT 1

struct ktermios tty_std_termios = {     /* for the benefit of tty drivers  */
        .c_iflag = ICRNL | IXON,
        .c_oflag = OPOST | ONLCR,
        .c_cflag = B38400 | CS8 | CREAD | HUPCL,
        .c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK |
                   ECHOCTL | ECHOKE | IEXTEN,
        .c_cc = INIT_C_CC,
        .c_ispeed = 38400,
        .c_ospeed = 38400
};

EXPORT_SYMBOL(tty_std_termios);

/* This list gets poked at by procfs and various bits of boot up code. This
   could do with some rationalisation such as pulling the tty proc function
   into this file */
   
LIST_HEAD(tty_drivers);                 /* linked list of tty drivers */

/* Mutex to protect creating and releasing a tty. This is shared with
   vt.c for deeply disgusting hack reasons */
DEFINE_MUTEX(tty_mutex);
EXPORT_SYMBOL(tty_mutex);

#ifdef CONFIG_UNIX98_PTYS
extern struct tty_driver *ptm_driver;   /* Unix98 pty masters; for /dev/ptmx */
extern int pty_limit;           /* Config limit on Unix98 ptys */
static DEFINE_IDR(allocated_ptys);
static DECLARE_MUTEX(allocated_ptys_lock);
static int ptmx_open(struct inode *, struct file *);
#endif

static void initialize_tty_struct(struct tty_struct *tty);

static ssize_t tty_read(struct file *, char __user *, size_t, loff_t *);
static ssize_t tty_write(struct file *, const char __user *, size_t, loff_t *);
ssize_t redirected_tty_write(struct file *, const char __user *, size_t, loff_t *);
static unsigned int tty_poll(struct file *, poll_table *);
static int tty_open(struct inode *, struct file *);
static int tty_release(struct inode *, struct file *);
int tty_ioctl(struct inode * inode, struct file * file,
              unsigned int cmd, unsigned long arg);
static int tty_fasync(int fd, struct file * filp, int on);
static void release_tty(struct tty_struct *tty, int idx);
static void __proc_set_tty(struct task_struct *tsk, struct tty_struct *tty);
static void proc_set_tty(struct task_struct *tsk, struct tty_struct *tty);

/**
 *      alloc_tty_struct        -       allocate a tty object
 *
 *      Return a new empty tty structure. The data fields have not
 *      been initialized in any way but has been zeroed
 *
 *      Locking: none
 */

static struct tty_struct *alloc_tty_struct(void)
{
        return kzalloc(sizeof(struct tty_struct), GFP_KERNEL);
}

static void tty_buffer_free_all(struct tty_struct *);

/**
 *      free_tty_struct         -       free a disused tty
 *      @tty: tty struct to free
 *
 *      Free the write buffers, tty queue and tty memory itself.
 *
 *      Locking: none. Must be called after tty is definitely unused
 */

static inline void free_tty_struct(struct tty_struct *tty)
{
        kfree(tty->write_buf);
        tty_buffer_free_all(tty);
        kfree(tty);
}

#define TTY_NUMBER(tty) ((tty)->index + (tty)->driver->name_base)

/**
 *      tty_name        -       return tty naming
 *      @tty: tty structure
 *      @buf: buffer for output
 *
 *      Convert a tty structure into a name. The name reflects the kernel
 *      naming policy and if udev is in use may not reflect user space
 *
 *      Locking: none
 */

char *tty_name(struct tty_struct *tty, char *buf)
{
        if (!tty) /* Hmm.  NULL pointer.  That's fun. */
                strcpy(buf, "NULL tty");
        else
                strcpy(buf, tty->name);
        return buf;
}

EXPORT_SYMBOL(tty_name);

int tty_paranoia_check(struct tty_struct *tty, struct inode *inode,
                              const char *routine)
{
#ifdef TTY_PARANOIA_CHECK
        if (!tty) {
                printk(KERN_WARNING
                        "null TTY for (%d:%d) in %s\n",
                        imajor(inode), iminor(inode), routine);
                return 1;
        }
        if (tty->magic != TTY_MAGIC) {
                printk(KERN_WARNING
                        "bad magic number for tty struct (%d:%d) in %s\n",
                        imajor(inode), iminor(inode), routine);
                return 1;
        }
#endif
        return 0;
}

static int check_tty_count(struct tty_struct *tty, const char *routine)
{
#ifdef CHECK_TTY_COUNT
        struct list_head *p;
        int count = 0;
        
        file_list_lock();
        list_for_each(p, &tty->tty_files) {
                count++;
        }
        file_list_unlock();
        if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
            tty->driver->subtype == PTY_TYPE_SLAVE &&
            tty->link && tty->link->count)
                count++;
        if (tty->count != count) {
                printk(KERN_WARNING "Warning: dev (%s) tty->count(%d) "
                                    "!= #fd's(%d) in %s\n",
                       tty->name, tty->count, count, routine);
                return count;
        }
#endif
        return 0;
}

/*
 * Tty buffer allocation management
 */

/**
 *      tty_buffer_free_all             -       free buffers used by a tty
 *      @tty: tty to free from
 *
 *      Remove all the buffers pending on a tty whether queued with data
 *      or in the free ring. Must be called when the tty is no longer in use
 *
 *      Locking: none
 */

static void tty_buffer_free_all(struct tty_struct *tty)
{
        struct tty_buffer *thead;
        while((thead = tty->buf.head) != NULL) {
                tty->buf.head = thead->next;
                kfree(thead);
        }
        while((thead = tty->buf.free) != NULL) {
                tty->buf.free = thead->next;
                kfree(thead);
        }
        tty->buf.tail = NULL;
        tty->buf.memory_used = 0;
}

/**
 *      tty_buffer_init         -       prepare a tty buffer structure
 *      @tty: tty to initialise
 *
 *      Set up the initial state of the buffer management for a tty device.
 *      Must be called before the other tty buffer functions are used.
 *
 *      Locking: none
 */

static void tty_buffer_init(struct tty_struct *tty)
{
        spin_lock_init(&tty->buf.lock);
        tty->buf.head = NULL;
        tty->buf.tail = NULL;
        tty->buf.free = NULL;
        tty->buf.memory_used = 0;
}

/**
 *      tty_buffer_alloc        -       allocate a tty buffer
 *      @tty: tty device
 *      @size: desired size (characters)
 *
 *      Allocate a new tty buffer to hold the desired number of characters.
 *      Return NULL if out of memory or the allocation would exceed the
 *      per device queue
 *
 *      Locking: Caller must hold tty->buf.lock
 */

static struct tty_buffer *tty_buffer_alloc(struct tty_struct *tty, size_t size)
{
        struct tty_buffer *p;

        if (tty->buf.memory_used + size > 65536)
                return NULL;
        p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
        if(p == NULL)
                return NULL;
        p->used = 0;
        p->size = size;
        p->next = NULL;
        p->commit = 0;
        p->read = 0;
        p->char_buf_ptr = (char *)(p->data);
        p->flag_buf_ptr = (unsigned char *)p->char_buf_ptr + size;
        tty->buf.memory_used += size;
        return p;
}

/**
 *      tty_buffer_free         -       free a tty buffer
 *      @tty: tty owning the buffer
 *      @b: the buffer to free
 *
 *      Free a tty buffer, or add it to the free list according to our
 *      internal strategy
 *
 *      Locking: Caller must hold tty->buf.lock
 */

static void tty_buffer_free(struct tty_struct *tty, struct tty_buffer *b)
{
        /* Dumb strategy for now - should keep some stats */
        tty->buf.memory_used -= b->size;
        WARN_ON(tty->buf.memory_used < 0);

        if(b->size >= 512)
                kfree(b);
        else {
                b->next = tty->buf.free;
                tty->buf.free = b;
        }
}

/**
 *      tty_buffer_find         -       find a free tty buffer
 *      @tty: tty owning the buffer
 *      @size: characters wanted
 *
 *      Locate an existing suitable tty buffer or if we are lacking one then
 *      allocate a new one. We round our buffers off in 256 character chunks
 *      to get better allocation behaviour.
 *
 *      Locking: Caller must hold tty->buf.lock
 */

static struct tty_buffer *tty_buffer_find(struct tty_struct *tty, size_t size)
{
        struct tty_buffer **tbh = &tty->buf.free;
        while((*tbh) != NULL) {
                struct tty_buffer *t = *tbh;
                if(t->size >= size) {
                        *tbh = t->next;
                        t->next = NULL;
                        t->used = 0;
                        t->commit = 0;
                        t->read = 0;
                        tty->buf.memory_used += t->size;
                        return t;
                }
                tbh = &((*tbh)->next);
        }
        /* Round the buffer size out */
        size = (size + 0xFF) & ~ 0xFF;
        return tty_buffer_alloc(tty, size);
        /* Should possibly check if this fails for the largest buffer we
           have queued and recycle that ? */
}

/**
 *      tty_buffer_request_room         -       grow tty buffer if needed
 *      @tty: tty structure
 *      @size: size desired
 *
 *      Make at least size bytes of linear space available for the tty
 *      buffer. If we fail return the size we managed to find.
 *
 *      Locking: Takes tty->buf.lock
 */
int tty_buffer_request_room(struct tty_struct *tty, size_t size)
{
        struct tty_buffer *b, *n;
        int left;
        unsigned long flags;

        spin_lock_irqsave(&tty->buf.lock, flags);

        /* OPTIMISATION: We could keep a per tty "zero" sized buffer to
           remove this conditional if its worth it. This would be invisible
           to the callers */
        if ((b = tty->buf.tail) != NULL)
                left = b->size - b->used;
        else
                left = 0;

        if (left < size) {
                /* This is the slow path - looking for new buffers to use */
                if ((n = tty_buffer_find(tty, size)) != NULL) {
                        if (b != NULL) {
                                b->next = n;
                                b->commit = b->used;
                        } else
                                tty->buf.head = n;
                        tty->buf.tail = n;
                } else
                        size = left;
        }

        spin_unlock_irqrestore(&tty->buf.lock, flags);
        return size;
}
EXPORT_SYMBOL_GPL(tty_buffer_request_room);

/**
 *      tty_insert_flip_string  -       Add characters to the tty buffer
 *      @tty: tty structure
 *      @chars: characters
 *      @size: size
 *
 *      Queue a series of bytes to the tty buffering. All the characters
 *      passed are marked as without error. Returns the number added.
 *
 *      Locking: Called functions may take tty->buf.lock
 */

int tty_insert_flip_string(struct tty_struct *tty, const unsigned char *chars,
                                size_t size)
{
        int copied = 0;
        do {
                int space = tty_buffer_request_room(tty, size - copied);
                struct tty_buffer *tb = tty->buf.tail;
                /* If there is no space then tb may be NULL */
                if(unlikely(space == 0))
                        break;
                memcpy(tb->char_buf_ptr + tb->used, chars, space);
                memset(tb->flag_buf_ptr + tb->used, TTY_NORMAL, space);
                tb->used += space;
                copied += space;
                chars += space;
                /* There is a small chance that we need to split the data over
                   several buffers. If this is the case we must loop */
        } while (unlikely(size > copied));
        return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string);

/**
 *      tty_insert_flip_string_flags    -       Add characters to the tty buffer
 *      @tty: tty structure
 *      @chars: characters
 *      @flags: flag bytes
 *      @size: size
 *
 *      Queue a series of bytes to the tty buffering. For each character
 *      the flags array indicates the status of the character. Returns the
 *      number added.
 *
 *      Locking: Called functions may take tty->buf.lock
 */

int tty_insert_flip_string_flags(struct tty_struct *tty,
                const unsigned char *chars, const char *flags, size_t size)
{
        int copied = 0;
        do {
                int space = tty_buffer_request_room(tty, size - copied);
                struct tty_buffer *tb = tty->buf.tail;
                /* If there is no space then tb may be NULL */
                if(unlikely(space == 0))
                        break;
                memcpy(tb->char_buf_ptr + tb->used, chars, space);
                memcpy(tb->flag_buf_ptr + tb->used, flags, space);
                tb->used += space;
                copied += space;
                chars += space;
                flags += space;
                /* There is a small chance that we need to split the data over
                   several buffers. If this is the case we must loop */
        } while (unlikely(size > copied));
        return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_flags);

/**
 *      tty_schedule_flip       -       push characters to ldisc
 *      @tty: tty to push from
 *
 *      Takes any pending buffers and transfers their ownership to the
 *      ldisc side of the queue. It then schedules those characters for
 *      processing by the line discipline.
 *
 *      Locking: Takes tty->buf.lock
 */

void tty_schedule_flip(struct tty_struct *tty)
{
        unsigned long flags;
        spin_lock_irqsave(&tty->buf.lock, flags);
        if (tty->buf.tail != NULL)
                tty->buf.tail->commit = tty->buf.tail->used;
        spin_unlock_irqrestore(&tty->buf.lock, flags);
        schedule_delayed_work(&tty->buf.work, 1);
}
EXPORT_SYMBOL(tty_schedule_flip);

/**
 *      tty_prepare_flip_string         -       make room for characters
 *      @tty: tty
 *      @chars: return pointer for character write area
 *      @size: desired size
 *
 *      Prepare a block of space in the buffer for data. Returns the length
 *      available and buffer pointer to the space which is now allocated and
 *      accounted for as ready for normal characters. This is used for drivers
 *      that need their own block copy routines into the buffer. There is no
 *      guarantee the buffer is a DMA target!
 *
 *      Locking: May call functions taking tty->buf.lock
 */

int tty_prepare_flip_string(struct tty_struct *tty, unsigned char **chars, size_t size)
{
        int space = tty_buffer_request_room(tty, size);
        if (likely(space)) {
                struct tty_buffer *tb = tty->buf.tail;
                *chars = tb->char_buf_ptr + tb->used;
                memset(tb->flag_buf_ptr + tb->used, TTY_NORMAL, space);
                tb->used += space;
        }
        return space;
}

EXPORT_SYMBOL_GPL(tty_prepare_flip_string);

/**
 *      tty_prepare_flip_string_flags   -       make room for characters
 *      @tty: tty
 *      @chars: return pointer for character write area
 *      @flags: return pointer for status flag write area
 *      @size: desired size
 *
 *      Prepare a block of space in the buffer for data. Returns the length
 *      available and buffer pointer to the space which is now allocated and
 *      accounted for as ready for characters. This is used for drivers
 *      that need their own block copy routines into the buffer. There is no
 *      guarantee the buffer is a DMA target!
 *
 *      Locking: May call functions taking tty->buf.lock
 */

int tty_prepare_flip_string_flags(struct tty_struct *tty, unsigned char **chars, char **flags, size_t size)
{
        int space = tty_buffer_request_room(tty, size);
        if (likely(space)) {
                struct tty_buffer *tb = tty->buf.tail;
                *chars = tb->char_buf_ptr + tb->used;
                *flags = tb->flag_buf_ptr + tb->used;
                tb->used += space;
        }
        return space;
}

EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);



/**
 *      tty_set_termios_ldisc           -       set ldisc field
 *      @tty: tty structure
 *      @num: line discipline number
 *
 *      This is probably overkill for real world processors but
 *      they are not on hot paths so a little discipline won't do 
 *      any harm.
 *
 *      Locking: takes termios_mutex
 */
 
static void tty_set_termios_ldisc(struct tty_struct *tty, int num)
{
        mutex_lock(&tty->termios_mutex);
        tty->termios->c_line = num;
        mutex_unlock(&tty->termios_mutex);
}

/*
 *      This guards the refcounted line discipline lists. The lock
 *      must be taken with irqs off because there are hangup path
 *      callers who will do ldisc lookups and cannot sleep.
 */
 
static DEFINE_SPINLOCK(tty_ldisc_lock);
static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
static struct tty_ldisc tty_ldiscs[NR_LDISCS];  /* line disc dispatch table */

/**
 *      tty_register_ldisc      -       install a line discipline
 *      @disc: ldisc number
 *      @new_ldisc: pointer to the ldisc object
 *
 *      Installs a new line discipline into the kernel. The discipline
 *      is set up as unreferenced and then made available to the kernel
 *      from this point onwards.
 *
 *      Locking:
 *              takes tty_ldisc_lock to guard against ldisc races
 */

int tty_register_ldisc(int disc, struct tty_ldisc *new_ldisc)
{
        unsigned long flags;
        int ret = 0;
        
        if (disc < N_TTY || disc >= NR_LDISCS)
                return -EINVAL;
        
        spin_lock_irqsave(&tty_ldisc_lock, flags);
        tty_ldiscs[disc] = *new_ldisc;
        tty_ldiscs[disc].num = disc;
        tty_ldiscs[disc].flags |= LDISC_FLAG_DEFINED;
        tty_ldiscs[disc].refcount = 0;
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        
        return ret;
}
EXPORT_SYMBOL(tty_register_ldisc);

/**
 *      tty_unregister_ldisc    -       unload a line discipline
 *      @disc: ldisc number
 *      @new_ldisc: pointer to the ldisc object
 *
 *      Remove a line discipline from the kernel providing it is not
 *      currently in use.
 *
 *      Locking:
 *              takes tty_ldisc_lock to guard against ldisc races
 */

int tty_unregister_ldisc(int disc)
{
        unsigned long flags;
        int ret = 0;

        if (disc < N_TTY || disc >= NR_LDISCS)
                return -EINVAL;

        spin_lock_irqsave(&tty_ldisc_lock, flags);
        if (tty_ldiscs[disc].refcount)
                ret = -EBUSY;
        else
                tty_ldiscs[disc].flags &= ~LDISC_FLAG_DEFINED;
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);

        return ret;
}
EXPORT_SYMBOL(tty_unregister_ldisc);

/**
 *      tty_ldisc_get           -       take a reference to an ldisc
 *      @disc: ldisc number
 *
 *      Takes a reference to a line discipline. Deals with refcounts and
 *      module locking counts. Returns NULL if the discipline is not available.
 *      Returns a pointer to the discipline and bumps the ref count if it is
 *      available
 *
 *      Locking:
 *              takes tty_ldisc_lock to guard against ldisc races
 */

struct tty_ldisc *tty_ldisc_get(int disc)
{
        unsigned long flags;
        struct tty_ldisc *ld;

        if (disc < N_TTY || disc >= NR_LDISCS)
                return NULL;
        
        spin_lock_irqsave(&tty_ldisc_lock, flags);

        ld = &tty_ldiscs[disc];
        /* Check the entry is defined */
        if(ld->flags & LDISC_FLAG_DEFINED)
        {
                /* If the module is being unloaded we can't use it */
                if (!try_module_get(ld->owner))
                        ld = NULL;
                else /* lock it */
                        ld->refcount++;
        }
        else
                ld = NULL;
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return ld;
}

EXPORT_SYMBOL_GPL(tty_ldisc_get);

/**
 *      tty_ldisc_put           -       drop ldisc reference
 *      @disc: ldisc number
 *
 *      Drop a reference to a line discipline. Manage refcounts and
 *      module usage counts
 *
 *      Locking:
 *              takes tty_ldisc_lock to guard against ldisc races
 */

void tty_ldisc_put(int disc)
{
        struct tty_ldisc *ld;
        unsigned long flags;
        
        BUG_ON(disc < N_TTY || disc >= NR_LDISCS);
                
        spin_lock_irqsave(&tty_ldisc_lock, flags);
        ld = &tty_ldiscs[disc];
        BUG_ON(ld->refcount == 0);
        ld->refcount--;
        module_put(ld->owner);
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
}
        
EXPORT_SYMBOL_GPL(tty_ldisc_put);

/**
 *      tty_ldisc_assign        -       set ldisc on a tty
 *      @tty: tty to assign
 *      @ld: line discipline
 *
 *      Install an instance of a line discipline into a tty structure. The
 *      ldisc must have a reference count above zero to ensure it remains/
 *      The tty instance refcount starts at zero.
 *
 *      Locking:
 *              Caller must hold references
 */

static void tty_ldisc_assign(struct tty_struct *tty, struct tty_ldisc *ld)
{
        tty->ldisc = *ld;
        tty->ldisc.refcount = 0;
}

/**
 *      tty_ldisc_try           -       internal helper
 *      @tty: the tty
 *
 *      Make a single attempt to grab and bump the refcount on
 *      the tty ldisc. Return 0 on failure or 1 on success. This is
 *      used to implement both the waiting and non waiting versions
 *      of tty_ldisc_ref
 *
 *      Locking: takes tty_ldisc_lock
 */

static int tty_ldisc_try(struct tty_struct *tty)
{
        unsigned long flags;
        struct tty_ldisc *ld;
        int ret = 0;
        
        spin_lock_irqsave(&tty_ldisc_lock, flags);
        ld = &tty->ldisc;
        if(test_bit(TTY_LDISC, &tty->flags))
        {
                ld->refcount++;
                ret = 1;
        }
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return ret;
}

/**
 *      tty_ldisc_ref_wait      -       wait for the tty ldisc
 *      @tty: tty device
 *
 *      Dereference the line discipline for the terminal and take a 
 *      reference to it. If the line discipline is in flux then 
 *      wait patiently until it changes.
 *
 *      Note: Must not be called from an IRQ/timer context. The caller
 *      must also be careful not to hold other locks that will deadlock
 *      against a discipline change, such as an existing ldisc reference
 *      (which we check for)
 *
 *      Locking: call functions take tty_ldisc_lock
 */
 
struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
        /* wait_event is a macro */
        wait_event(tty_ldisc_wait, tty_ldisc_try(tty));
        if(tty->ldisc.refcount == 0)
                printk(KERN_ERR "tty_ldisc_ref_wait\n");
        return &tty->ldisc;
}

EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);

/**
 *      tty_ldisc_ref           -       get the tty ldisc
 *      @tty: tty device
 *
 *      Dereference the line discipline for the terminal and take a 
 *      reference to it. If the line discipline is in flux then 
 *      return NULL. Can be called from IRQ and timer functions.
 *
 *      Locking: called functions take tty_ldisc_lock
 */
 
struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
{
        if(tty_ldisc_try(tty))
                return &tty->ldisc;
        return NULL;
}

EXPORT_SYMBOL_GPL(tty_ldisc_ref);

/**
 *      tty_ldisc_deref         -       free a tty ldisc reference
 *      @ld: reference to free up
 *
 *      Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
 *      be called in IRQ context.
 *
 *      Locking: takes tty_ldisc_lock
 */
 
void tty_ldisc_deref(struct tty_ldisc *ld)
{
        unsigned long flags;

        BUG_ON(ld == NULL);
                
        spin_lock_irqsave(&tty_ldisc_lock, flags);
        if(ld->refcount == 0)
                printk(KERN_ERR "tty_ldisc_deref: no references.\n");
        else
                ld->refcount--;
        if(ld->refcount == 0)
                wake_up(&tty_ldisc_wait);
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
}

EXPORT_SYMBOL_GPL(tty_ldisc_deref);

/**
 *      tty_ldisc_enable        -       allow ldisc use
 *      @tty: terminal to activate ldisc on
 *
 *      Set the TTY_LDISC flag when the line discipline can be called
 *      again. Do neccessary wakeups for existing sleepers.
 *
 *      Note: nobody should set this bit except via this function. Clearing
 *      directly is allowed.
 */

static void tty_ldisc_enable(struct tty_struct *tty)
{
        set_bit(TTY_LDISC, &tty->flags);
        wake_up(&tty_ldisc_wait);
}
        
/**
 *      tty_set_ldisc           -       set line discipline
 *      @tty: the terminal to set
 *      @ldisc: the line discipline
 *
 *      Set the discipline of a tty line. Must be called from a process
 *      context.
 *
 *      Locking: takes tty_ldisc_lock.
 *               called functions take termios_mutex
 */
 
static int tty_set_ldisc(struct tty_struct *tty, int ldisc)
{
        int retval = 0;
        struct tty_ldisc o_ldisc;
        char buf[64];
        int work;
        unsigned long flags;
        struct tty_ldisc *ld;
        struct tty_struct *o_tty;

        if ((ldisc < N_TTY) || (ldisc >= NR_LDISCS))
                return -EINVAL;

restart:

        ld = tty_ldisc_get(ldisc);
        /* Eduardo Blanco <ejbs@cs.cs.com.uy> */
        /* Cyrus Durgin <cider@speakeasy.org> */
        if (ld == NULL) {
                request_module("tty-ldisc-%d", ldisc);
                ld = tty_ldisc_get(ldisc);
        }
        if (ld == NULL)
                return -EINVAL;

        /*
         *      No more input please, we are switching. The new ldisc
         *      will update this value in the ldisc open function
         */

        tty->receive_room = 0;

        /*
         *      Problem: What do we do if this blocks ?
         */

        tty_wait_until_sent(tty, 0);

        if (tty->ldisc.num == ldisc) {
                tty_ldisc_put(ldisc);
                return 0;
        }

        o_ldisc = tty->ldisc;
        o_tty = tty->link;

        /*
         *      Make sure we don't change while someone holds a
         *      reference to the line discipline. The TTY_LDISC bit
         *      prevents anyone taking a reference once it is clear.
         *      We need the lock to avoid racing reference takers.
         */

        spin_lock_irqsave(&tty_ldisc_lock, flags);
        if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) {
                if(tty->ldisc.refcount) {
                        /* Free the new ldisc we grabbed. Must drop the lock
                           first. */
                        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
                        tty_ldisc_put(ldisc);
                        /*
                         * There are several reasons we may be busy, including
                         * random momentary I/O traffic. We must therefore
                         * retry. We could distinguish between blocking ops
                         * and retries if we made tty_ldisc_wait() smarter. That
                         * is up for discussion.
                         */
                        if (wait_event_interruptible(tty_ldisc_wait, tty->ldisc.refcount == 0) < 0)
                                return -ERESTARTSYS;
                        goto restart;
                }
                if(o_tty && o_tty->ldisc.refcount) {
                        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
                        tty_ldisc_put(ldisc);
                        if (wait_event_interruptible(tty_ldisc_wait, o_tty->ldisc.refcount == 0) < 0)
                                return -ERESTARTSYS;
                        goto restart;
                }
        }

        /* if the TTY_LDISC bit is set, then we are racing against another ldisc change */

        if (!test_bit(TTY_LDISC, &tty->flags)) {
                spin_unlock_irqrestore(&tty_ldisc_lock, flags);
                tty_ldisc_put(ldisc);
                ld = tty_ldisc_ref_wait(tty);
                tty_ldisc_deref(ld);
                goto restart;
        }

        clear_bit(TTY_LDISC, &tty->flags);
        if (o_tty)
                clear_bit(TTY_LDISC, &o_tty->flags);
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);

        /*
         *      From this point on we know nobody has an ldisc
         *      usage reference, nor can they obtain one until
         *      we say so later on.
         */

        work = cancel_delayed_work(&tty->buf.work);
        /*
         * Wait for ->hangup_work and ->buf.work handlers to terminate
         */
         
        flush_scheduled_work();
        /* Shutdown the current discipline. */
        if (tty->ldisc.close)
                (tty->ldisc.close)(tty);

        /* Now set up the new line discipline. */
        tty_ldisc_assign(tty, ld);
        tty_set_termios_ldisc(tty, ldisc);
        if (tty->ldisc.open)
                retval = (tty->ldisc.open)(tty);
        if (retval < 0) {
                tty_ldisc_put(ldisc);
                /* There is an outstanding reference here so this is safe */
                tty_ldisc_assign(tty, tty_ldisc_get(o_ldisc.num));
                tty_set_termios_ldisc(tty, tty->ldisc.num);
                if (tty->ldisc.open && (tty->ldisc.open(tty) < 0)) {
                        tty_ldisc_put(o_ldisc.num);
                        /* This driver is always present */
                        tty_ldisc_assign(tty, tty_ldisc_get(N_TTY));
                        tty_set_termios_ldisc(tty, N_TTY);
                        if (tty->ldisc.open) {
                                int r = tty->ldisc.open(tty);

                                if (r < 0)
                                        panic("Couldn't open N_TTY ldisc for "
                                              "%s --- error %d.",
                                              tty_name(tty, buf), r);
                        }
                }
        }
        /* At this point we hold a reference to the new ldisc and a
           a reference to the old ldisc. If we ended up flipping back
           to the existing ldisc we have two references to it */
        
        if (tty->ldisc.num != o_ldisc.num && tty->driver->set_ldisc)
                tty->driver->set_ldisc(tty);
                
        tty_ldisc_put(o_ldisc.num);
        
        /*
         *      Allow ldisc referencing to occur as soon as the driver
         *      ldisc callback completes.
         */
         
        tty_ldisc_enable(tty);
        if (o_tty)
                tty_ldisc_enable(o_tty);
        
        /* Restart it in case no characters kick it off. Safe if
           already running */
        if (work)
                schedule_delayed_work(&tty->buf.work, 1);
        return retval;
}

/**
 *      get_tty_driver          -       find device of a tty
 *      @dev_t: device identifier
 *      @index: returns the index of the tty
 *
 *      This routine returns a tty driver structure, given a device number
 *      and also passes back the index number.
 *
 *      Locking: caller must hold tty_mutex
 */

static struct tty_driver *get_tty_driver(dev_t device, int *index)
{
        struct tty_driver *p;

        list_for_each_entry(p, &tty_drivers, tty_drivers) {
                dev_t base = MKDEV(p->major, p->minor_start);
                if (device < base || device >= base + p->num)
                        continue;
                *index = device - base;
                return p;
        }
        return NULL;
}

/**
 *      tty_check_change        -       check for POSIX terminal changes
 *      @tty: tty to check
 *
 *      If we try to write to, or set the state of, a terminal and we're
 *      not in the foreground, send a SIGTTOU.  If the signal is blocked or
 *      ignored, go ahead and perform the operation.  (POSIX 7.2)
 *
 *      Locking: none
 */

int tty_check_change(struct tty_struct * tty)
{
        if (current->signal->tty != tty)
                return 0;
        if (!tty->pgrp) {
                printk(KERN_WARNING "tty_check_change: tty->pgrp == NULL!\n");
                return 0;
        }
        if (task_pgrp(current) == tty->pgrp)
                return 0;
        if (is_ignored(SIGTTOU))
                return 0;
        if (is_current_pgrp_orphaned())
                return -EIO;
        (void) kill_pgrp(task_pgrp(current), SIGTTOU, 1);
        return -ERESTARTSYS;
}

EXPORT_SYMBOL(tty_check_change);

static ssize_t hung_up_tty_read(struct file * file, char __user * buf,
                                size_t count, loff_t *ppos)
{
        return 0;
}

static ssize_t hung_up_tty_write(struct file * file, const char __user * buf,
                                 size_t count, loff_t *ppos)
{
        return -EIO;
}

/* No kernel lock held - none needed ;) */
static unsigned int hung_up_tty_poll(struct file * filp, poll_table * wait)
{
        return POLLIN | POLLOUT | POLLERR | POLLHUP | POLLRDNORM | POLLWRNORM;
}

static int hung_up_tty_ioctl(struct inode * inode, struct file * file,
                             unsigned int cmd, unsigned long arg)
{
        return cmd == TIOCSPGRP ? -ENOTTY : -EIO;
}

static const struct file_operations tty_fops = {
        .llseek         = no_llseek,
        .read           = tty_read,
        .write          = tty_write,
        .poll           = tty_poll,
        .ioctl          = tty_ioctl,
        .open           = tty_open,
        .release        = tty_release,
        .fasync         = tty_fasync,
};

#ifdef CONFIG_UNIX98_PTYS
static const struct file_operations ptmx_fops = {
        .llseek         = no_llseek,
        .read           = tty_read,
        .write          = tty_write,
        .poll           = tty_poll,
        .ioctl          = tty_ioctl,
        .open           = ptmx_open,
        .release        = tty_release,
        .fasync         = tty_fasync,
};
#endif

static const struct file_operations console_fops = {
        .llseek         = no_llseek,
        .read           = tty_read,
        .write          = redirected_tty_write,
        .poll           = tty_poll,
        .ioctl          = tty_ioctl,
        .open           = tty_open,
        .release        = tty_release,
        .fasync         = tty_fasync,
};

static const struct file_operations hung_up_tty_fops = {
        .llseek         = no_llseek,
        .read           = hung_up_tty_read,
        .write          = hung_up_tty_write,
        .poll           = hung_up_tty_poll,
        .ioctl          = hung_up_tty_ioctl,
        .release        = tty_release,
};

static DEFINE_SPINLOCK(redirect_lock);
static struct file *redirect;

/**
 *      tty_wakeup      -       request more data
 *      @tty: terminal
 *
 *      Internal and external helper for wakeups of tty. This function
 *      informs the line discipline if present that the driver is ready
 *      to receive more output data.
 */
 
void tty_wakeup(struct tty_struct *tty)
{
        struct tty_ldisc *ld;
        
        if (test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) {
                ld = tty_ldisc_ref(tty);
                if(ld) {
                        if(ld->write_wakeup)
                                ld->write_wakeup(tty);
                        tty_ldisc_deref(ld);
                }
        }
        wake_up_interruptible(&tty->write_wait);
}

EXPORT_SYMBOL_GPL(tty_wakeup);

/**
 *      tty_ldisc_flush -       flush line discipline queue
 *      @tty: tty
 *
 *      Flush the line discipline queue (if any) for this tty. If there
 *      is no line discipline active this is a no-op.
 */
 
void tty_ldisc_flush(struct tty_struct *tty)
{
        struct tty_ldisc *ld = tty_ldisc_ref(tty);
        if(ld) {
                if(ld->flush_buffer)
                        ld->flush_buffer(tty);
                tty_ldisc_deref(ld);
        }
}

EXPORT_SYMBOL_GPL(tty_ldisc_flush);

/**
 *      tty_reset_termios       -       reset terminal state
 *      @tty: tty to reset
 *
 *      Restore a terminal to the driver default state
 */

static void tty_reset_termios(struct tty_struct *tty)
{
        mutex_lock(&tty->termios_mutex);
        *tty->termios = tty->driver->init_termios;
        tty->termios->c_ispeed = tty_termios_input_baud_rate(tty->termios);
        tty->termios->c_ospeed = tty_termios_baud_rate(tty->termios);
        mutex_unlock(&tty->termios_mutex);
}
        
/**
 *      do_tty_hangup           -       actual handler for hangup events
 *      @work: tty device
 *
 *      This can be called by the "eventd" kernel thread.  That is process
 *      synchronous but doesn't hold any locks, so we need to make sure we
 *      have the appropriate locks for what we're doing.
 *
 *      The hangup event clears any pending redirections onto the hung up
 *      device. It ensures future writes will error and it does the needed
 *      line discipline hangup and signal delivery. The tty object itself
 *      remains intact.
 *
 *      Locking:
 *              BKL
 *                redirect lock for undoing redirection
 *                file list lock for manipulating list of ttys
 *                tty_ldisc_lock from called functions
 *                termios_mutex resetting termios data
 *                tasklist_lock to walk task list for hangup event
 *                  ->siglock to protect ->signal/->sighand
 */
static void do_tty_hangup(struct work_struct *work)
{
        struct tty_struct *tty =
                container_of(work, struct tty_struct, hangup_work);
        struct file * cons_filp = NULL;
        struct file *filp, *f = NULL;
        struct task_struct *p;
        struct tty_ldisc *ld;
        int    closecount = 0, n;

        if (!tty)
                return;

        /* inuse_filps is protected by the single kernel lock */
        lock_kernel();

        spin_lock(&redirect_lock);
        if (redirect && redirect->private_data == tty) {
                f = redirect;
                redirect = NULL;
        }
        spin_unlock(&redirect_lock);
        
        check_tty_count(tty, "do_tty_hangup");
        file_list_lock();
        /* This breaks for file handles being sent over AF_UNIX sockets ? */
        list_for_each_entry(filp, &tty->tty_files, f_u.fu_list) {
                if (filp->f_op->write == redirected_tty_write)
                        cons_filp = filp;
                if (filp->f_op->write != tty_write)
                        continue;
                closecount++;
                tty_fasync(-1, filp, 0);        /* can't block */
                filp->f_op = &hung_up_tty_fops;
        }
        file_list_unlock();
        
        /* FIXME! What are the locking issues here? This may me overdoing things..
         * this question is especially important now that we've removed the irqlock. */

        ld = tty_ldisc_ref(tty);
        if(ld != NULL)  /* We may have no line discipline at this point */
        {
                if (ld->flush_buffer)
                        ld->flush_buffer(tty);
                if (tty->driver->flush_buffer)
                        tty->driver->flush_buffer(tty);
                if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
                    ld->write_wakeup)
                        ld->write_wakeup(tty);
                if (ld->hangup)
                        ld->hangup(tty);
        }

        /* FIXME: Once we trust the LDISC code better we can wait here for
           ldisc completion and fix the driver call race */
           
        wake_up_interruptible(&tty->write_wait);
        wake_up_interruptible(&tty->read_wait);

        /*
         * Shutdown the current line discipline, and reset it to
         * N_TTY.
         */
        if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
                tty_reset_termios(tty);
        
        /* Defer ldisc switch */
        /* tty_deferred_ldisc_switch(N_TTY);
        
          This should get done automatically when the port closes and
          tty_release is called */
        
        read_lock(&tasklist_lock);
        if (tty->session) {
                do_each_pid_task(tty->session, PIDTYPE_SID, p) {
                        spin_lock_irq(&p->sighand->siglock);
                        if (p->signal->tty == tty)
                                p->signal->tty = NULL;
                        if (!p->signal->leader) {
                                spin_unlock_irq(&p->sighand->siglock);
                                continue;
                        }
                        __group_send_sig_info(SIGHUP, SEND_SIG_PRIV, p);
                        __group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
                        put_pid(p->signal->tty_old_pgrp);  /* A noop */
                        if (tty->pgrp)
                                p->signal->tty_old_pgrp = get_pid(tty->pgrp);
                        spin_unlock_irq(&p->sighand->siglock);
                } while_each_pid_task(tty->session, PIDTYPE_SID, p);
        }
        read_unlock(&tasklist_lock);

        tty->flags = 0;
        put_pid(tty->session);
        put_pid(tty->pgrp);
        tty->session = NULL;
        tty->pgrp = NULL;
        tty->ctrl_status = 0;
        /*
         *      If one of the devices matches a console pointer, we
         *      cannot just call hangup() because that will cause
         *      tty->count and state->count to go out of sync.
         *      So we just call close() the right number of times.
         */
        if (cons_filp) {
                if (tty->driver->close)
                        for (n = 0; n < closecount; n++)
                                tty->driver->close(tty, cons_filp);
        } else if (tty->driver->hangup)
                (tty->driver->hangup)(tty);
                
        /* We don't want to have driver/ldisc interactions beyond
           the ones we did here. The driver layer expects no
           calls after ->hangup() from the ldisc side. However we
           can't yet guarantee all that */

        set_bit(TTY_HUPPED, &tty->flags);
        if (ld) {
                tty_ldisc_enable(tty);
                tty_ldisc_deref(ld);
        }
        unlock_kernel();
        if (f)
                fput(f);
}

/**
 *      tty_hangup              -       trigger a hangup event
 *      @tty: tty to hangup
 *
 *      A carrier loss (virtual or otherwise) has occurred on this like
 *      schedule a hangup sequence to run after this event.
 */

void tty_hangup(struct tty_struct * tty)
{
#ifdef TTY_DEBUG_HANGUP
        char    buf[64];
        
        printk(KERN_DEBUG "%s hangup...\n", tty_name(tty, buf));
#endif
        schedule_work(&tty->hangup_work);
}

EXPORT_SYMBOL(tty_hangup);

/**
 *      tty_vhangup             -       process vhangup
 *      @tty: tty to hangup
 *
 *      The user has asked via system call for the terminal to be hung up.
 *      We do this synchronously so that when the syscall returns the process
 *      is complete. That guarantee is neccessary for security reasons.
 */

void tty_vhangup(struct tty_struct * tty)
{
#ifdef TTY_DEBUG_HANGUP
        char    buf[64];

        printk(KERN_DEBUG "%s vhangup...\n", tty_name(tty, buf));
#endif
        do_tty_hangup(&tty->hangup_work);
}
EXPORT_SYMBOL(tty_vhangup);

/**
 *      tty_hung_up_p           -       was tty hung up
 *      @filp: file pointer of tty
 *
 *      Return true if the tty has been subject to a vhangup or a carrier
 *      loss
 */

int tty_hung_up_p(struct file * filp)
{
        return (filp->f_op == &hung_up_tty_fops);
}

EXPORT_SYMBOL(tty_hung_up_p);

static void session_clear_tty(struct pid *session)
{
        struct task_struct *p;
        do_each_pid_task(session, PIDTYPE_SID, p) {
                proc_clear_tty(p);
        } while_each_pid_task(session, PIDTYPE_SID, p);
}

/**
 *      disassociate_ctty       -       disconnect controlling tty
 *      @on_exit: true if exiting so need to "hang up" the session
 *
 *      This function is typically called only by the session leader, when
 *      it wants to disassociate itself from its controlling tty.
 *
 *      It performs the following functions:
 *      (1)  Sends a SIGHUP and SIGCONT to the foreground process group
 *      (2)  Clears the tty from being controlling the session
 *      (3)  Clears the controlling tty for all processes in the
 *              session group.
 *
 *      The argument on_exit is set to 1 if called when a process is
 *      exiting; it is 0 if called by the ioctl TIOCNOTTY.
 *
 *      Locking:
 *              BKL is taken for hysterical raisins
 *                tty_mutex is taken to protect tty
 *                ->siglock is taken to protect ->signal/->sighand
 *                tasklist_lock is taken to walk process list for sessions
 *                  ->siglock is taken to protect ->signal/->sighand
 */

void disassociate_ctty(int on_exit)
{
        struct tty_struct *tty;
        struct pid *tty_pgrp = NULL;

        lock_kernel();

        mutex_lock(&tty_mutex);
        tty = get_current_tty();
        if (tty) {
                tty_pgrp = get_pid(tty->pgrp);
                mutex_unlock(&tty_mutex);
                /* XXX: here we race, there is nothing protecting tty */
                if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
                        tty_vhangup(tty);
        } else if (on_exit) {
                struct pid *old_pgrp;
                spin_lock_irq(&current->sighand->siglock);
                old_pgrp = current->signal->tty_old_pgrp;
                current->signal->tty_old_pgrp = NULL;
                spin_unlock_irq(&current->sighand->siglock);
                if (old_pgrp) {
                        kill_pgrp(old_pgrp, SIGHUP, on_exit);
                        kill_pgrp(old_pgrp, SIGCONT, on_exit);
                        put_pid(old_pgrp);
                }
                mutex_unlock(&tty_mutex);
                unlock_kernel();        
                return;
        }
        if (tty_pgrp) {
                kill_pgrp(tty_pgrp, SIGHUP, on_exit);
                if (!on_exit)
                        kill_pgrp(tty_pgrp, SIGCONT, on_exit);
                put_pid(tty_pgrp);
        }

        spin_lock_irq(&current->sighand->siglock);
        put_pid(current->signal->tty_old_pgrp);
        current->signal->tty_old_pgrp = NULL;
        spin_unlock_irq(&current->sighand->siglock);

        mutex_lock(&tty_mutex);
        /* It is possible that do_tty_hangup has free'd this tty */
        tty = get_current_tty();
        if (tty) {
                put_pid(tty->session);
                put_pid(tty->pgrp);
                tty->session = NULL;
                tty->pgrp = NULL;
        } else {
#ifdef TTY_DEBUG_HANGUP
                printk(KERN_DEBUG "error attempted to write to tty [0x%p]"
                       " = NULL", tty);
#endif
        }
        mutex_unlock(&tty_mutex);

        /* Now clear signal->tty under the lock */
        read_lock(&tasklist_lock);
        session_clear_tty(task_session(current));
        read_unlock(&tasklist_lock);
        unlock_kernel();
}

/**
 *
 *      no_tty  - Ensure the current process does not have a controlling tty
 */
void no_tty(void)
{
        struct task_struct *tsk = current;
        if (tsk->signal->leader)
                disassociate_ctty(0);
        proc_clear_tty(tsk);
}


/**
 *      stop_tty        -       propogate flow control
 *      @tty: tty to stop
 *
 *      Perform flow control to the driver. For PTY/TTY pairs we
 *      must also propogate the TIOCKPKT status. May be called
 *      on an already stopped device and will not re-call the driver
 *      method.
 *
 *      This functionality is used by both the line disciplines for
 *      halting incoming flow and by the driver. It may therefore be
 *      called from any context, may be under the tty atomic_write_lock
 *      but not always.
 *
 *      Locking:
 *              Broken. Relies on BKL which is unsafe here.
 */

void stop_tty(struct tty_struct *tty)
{
        if (tty->stopped)
                return;
        tty->stopped = 1;
        if (tty->link && tty->link->packet) {
                tty->ctrl_status &= ~TIOCPKT_START;
                tty->ctrl_status |= TIOCPKT_STOP;
                wake_up_interruptible(&tty->link->read_wait);
        }
        if (tty->driver->stop)
                (tty->driver->stop)(tty);
}

EXPORT_SYMBOL(stop_tty);

/**
 *      start_tty       -       propogate flow control
 *      @tty: tty to start
 *
 *      Start a tty that has been stopped if at all possible. Perform
 *      any neccessary wakeups and propogate the TIOCPKT status. If this
 *      is the tty was previous stopped and is being started then the
 *      driver start method is invoked and the line discipline woken.
 *
 *      Locking:
 *              Broken. Relies on BKL which is unsafe here.
 */

void start_tty(struct tty_struct *tty)
{
        if (!tty->stopped || tty->flow_stopped)
                return;
        tty->stopped = 0;
        if (tty->link && tty->link->packet) {
                tty->ctrl_status &= ~TIOCPKT_STOP;
                tty->ctrl_status |= TIOCPKT_START;
                wake_up_interruptible(&tty->link->read_wait);
        }
        if (tty->driver->start)
                (tty->driver->start)(tty);

        /* If we have a running line discipline it may need kicking */
        tty_wakeup(tty);
}

EXPORT_SYMBOL(start_tty);

/**
 *      tty_read        -       read method for tty device files
 *      @file: pointer to tty file
 *      @buf: user buffer
 *      @count: size of user buffer
 *      @ppos: unused
 *
 *      Perform the read system call function on this terminal device. Checks
 *      for hung up devices before calling the line discipline method.
 *
 *      Locking:
 *              Locks the line discipline internally while needed
 *              For historical reasons the line discipline read method is
 *      invoked under the BKL. This will go away in time so do not rely on it
 *      in new code. Multiple read calls may be outstanding in parallel.
 */

static ssize_t tty_read(struct file * file, char __user * buf, size_t count, 
                        loff_t *ppos)
{
        int i;
        struct tty_struct * tty;
        struct inode *inode;
        struct tty_ldisc *ld;

        tty = (struct tty_struct *)file->private_data;
        inode = file->f_path.dentry->d_inode;
        if (tty_paranoia_check(tty, inode, "tty_read"))
                return -EIO;
        if (!tty || (test_bit(TTY_IO_ERROR, &tty->flags)))
                return -EIO;

        /* We want to wait for the line discipline to sort out in this
           situation */
        ld = tty_ldisc_ref_wait(tty);
        lock_kernel();
        if (ld->read)
                i = (ld->read)(tty,file,buf,count);
        else
                i = -EIO;
        tty_ldisc_deref(ld);
        unlock_kernel();
        if (i > 0)
                inode->i_atime = current_fs_time(inode->i_sb);
        return i;
}

/*
 * Split writes up in sane blocksizes to avoid
 * denial-of-service type attacks
 */
static inline ssize_t do_tty_write(
        ssize_t (*write)(struct tty_struct *, struct file *, const unsigned char *, size_t),
        struct tty_struct *tty,
        struct file *file,
        const char __user *buf,
        size_t count)
{
        ssize_t ret = 0, written = 0;
        unsigned int chunk;
        
        /* FIXME: O_NDELAY ... */
        if (mutex_lock_interruptible(&tty->atomic_write_lock)) {
                return -ERESTARTSYS;
        }

        /*
         * We chunk up writes into a temporary buffer. This
         * simplifies low-level drivers immensely, since they
         * don't have locking issues and user mode accesses.
         *
         * But if TTY_NO_WRITE_SPLIT is set, we should use a
         * big chunk-size..
         *
         * The default chunk-size is 2kB, because the NTTY
         * layer has problems with bigger chunks. It will
         * claim to be able to handle more characters than
         * it actually does.
         *
         * FIXME: This can probably go away now except that 64K chunks
         * are too likely to fail unless switched to vmalloc...
         */
        chunk = 2048;
        if (test_bit(TTY_NO_WRITE_SPLIT, &tty->flags))
                chunk = 65536;
        if (count < chunk)
                chunk = count;

        /* write_buf/write_cnt is protected by the atomic_write_lock mutex */
        if (tty->write_cnt < chunk) {
                unsigned char *buf;

                if (chunk < 1024)
                        chunk = 1024;

                buf = kmalloc(chunk, GFP_KERNEL);
                if (!buf) {
                        mutex_unlock(&tty->atomic_write_lock);
                        return -ENOMEM;
                }
                kfree(tty->write_buf);
                tty->write_cnt = chunk;
                tty->write_buf = buf;
        }

        /* Do the write .. */
        for (;;) {
                size_t size = count;
                if (size > chunk)
                        size = chunk;
                ret = -EFAULT;
                if (copy_from_user(tty->write_buf, buf, size))
                        break;
                lock_kernel();
                ret = write(tty, file, tty->write_buf, size);
                unlock_kernel();
                if (ret <= 0)
                        break;
                written += ret;
                buf += ret;
                count -= ret;
                if (!count)
                        break;
                ret = -ERESTARTSYS;
                if (signal_pending(current))
                        break;
                cond_resched();
        }
        if (written) {
                struct inode *inode = file->f_path.dentry->d_inode;
                inode->i_mtime = current_fs_time(inode->i_sb);
                ret = written;
        }
        mutex_unlock(&tty->atomic_write_lock);
        return ret;
}


/**
 *      tty_write               -       write method for tty device file
 *      @file: tty file pointer
 *      @buf: user data to write
 *      @count: bytes to write
 *      @ppos: unused
 *
 *      Write data to a tty device via the line discipline.
 *
 *      Locking:
 *              Locks the line discipline as required
 *              Writes to the tty driver are serialized by the atomic_write_lock
 *      and are then processed in chunks to the device. The line discipline
 *      write method will not be involked in parallel for each device
 *              The line discipline write method is called under the big
 *      kernel lock for historical reasons. New code should not rely on this.
 */

static ssize_t tty_write(struct file * file, const char __user * buf, size_t count,
                         loff_t *ppos)
{
        struct tty_struct * tty;
        struct inode *inode = file->f_path.dentry->d_inode;
        ssize_t ret;
        struct tty_ldisc *ld;
        
        tty = (struct tty_struct *)file->private_data;
        if (tty_paranoia_check(tty, inode, "tty_write"))
                return -EIO;
        if (!tty || !tty->driver->write || (test_bit(TTY_IO_ERROR, &tty->flags)))
                return -EIO;

        ld = tty_ldisc_ref_wait(tty);           
        if (!ld->write)
                ret = -EIO;
        else
                ret = do_tty_write(ld->write, tty, file, buf, count);
        tty_ldisc_deref(ld);
        return ret;
}

ssize_t redirected_tty_write(struct file * file, const char __user * buf, size_t count,
                         loff_t *ppos)
{
        struct file *p = NULL;

        spin_lock(&redirect_lock);
        if (redirect) {
                get_file(redirect);
                p = redirect;
        }
        spin_unlock(&redirect_lock);

        if (p) {
                ssize_t res;
                res = vfs_write(p, buf, count, &p->f_pos);
                fput(p);
                return res;
        }

        return tty_write(file, buf, count, ppos);
}

static char ptychar[] = "pqrstuvwxyzabcde";

/**
 *      pty_line_name   -       generate name for a pty
 *      @driver: the tty driver in use
 *      @index: the minor number
 *      @p: output buffer of at least 6 bytes
 *
 *      Generate a name from a driver reference and write it to the output
 *      buffer.
 *
 *      Locking: None
 */
static void pty_line_name(struct tty_driver *driver, int index, char *p)
{
        int i = index + driver->name_base;
        /* ->name is initialized to "ttyp", but "tty" is expected */
        sprintf(p, "%s%c%x",
                        driver->subtype == PTY_TYPE_SLAVE ? "tty" : driver->name,
                        ptychar[i >> 4 & 0xf], i & 0xf);
}

/**
 *      pty_line_name   -       generate name for a tty
 *      @driver: the tty driver in use
 *      @index: the minor number
 *      @p: output buffer of at least 7 bytes
 *
 *      Generate a name from a driver reference and write it to the output
 *      buffer.
 *
 *      Locking: None
 */
static void tty_line_name(struct tty_driver *driver, int index, char *p)
{
        sprintf(p, "%s%d", driver->name, index + driver->name_base);
}

/**
 *      init_dev                -       initialise a tty device
 *      @driver: tty driver we are opening a device on
 *      @idx: device index
 *      @tty: returned tty structure
 *
 *      Prepare a tty device. This may not be a "new" clean device but
 *      could also be an active device. The pty drivers require special
 *      handling because of this.
 *
 *      Locking:
 *              The function is called under the tty_mutex, which
 *      protects us from the tty struct or driver itself going away.
 *
 *      On exit the tty device has the line discipline attached and
 *      a reference count of 1. If a pair was created for pty/tty use
 *      and the other was a pty master then it too has a reference count of 1.
 *
 * WSH 06/09/97: Rewritten to remove races and properly clean up after a
 * failed open.  The new code protects the open with a mutex, so it's
 * really quite straightforward.  The mutex locking can probably be
 * relaxed for the (most common) case of reopening a tty.
 */

static int init_dev(struct tty_driver *driver, int idx,
        struct tty_struct **ret_tty)
{
        struct tty_struct *tty, *o_tty;
        struct ktermios *tp, **tp_loc, *o_tp, **o_tp_loc;
        struct ktermios *ltp, **ltp_loc, *o_ltp, **o_ltp_loc;
        int retval = 0;

        /* check whether we're reopening an existing tty */
        if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
                tty = devpts_get_tty(idx);
                /*
                 * If we don't have a tty here on a slave open, it's because
                 * the master already started the close process and there's
                 * no relation between devpts file and tty anymore.
                 */
                if (!tty && driver->subtype == PTY_TYPE_SLAVE) {
                        retval = -EIO;
                        goto end_init;
                }
                /*
                 * It's safe from now on because init_dev() is called with
                 * tty_mutex held and release_dev() won't change tty->count
                 * or tty->flags without having to grab tty_mutex
                 */
                if (tty && driver->subtype == PTY_TYPE_MASTER)
                        tty = tty->link;
        } else {
                tty = driver->ttys[idx];
        }
        if (tty) goto fast_track;

        /*
         * First time open is complex, especially for PTY devices.
         * This code guarantees that either everything succeeds and the
         * TTY is ready for operation, or else the table slots are vacated
         * and the allocated memory released.  (Except that the termios 
         * and locked termios may be retained.)
         */

        if (!try_module_get(driver->owner)) {
                retval = -ENODEV;
                goto end_init;
        }

        o_tty = NULL;
        tp = o_tp = NULL;
        ltp = o_ltp = NULL;

        tty = alloc_tty_struct();
        if(!tty)
                goto fail_no_mem;
        initialize_tty_struct(tty);
        tty->driver = driver;
        tty->index = idx;
        tty_line_name(driver, idx, tty->name);

        if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
                tp_loc = &tty->termios;
                ltp_loc = &tty->termios_locked;
        } else {
                tp_loc = &driver->termios[idx];
                ltp_loc = &driver->termios_locked[idx];
        }

        if (!*tp_loc) {
                tp = (struct ktermios *) kmalloc(sizeof(struct ktermios),
                                                GFP_KERNEL);
                if (!tp)
                        goto free_mem_out;
                *tp = driver->init_termios;
        }

        if (!*ltp_loc) {
                ltp = (struct ktermios *) kmalloc(sizeof(struct ktermios),
                                                 GFP_KERNEL);
                if (!ltp)
                        goto free_mem_out;
                memset(ltp, 0, sizeof(struct ktermios));
        }

        if (driver->type == TTY_DRIVER_TYPE_PTY) {
                o_tty = alloc_tty_struct();
                if (!o_tty)
                        goto free_mem_out;
                initialize_tty_struct(o_tty);
                o_tty->driver = driver->other;
                o_tty->index = idx;
                tty_line_name(driver->other, idx, o_tty->name);

                if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
                        o_tp_loc = &o_tty->termios;
                        o_ltp_loc = &o_tty->termios_locked;
                } else {
                        o_tp_loc = &driver->other->termios[idx];
                        o_ltp_loc = &driver->other->termios_locked[idx];
                }

                if (!*o_tp_loc) {
                        o_tp = (struct ktermios *)
                                kmalloc(sizeof(struct ktermios), GFP_KERNEL);
                        if (!o_tp)
                                goto free_mem_out;
                        *o_tp = driver->other->init_termios;
                }

                if (!*o_ltp_loc) {
                        o_ltp = (struct ktermios *)
                                kmalloc(sizeof(struct ktermios), GFP_KERNEL);
                        if (!o_ltp)
                                goto free_mem_out;
                        memset(o_ltp, 0, sizeof(struct ktermios));
                }

                /*
                 * Everything allocated ... set up the o_tty structure.
                 */
                if (!(driver->other->flags & TTY_DRIVER_DEVPTS_MEM)) {
                        driver->other->ttys[idx] = o_tty;
                }
                if (!*o_tp_loc)
                        *o_tp_loc = o_tp;
                if (!*o_ltp_loc)
                        *o_ltp_loc = o_ltp;
                o_tty->termios = *o_tp_loc;
                o_tty->termios_locked = *o_ltp_loc;
                driver->other->refcount++;
                if (driver->subtype == PTY_TYPE_MASTER)
                        o_tty->count++;

                /* Establish the links in both directions */
                tty->link   = o_tty;
                o_tty->link = tty;
        }

        /* 
         * All structures have been allocated, so now we install them.
         * Failures after this point use release_tty to clean up, so
         * there's no need to null out the local pointers.
         */
        if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM)) {
                driver->ttys[idx] = tty;
        }
        
        if (!*tp_loc)
                *tp_loc = tp;
        if (!*ltp_loc)
                *ltp_loc = ltp;
        tty->termios = *tp_loc;
        tty->termios_locked = *ltp_loc;
        /* Compatibility until drivers always set this */
        tty->termios->c_ispeed = tty_termios_input_baud_rate(tty->termios);
        tty->termios->c_ospeed = tty_termios_baud_rate(tty->termios);
        driver->refcount++;
        tty->count++;

        /* 
         * Structures all installed ... call the ldisc open routines.
         * If we fail here just call release_tty to clean up.  No need
         * to decrement the use counts, as release_tty doesn't care.
         */

        if (tty->ldisc.open) {
                retval = (tty->ldisc.open)(tty);
                if (retval)
                        goto release_mem_out;
        }
        if (o_tty && o_tty->ldisc.open) {
                retval = (o_tty->ldisc.open)(o_tty);
                if (retval) {
                        if (tty->ldisc.close)
                                (tty->ldisc.close)(tty);
                        goto release_mem_out;
                }
                tty_ldisc_enable(o_tty);
        }
        tty_ldisc_enable(tty);
        goto success;

        /*
         * This fast open can be used if the tty is already open.
         * No memory is allocated, and the only failures are from
         * attempting to open a closing tty or attempting multiple
         * opens on a pty master.
         */
fast_track:
        if (test_bit(TTY_CLOSING, &tty->flags)) {
                retval = -EIO;
                goto end_init;
        }
        if (driver->type == TTY_DRIVER_TYPE_PTY &&
            driver->subtype == PTY_TYPE_MASTER) {
                /*
                 * special case for PTY masters: only one open permitted, 
                 * and the slave side open count is incremented as well.
                 */
                if (tty->count) {
                        retval = -EIO;
                        goto end_init;
                }
                tty->link->count++;
        }
        tty->count++;
        tty->driver = driver; /* N.B. why do this every time?? */

        /* FIXME */
        if(!test_bit(TTY_LDISC, &tty->flags))
                printk(KERN_ERR "init_dev but no ldisc\n");
success:
        *ret_tty = tty;
        
        /* All paths come through here to release the mutex */
end_init:
        return retval;

        /* Release locally allocated memory ... nothing placed in slots */
free_mem_out:
        kfree(o_tp);
        if (o_tty)
                free_tty_struct(o_tty);
        kfree(ltp);
        kfree(tp);
        free_tty_struct(tty);

fail_no_mem:
        module_put(driver->owner);
        retval = -ENOMEM;
        goto end_init;

        /* call the tty release_tty routine to clean out this slot */
release_mem_out:
        if (printk_ratelimit())
                printk(KERN_INFO "init_dev: ldisc open failed, "
                                 "clearing slot %d\n", idx);
        release_tty(tty, idx);
        goto end_init;
}

/**
 *      release_one_tty         -       release tty structure memory
 *
 *      Releases memory associated with a tty structure, and clears out the
 *      driver table slots. This function is called when a device is no longer
 *      in use. It also gets called when setup of a device fails.
 *
 *      Locking:
 *              tty_mutex - sometimes only
 *              takes the file list lock internally when working on the list
 *      of ttys that the driver keeps.
 *              FIXME: should we require tty_mutex is held here ??
 */
static void release_one_tty(struct tty_struct *tty, int idx)
{
        int devpts = tty->driver->flags & TTY_DRIVER_DEVPTS_MEM;
        struct ktermios *tp;

        if (!devpts)
                tty->driver->ttys[idx] = NULL;

        if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
                tp = tty->termios;
                if (!devpts)
                        tty->driver->termios[idx] = NULL;
                kfree(tp);

                tp = tty->termios_locked;
                if (!devpts)
                        tty->driver->termios_locked[idx] = NULL;
                kfree(tp);
        }


        tty->magic = 0;
        tty->driver->refcount--;

        file_list_lock();
        list_del_init(&tty->tty_files);
        file_list_unlock();

        free_tty_struct(tty);
}

/**
 *      release_tty             -       release tty structure memory
 *
 *      Release both @tty and a possible linked partner (think pty pair),
 *      and decrement the refcount of the backing module.
 *
 *      Locking:
 *              tty_mutex - sometimes only
 *              takes the file list lock internally when working on the list
 *      of ttys that the driver keeps.
 *              FIXME: should we require tty_mutex is held here ??
 */
static void release_tty(struct tty_struct *tty, int idx)
{
        struct tty_driver *driver = tty->driver;

        if (tty->link)
                release_one_tty(tty->link, idx);
        release_one_tty(tty, idx);
        module_put(driver->owner);
}

/*
 * Even releasing the tty structures is a tricky business.. We have
 * to be very careful that the structures are all released at the
 * same time, as interrupts might otherwise get the wrong pointers.
 *
 * WSH 09/09/97: rewritten to avoid some nasty race conditions that could
 * lead to double frees or releasing memory still in use.
 */
static void release_dev(struct file * filp)
{
        struct tty_struct *tty, *o_tty;
        int     pty_master, tty_closing, o_tty_closing, do_sleep;
        int     devpts;
        int     idx;
        char    buf[64];
        unsigned long flags;
        
        tty = (struct tty_struct *)filp->private_data;
        if (tty_paranoia_check(tty, filp->f_path.dentry->d_inode, "release_dev"))
                return;

        check_tty_count(tty, "release_dev");

        tty_fasync(-1, filp, 0);

        idx = tty->index;
        pty_master = (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
                      tty->driver->subtype == PTY_TYPE_MASTER);
        devpts = (tty->driver->flags & TTY_DRIVER_DEVPTS_MEM) != 0;
        o_tty = tty->link;

#ifdef TTY_PARANOIA_CHECK
        if (idx < 0 || idx >= tty->driver->num) {
                printk(KERN_DEBUG "release_dev: bad idx when trying to "
                                  "free (%s)\n", tty->name);
                return;
        }
        if (!(tty->driver->flags & TTY_DRIVER_DEVPTS_MEM)) {
                if (tty != tty->driver->ttys[idx]) {
                        printk(KERN_DEBUG "release_dev: driver.table[%d] not tty "
                               "for (%s)\n", idx, tty->name);
                        return;
                }
                if (tty->termios != tty->driver->termios[idx]) {
                        printk(KERN_DEBUG "release_dev: driver.termios[%d] not termios "
                               "for (%s)\n",
                               idx, tty->name);
                        return;
                }
                if (tty->termios_locked != tty->driver->termios_locked[idx]) {
                        printk(KERN_DEBUG "release_dev: driver.termios_locked[%d] not "
                               "termios_locked for (%s)\n",
                               idx, tty->name);
                        return;
                }
        }
#endif

#ifdef TTY_DEBUG_HANGUP
        printk(KERN_DEBUG "release_dev of %s (tty count=%d)...",
               tty_name(tty, buf), tty->count);
#endif

#ifdef TTY_PARANOIA_CHECK
        if (tty->driver->other &&
             !(tty->driver->flags & TTY_DRIVER_DEVPTS_MEM)) {
                if (o_tty != tty->driver->other->ttys[idx]) {
                        printk(KERN_DEBUG "release_dev: other->table[%d] "
                                          "not o_tty for (%s)\n",
                               idx, tty->name);
                        return;
                }
                if (o_tty->termios != tty->driver->other->termios[idx]) {
                        printk(KERN_DEBUG "release_dev: other->termios[%d] "
                                          "not o_termios for (%s)\n",
                               idx, tty->name);
                        return;
                }
                if (o_tty->termios_locked != 
                      tty->driver->other->termios_locked[idx]) {
                        printk(KERN_DEBUG "release_dev: other->termios_locked["
                                          "%d] not o_termios_locked for (%s)\n",
                               idx, tty->name);
                        return;
                }
                if (o_tty->link != tty) {
                        printk(KERN_DEBUG "release_dev: bad pty pointers\n");
                        return;
                }
        }
#endif
        if (tty->driver->close)
                tty->driver->close(tty, filp);

        /*
         * Sanity check: if tty->count is going to zero, there shouldn't be
         * any waiters on tty->read_wait or tty->write_wait.  We test the
         * wait queues and kick everyone out _before_ actually starting to
         * close.  This ensures that we won't block while releasing the tty
         * structure.
         *
         * The test for the o_tty closing is necessary, since the master and
         * slave sides may close in any order.  If the slave side closes out
         * first, its count will be one, since the master side holds an open.
         * Thus this test wouldn't be triggered at the time the slave closes,
         * so we do it now.
         *
         * Note that it's possible for the tty to be opened again while we're
         * flushing out waiters.  By recalculating the closing flags before
         * each iteration we avoid any problems.
         */
        while (1) {
                /* Guard against races with tty->count changes elsewhere and
                   opens on /dev/tty */
                   
                mutex_lock(&tty_mutex);
                tty_closing = tty->count <= 1;
                o_tty_closing = o_tty &&
                        (o_tty->count <= (pty_master ? 1 : 0));
                do_sleep = 0;

                if (tty_closing) {
                        if (waitqueue_active(&tty->read_wait)) {
                                wake_up(&tty->read_wait);
                                do_sleep++;
                        }
                        if (waitqueue_active(&tty->write_wait)) {
                                wake_up(&tty->write_wait);
                                do_sleep++;
                        }
                }
                if (o_tty_closing) {
                        if (waitqueue_active(&o_tty->read_wait)) {
                                wake_up(&o_tty->read_wait);
                                do_sleep++;
                        }
                        if (waitqueue_active(&o_tty->write_wait)) {
                                wake_up(&o_tty->write_wait);
                                do_sleep++;
                        }
                }
                if (!do_sleep)
                        break;

                printk(KERN_WARNING "release_dev: %s: read/write wait queue "
                                    "active!\n", tty_name(tty, buf));
                mutex_unlock(&tty_mutex);
                schedule();
        }       

        /*
         * The closing flags are now consistent with the open counts on 
         * both sides, and we've completed the last operation that could 
         * block, so it's safe to proceed with closing.
         */
        if (pty_master) {
                if (--o_tty->count < 0) {
                        printk(KERN_WARNING "release_dev: bad pty slave count "
                                            "(%d) for %s\n",
                               o_tty->count, tty_name(o_tty, buf));
                        o_tty->count = 0;
                }
        }
        if (--tty->count < 0) {
                printk(KERN_WARNING "release_dev: bad tty->count (%d) for %s\n",
                       tty->count, tty_name(tty, buf));
                tty->count = 0;
        }
        
        /*
         * We've decremented tty->count, so we need to remove this file
         * descriptor off the tty->tty_files list; this serves two
         * purposes:
         *  - check_tty_count sees the correct number of file descriptors
         *    associated with this tty.
         *  - do_tty_hangup no longer sees this file descriptor as
         *    something that needs to be handled for hangups.
         */
        file_kill(filp);
        filp->private_data = NULL;

        /*
         * Perform some housekeeping before deciding whether to return.
         *
         * Set the TTY_CLOSING flag if this was the last open.  In the
         * case of a pty we may have to wait around for the other side
         * to close, and TTY_CLOSING makes sure we can't be reopened.
         */
        if(tty_closing)
                set_bit(TTY_CLOSING, &tty->flags);
        if(o_tty_closing)
                set_bit(TTY_CLOSING, &o_tty->flags);

        /*
         * If _either_ side is closing, make sure there aren't any
         * processes that still think tty or o_tty is their controlling
         * tty.
         */
        if (tty_closing || o_tty_closing) {
                read_lock(&tasklist_lock);
                session_clear_tty(tty->session);
                if (o_tty)
                        session_clear_tty(o_tty->session);
                read_unlock(&tasklist_lock);
        }

        mutex_unlock(&tty_mutex);

        /* check whether both sides are closing ... */
        if (!tty_closing || (o_tty && !o_tty_closing))
                return;
        
#ifdef TTY_DEBUG_HANGUP
        printk(KERN_DEBUG "freeing tty structure...");
#endif
        /*
         * Prevent flush_to_ldisc() from rescheduling the work for later.  Then
         * kill any delayed work. As this is the final close it does not
         * race with the set_ldisc code path.
         */
        clear_bit(TTY_LDISC, &tty->flags);
        cancel_delayed_work(&tty->buf.work);

        /*
         * Wait for ->hangup_work and ->buf.work handlers to terminate
         */
         
        flush_scheduled_work();
        
        /*
         * Wait for any short term users (we know they are just driver
         * side waiters as the file is closing so user count on the file
         * side is zero.
         */
        spin_lock_irqsave(&tty_ldisc_lock, flags);
        while(tty->ldisc.refcount)
        {
                spin_unlock_irqrestore(&tty_ldisc_lock, flags);
                wait_event(tty_ldisc_wait, tty->ldisc.refcount == 0);
                spin_lock_irqsave(&tty_ldisc_lock, flags);
        }
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        /*
         * Shutdown the current line discipline, and reset it to N_TTY.
         * N.B. why reset ldisc when we're releasing the memory??
         *
         * FIXME: this MUST get fixed for the new reflocking
         */
        if (tty->ldisc.close)
                (tty->ldisc.close)(tty);
        tty_ldisc_put(tty->ldisc.num);
        
        /*
         *      Switch the line discipline back
         */
        tty_ldisc_assign(tty, tty_ldisc_get(N_TTY));
        tty_set_termios_ldisc(tty,N_TTY); 
        if (o_tty) {
                /* FIXME: could o_tty be in setldisc here ? */
                clear_bit(TTY_LDISC, &o_tty->flags);
                if (o_tty->ldisc.close)
                        (o_tty->ldisc.close)(o_tty);
                tty_ldisc_put(o_tty->ldisc.num);
                tty_ldisc_assign(o_tty, tty_ldisc_get(N_TTY));
                tty_set_termios_ldisc(o_tty,N_TTY); 
        }
        /*
         * The release_tty function takes care of the details of clearing
         * the slots and preserving the termios structure.
         */
        release_tty(tty, idx);

#ifdef CONFIG_UNIX98_PTYS
        /* Make this pty number available for reallocation */
        if (devpts) {
                down(&allocated_ptys_lock);
                idr_remove(&allocated_ptys, idx);
                up(&allocated_ptys_lock);
        }
#endif

}

/**
 *      tty_open                -       open a tty device
 *      @inode: inode of device file
 *      @filp: file pointer to tty
 *
 *      tty_open and tty_release keep up the tty count that contains the
 *      number of opens done on a tty. We cannot use the inode-count, as
 *      different inodes might point to the same tty.
 *
 *      Open-counting is needed for pty masters, as well as for keeping
 *      track of serial lines: DTR is dropped when the last close happens.
 *      (This is not done solely through tty->count, now.  - Ted 1/27/92)
 *
 *      The termios state of a pty is reset on first open so that
 *      settings don't persist across reuse.
 *
 *      Locking: tty_mutex protects tty, get_tty_driver and init_dev work.
 *               tty->count should protect the rest.
 *               ->siglock protects ->signal/->sighand
 */

static int tty_open(struct inode * inode, struct file * filp)
{
        struct tty_struct *tty;
        int noctty, retval;
        struct tty_driver *driver;
        int index;
        dev_t device = inode->i_rdev;
        unsigned short saved_flags = filp->f_flags;

        nonseekable_open(inode, filp);
        
retry_open:
        noctty = filp->f_flags & O_NOCTTY;
        index  = -1;
        retval = 0;
        
        mutex_lock(&tty_mutex);

        if (device == MKDEV(TTYAUX_MAJOR,0)) {
                tty = get_current_tty();
                if (!tty) {
                        mutex_unlock(&tty_mutex);
                        return -ENXIO;
                }
                driver = tty->driver;
                index = tty->index;
                filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
                /* noctty = 1; */
                goto got_driver;
        }
#ifdef CONFIG_VT
        if (device == MKDEV(TTY_MAJOR,0)) {
                extern struct tty_driver *console_driver;
                driver = console_driver;
                index = fg_console;
                noctty = 1;
                goto got_driver;
        }
#endif
        if (device == MKDEV(TTYAUX_MAJOR,1)) {
                driver = console_device(&index);
                if (driver) {
                        /* Don't let /dev/console block */
                        filp->f_flags |= O_NONBLOCK;
                        noctty = 1;
                        goto got_driver;
                }
                mutex_unlock(&tty_mutex);
                return -ENODEV;
        }

        driver = get_tty_driver(device, &index);
        if (!driver) {
                mutex_unlock(&tty_mutex);
                return -ENODEV;
        }
got_driver:
        retval = init_dev(driver, index, &tty);
        mutex_unlock(&tty_mutex);
        if (retval)
                return retval;

        filp->private_data = tty;
        file_move(filp, &tty->tty_files);
        check_tty_count(tty, "tty_open");
        if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
            tty->driver->subtype == PTY_TYPE_MASTER)
                noctty = 1;
#ifdef TTY_DEBUG_HANGUP
        printk(KERN_DEBUG "opening %s...", tty->name);
#endif
        if (!retval) {
                if (tty->driver->open)
                        retval = tty->driver->open(tty, filp);
                else
                        retval = -ENODEV;
        }
        filp->f_flags = saved_flags;

        if (!retval && test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
                retval = -EBUSY;

        if (retval) {
#ifdef TTY_DEBUG_HANGUP
                printk(KERN_DEBUG "error %d in opening %s...", retval,
                       tty->name);
#endif
                release_dev(filp);
                if (retval != -ERESTARTSYS)
                        return retval;
                if (signal_pending(current))
                        return retval;
                schedule();
                /*
                 * Need to reset f_op in case a hangup happened.
                 */
                if (filp->f_op == &hung_up_tty_fops)
                        filp->f_op = &tty_fops;
                goto retry_open;
        }

        mutex_lock(&tty_mutex);
        spin_lock_irq(&current->sighand->siglock);
        if (!noctty &&
            current->signal->leader &&
            !current->signal->tty &&
            tty->session == NULL)
                __proc_set_tty(current, tty);
        spin_unlock_irq(&current->sighand->siglock);
        mutex_unlock(&tty_mutex);
        return 0;
}

#ifdef CONFIG_UNIX98_PTYS
/**
 *      ptmx_open               -       open a unix 98 pty master
 *      @inode: inode of device file
 *      @filp: file pointer to tty
 *
 *      Allocate a unix98 pty master device from the ptmx driver.
 *
 *      Locking: tty_mutex protects theinit_dev work. tty->count should
                protect the rest.
 *              allocated_ptys_lock handles the list of free pty numbers
 */

static int ptmx_open(struct inode * inode, struct file * filp)
{
        struct tty_struct *tty;
        int retval;
        int index;
        int idr_ret;

        nonseekable_open(inode, filp);

        /* find a device that is not in use. */
        down(&allocated_ptys_lock);
        if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
                up(&allocated_ptys_lock);
                return -ENOMEM;
        }
        idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
        if (idr_ret < 0) {
                up(&allocated_ptys_lock);
                if (idr_ret == -EAGAIN)
                        return -ENOMEM;
                return -EIO;
        }
        if (index >= pty_limit) {
                idr_remove(&allocated_ptys, index);
                up(&allocated_ptys_lock);
                return -EIO;
        }
        up(&allocated_ptys_lock);

        mutex_lock(&tty_mutex);
        retval = init_dev(ptm_driver, index, &tty);
        mutex_unlock(&tty_mutex);
        
        if (retval)
                goto out;

        set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */
        filp->private_data = tty;
        file_move(filp, &tty->tty_files);

        retval = -ENOMEM;
        if (devpts_pty_new(tty->link))
                goto out1;

        check_tty_count(tty, "tty_open");
        retval = ptm_driver->open(tty, filp);
        if (!retval)
                return 0;
out1:
        release_dev(filp);
        return retval;
out:
        down(&allocated_ptys_lock);
        idr_remove(&allocated_ptys, index);
        up(&allocated_ptys_lock);
        return retval;
}
#endif

/**
 *      tty_release             -       vfs callback for close
 *      @inode: inode of tty
 *      @filp: file pointer for handle to tty
 *
 *      Called the last time each file handle is closed that references
 *      this tty. There may however be several such references.
 *
 *      Locking:
 *              Takes bkl. See release_dev
 */

static int tty_release(struct inode * inode, struct file * filp)
{
        lock_kernel();
        release_dev(filp);
        unlock_kernel();
        return 0;
}

/**
 *      tty_poll        -       check tty status
 *      @filp: file being polled
 *      @wait: poll wait structures to update
 *
 *      Call the line discipline polling method to obtain the poll
 *      status of the device.
 *
 *      Locking: locks called line discipline but ldisc poll method
 *      may be re-entered freely by other callers.
 */

static unsigned int tty_poll(struct file * filp, poll_table * wait)
{
        struct tty_struct * tty;
        struct tty_ldisc *ld;
        int ret = 0;

        tty = (struct tty_struct *)filp->private_data;
        if (tty_paranoia_check(tty, filp->f_path.dentry->d_inode, "tty_poll"))
                return 0;
                
        ld = tty_ldisc_ref_wait(tty);
        if (ld->poll)
                ret = (ld->poll)(tty, filp, wait);
        tty_ldisc_deref(ld);
        return ret;
}

static int tty_fasync(int fd, struct file * filp, int on)
{
        struct tty_struct * tty;
        int retval;

        tty = (struct tty_struct *)filp->private_data;
        if (tty_paranoia_check(tty, filp->f_path.dentry->d_inode, "tty_fasync"))
                return 0;
        
        retval = fasync_helper(fd, filp, on, &tty->fasync);
        if (retval <= 0)
                return retval;

        if (on) {
                enum pid_type type;
                struct pid *pid;
                if (!waitqueue_active(&tty->read_wait))
                        tty->minimum_to_wake = 1;
                if (tty->pgrp) {
                        pid = tty->pgrp;
                        type = PIDTYPE_PGID;
                } else {
                        pid = task_pid(current);
                        type = PIDTYPE_PID;
                }
                retval = __f_setown(filp, pid, type, 0);
                if (retval)
                        return retval;
        } else {
                if (!tty->fasync && !waitqueue_active(&tty->read_wait))
                        tty->minimum_to_wake = N_TTY_BUF_SIZE;
        }
        return 0;
}

/**
 *      tiocsti                 -       fake input character
 *      @tty: tty to fake input into
 *      @p: pointer to character
 *
 *      Fake input to a tty device. Does the neccessary locking and
 *      input management.
 *
 *      FIXME: does not honour flow control ??
 *
 *      Locking:
 *              Called functions take tty_ldisc_lock
 *              current->signal->tty check is safe without locks
 *
 *      FIXME: may race normal receive processing
 */

static int tiocsti(struct tty_struct *tty, char __user *p)
{
        char ch, mbz = 0;
        struct tty_ldisc *ld;
        
        if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN))
                return -EPERM;
        if (get_user(ch, p))
                return -EFAULT;
        ld = tty_ldisc_ref_wait(tty);
        ld->receive_buf(tty, &ch, &mbz, 1);
        tty_ldisc_deref(ld);
        return 0;
}

/**
 *      tiocgwinsz              -       implement window query ioctl
 *      @tty; tty
 *      @arg: user buffer for result
 *
 *      Copies the kernel idea of the window size into the user buffer.
 *
 *      Locking: tty->termios_mutex is taken to ensure the winsize data
 *              is consistent.
 */

static int tiocgwinsz(struct tty_struct *tty, struct winsize __user * arg)
{
        int err;

        mutex_lock(&tty->termios_mutex);
        err = copy_to_user(arg, &tty->winsize, sizeof(*arg));
        mutex_unlock(&tty->termios_mutex);

        return err ? -EFAULT: 0;
}

/**
 *      tiocswinsz              -       implement window size set ioctl
 *      @tty; tty
 *      @arg: user buffer for result
 *
 *      Copies the user idea of the window size to the kernel. Traditionally
 *      this is just advisory information but for the Linux console it
 *      actually has driver level meaning and triggers a VC resize.
 *
 *      Locking:
 *              Called function use the console_sem is used to ensure we do
 *      not try and resize the console twice at once.
 *              The tty->termios_mutex is used to ensure we don't double
 *      resize and get confused. Lock order - tty->termios_mutex before
 *      console sem
 */

static int tiocswinsz(struct tty_struct *tty, struct tty_struct *real_tty,
        struct winsize __user * arg)
{
        struct winsize tmp_ws;

        if (copy_from_user(&tmp_ws, arg, sizeof(*arg)))
                return -EFAULT;

        mutex_lock(&tty->termios_mutex);
        if (!memcmp(&tmp_ws, &tty->winsize, sizeof(*arg)))
                goto done;

#ifdef CONFIG_VT
        if (tty->driver->type == TTY_DRIVER_TYPE_CONSOLE) {
                if (vc_lock_resize(tty->driver_data, tmp_ws.ws_col,
                                        tmp_ws.ws_row)) {
                        mutex_unlock(&tty->termios_mutex);
                        return -ENXIO;
                }
        }
#endif
        if (tty->pgrp)
                kill_pgrp(tty->pgrp, SIGWINCH, 1);
        if ((real_tty->pgrp != tty->pgrp) && real_tty->pgrp)
                kill_pgrp(real_tty->pgrp, SIGWINCH, 1);
        tty->winsize = tmp_ws;
        real_tty->winsize = tmp_ws;
done:
        mutex_unlock(&tty->termios_mutex);
        return 0;
}

/**
 *      tioccons        -       allow admin to move logical console
 *      @file: the file to become console
 *
 *      Allow the adminstrator to move the redirected console device
 *
 *      Locking: uses redirect_lock to guard the redirect information
 */

static int tioccons(struct file *file)
{
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        if (file->f_op->write == redirected_tty_write) {
                struct file *f;
                spin_lock(&redirect_lock);
                f = redirect;
                redirect = NULL;
                spin_unlock(&redirect_lock);
                if (f)
                        fput(f);
                return 0;
        }
        spin_lock(&redirect_lock);
        if (redirect) {
                spin_unlock(&redirect_lock);
                return -EBUSY;
        }
        get_file(file);
        redirect = file;
        spin_unlock(&redirect_lock);
        return 0;
}

/**
 *      fionbio         -       non blocking ioctl
 *      @file: file to set blocking value
 *      @p: user parameter
 *
 *      Historical tty interfaces had a blocking control ioctl before
 *      the generic functionality existed. This piece of history is preserved
 *      in the expected tty API of posix OS's.
 *
 *      Locking: none, the open fle handle ensures it won't go away.
 */

static int fionbio(struct file *file, int __user *p)
{
        int nonblock;

        if (get_user(nonblock, p))
                return -EFAULT;

        if (nonblock)
                file->f_flags |= O_NONBLOCK;
        else
                file->f_flags &= ~O_NONBLOCK;
        return 0;
}

/**
 *      tiocsctty       -       set controlling tty
 *      @tty: tty structure
 *      @arg: user argument
 *
 *      This ioctl is used to manage job control. It permits a session
 *      leader to set this tty as the controlling tty for the session.
 *
 *      Locking:
 *              Takes tty_mutex() to protect tty instance
 *              Takes tasklist_lock internally to walk sessions
 *              Takes ->siglock() when updating signal->tty
 */

static int tiocsctty(struct tty_struct *tty, int arg)
{
        int ret = 0;
        if (current->signal->leader && (task_session(current) == tty->session))
                return ret;

        mutex_lock(&tty_mutex);
        /*
         * The process must be a session leader and
         * not have a controlling tty already.
         */
        if (!current->signal->leader || current->signal->tty) {
                ret = -EPERM;
                goto unlock;
        }

        if (tty->session) {
                /*
                 * This tty is already the controlling
                 * tty for another session group!
                 */
                if ((arg == 1) && capable(CAP_SYS_ADMIN)) {
                        /*
                         * Steal it away
                         */
                        read_lock(&tasklist_lock);
                        session_clear_tty(tty->session);
                        read_unlock(&tasklist_lock);
                } else {
                        ret = -EPERM;
                        goto unlock;
                }
        }
        proc_set_tty(current, tty);
unlock:
        mutex_unlock(&tty_mutex);
        return ret;
}

/**
 *      tiocgpgrp               -       get process group
 *      @tty: tty passed by user
 *      @real_tty: tty side of the tty pased by the user if a pty else the tty
 *      @p: returned pid
 *
 *      Obtain the process group of the tty. If there is no process group
 *      return an error.
 *
 *      Locking: none. Reference to current->signal->tty is safe.
 */

static int tiocgpgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
        /*
         * (tty == real_tty) is a cheap way of
         * testing if the tty is NOT a master pty.
         */
        if (tty == real_tty && current->signal->tty != real_tty)
                return -ENOTTY;
        return put_user(pid_nr(real_tty->pgrp), p);
}

/**
 *      tiocspgrp               -       attempt to set process group
 *      @tty: tty passed by user
 *      @real_tty: tty side device matching tty passed by user
 *      @p: pid pointer
 *
 *      Set the process group of the tty to the session passed. Only
 *      permitted where the tty session is our session.
 *
 *      Locking: None
 */

static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
        struct pid *pgrp;
        pid_t pgrp_nr;
        int retval = tty_check_change(real_tty);

        if (retval == -EIO)
                return -ENOTTY;
        if (retval)
                return retval;
        if (!current->signal->tty ||
            (current->signal->tty != real_tty) ||
            (real_tty->session != task_session(current)))
                return -ENOTTY;
        if (get_user(pgrp_nr, p))
                return -EFAULT;
        if (pgrp_nr < 0)
                return -EINVAL;
        rcu_read_lock();
        pgrp = find_pid(pgrp_nr);
        retval = -ESRCH;
        if (!pgrp)
                goto out_unlock;
        retval = -EPERM;
        if (session_of_pgrp(pgrp) != task_session(current))
                goto out_unlock;
        retval = 0;
        put_pid(real_tty->pgrp);
        real_tty->pgrp = get_pid(pgrp);
out_unlock:
        rcu_read_unlock();
        return retval;
}

/**
 *      tiocgsid                -       get session id
 *      @tty: tty passed by user
 *      @real_tty: tty side of the tty pased by the user if a pty else the tty
 *      @p: pointer to returned session id
 *
 *      Obtain the session id of the tty. If there is no session
 *      return an error.
 *
 *      Locking: none. Reference to current->signal->tty is safe.
 */

static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
        /*
         * (tty == real_tty) is a cheap way of
         * testing if the tty is NOT a master pty.
        */
        if (tty == real_tty && current->signal->tty != real_tty)
                return -ENOTTY;
        if (!real_tty->session)
                return -ENOTTY;
        return put_user(pid_nr(real_tty->session), p);
}

/**
 *      tiocsetd        -       set line discipline
 *      @tty: tty device
 *      @p: pointer to user data
 *
 *      Set the line discipline according to user request.
 *
 *      Locking: see tty_set_ldisc, this function is just a helper
 */

static int tiocsetd(struct tty_struct *tty, int __user *p)
{
        int ldisc;

        if (get_user(ldisc, p))
                return -EFAULT;
        return tty_set_ldisc(tty, ldisc);
}

/**
 *      send_break      -       performed time break
 *      @tty: device to break on
 *      @duration: timeout in mS
 *
 *      Perform a timed break on hardware that lacks its own driver level
 *      timed break functionality.
 *
 *      Locking:
 *              atomic_write_lock serializes
 *
 */

static int send_break(struct tty_struct *tty, unsigned int duration)
{
        if (mutex_lock_interruptible(&tty->atomic_write_lock))
                return -EINTR;
        tty->driver->break_ctl(tty, -1);
        if (!signal_pending(current)) {
                msleep_interruptible(duration);
        }
        tty->driver->break_ctl(tty, 0);
        mutex_unlock(&tty->atomic_write_lock);
        if (signal_pending(current))
                return -EINTR;
        return 0;
}

/**
 *      tiocmget                -       get modem status
 *      @tty: tty device
 *      @file: user file pointer
 *      @p: pointer to result
 *
 *      Obtain the modem status bits from the tty driver if the feature
 *      is supported. Return -EINVAL if it is not available.
 *
 *      Locking: none (up to the driver)
 */

static int tty_tiocmget(struct tty_struct *tty, struct file *file, int __user *p)
{
        int retval = -EINVAL;

        if (tty->driver->tiocmget) {
                retval = tty->driver->tiocmget(tty, file);

                if (retval >= 0)
                        retval = put_user(retval, p);
        }
        return retval;
}

/**
 *      tiocmset                -       set modem status
 *      @tty: tty device
 *      @file: user file pointer
 *      @cmd: command - clear bits, set bits or set all
 *      @p: pointer to desired bits
 *
 *      Set the modem status bits from the tty driver if the feature
 *      is supported. Return -EINVAL if it is not available.
 *
 *      Locking: none (up to the driver)
 */

static int tty_tiocmset(struct tty_struct *tty, struct file *file, unsigned int cmd,
             unsigned __user *p)
{
        int retval = -EINVAL;

        if (tty->driver->tiocmset) {
                unsigned int set, clear, val;

                retval = get_user(val, p);
                if (retval)
                        return retval;

                set = clear = 0;
                switch (cmd) {
                case TIOCMBIS:
                        set = val;
                        break;
                case TIOCMBIC:
                        clear = val;
                        break;
                case TIOCMSET:
                        set = val;
                        clear = ~val;
                        break;
                }

                set &= TIOCM_DTR|TIOCM_RTS|TIOCM_OUT1|TIOCM_OUT2|TIOCM_LOOP;
                clear &= TIOCM_DTR|TIOCM_RTS|TIOCM_OUT1|TIOCM_OUT2|TIOCM_LOOP;

                retval = tty->driver->tiocmset(tty, file, set, clear);
        }
        return retval;
}

/*
 * Split this up, as gcc can choke on it otherwise..
 */
int tty_ioctl(struct inode * inode, struct file * file,
              unsigned int cmd, unsigned long arg)
{
        struct tty_struct *tty, *real_tty;
        void __user *p = (void __user *)arg;
        int retval;
        struct tty_ldisc *ld;
        
        tty = (struct tty_struct *)file->private_data;
        if (tty_paranoia_check(tty, inode, "tty_ioctl"))
                return -EINVAL;

        /* CHECKME: is this safe as one end closes ? */

        real_tty = tty;
        if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
            tty->driver->subtype == PTY_TYPE_MASTER)
                real_tty = tty->link;

        /*
         * Break handling by driver
         */
        if (!tty->driver->break_ctl) {
                switch(cmd) {
                case TIOCSBRK:
                case TIOCCBRK:
                        if (tty->driver->ioctl)
                                return tty->driver->ioctl(tty, file, cmd, arg);
                        return -EINVAL;
                        
                /* These two ioctl's always return success; even if */
                /* the driver doesn't support them. */
                case TCSBRK:
                case TCSBRKP:
                        if (!tty->driver->ioctl)
                                return 0;
                        retval = tty->driver->ioctl(tty, file, cmd, arg);
                        if (retval == -ENOIOCTLCMD)
                                retval = 0;
                        return retval;
                }
        }

        /*
         * Factor out some common prep work
         */
        switch (cmd) {
        case TIOCSETD:
        case TIOCSBRK:
        case TIOCCBRK:
        case TCSBRK:
        case TCSBRKP:                   
                retval = tty_check_change(tty);
                if (retval)
                        return retval;
                if (cmd != TIOCCBRK) {
                        tty_wait_until_sent(tty, 0);
                        if (signal_pending(current))
                                return -EINTR;
                }
                break;
        }

        switch (cmd) {
                case TIOCSTI:
                        return tiocsti(tty, p);
                case TIOCGWINSZ:
                        return tiocgwinsz(tty, p);
                case TIOCSWINSZ:
                        return tiocswinsz(tty, real_tty, p);
                case TIOCCONS:
                        return real_tty!=tty ? -EINVAL : tioccons(file);
                case FIONBIO:
                        return fionbio(file, p);
                case TIOCEXCL:
                        set_bit(TTY_EXCLUSIVE, &tty->flags);
                        return 0;
                case TIOCNXCL:
                        clear_bit(TTY_EXCLUSIVE, &tty->flags);
                        return 0;
                case TIOCNOTTY:
                        if (current->signal->tty != tty)
                                return -ENOTTY;
                        no_tty();
                        return 0;
                case TIOCSCTTY:
                        return tiocsctty(tty, arg);
                case TIOCGPGRP:
                        return tiocgpgrp(tty, real_tty, p);
                case TIOCSPGRP:
                        return tiocspgrp(tty, real_tty, p);
                case TIOCGSID:
                        return tiocgsid(tty, real_tty, p);
                case TIOCGETD:
                        /* FIXME: check this is ok */
                        return put_user(tty->ldisc.num, (int __user *)p);
                case TIOCSETD:
                        return tiocsetd(tty, p);
#ifdef CONFIG_VT
                case TIOCLINUX:
                        return tioclinux(tty, arg);
#endif
                /*
                 * Break handling
                 */
                case TIOCSBRK:  /* Turn break on, unconditionally */
                        tty->driver->break_ctl(tty, -1);
                        return 0;
                        
                case TIOCCBRK:  /* Turn break off, unconditionally */
                        tty->driver->break_ctl(tty, 0);
                        return 0;
                case TCSBRK:   /* SVID version: non-zero arg --> no break */
                        /* non-zero arg means wait for all output data
                         * to be sent (performed above) but don't send break.
                         * This is used by the tcdrain() termios function.
                         */
                        if (!arg)
                                return send_break(tty, 250);
                        return 0;
                case TCSBRKP:   /* support for POSIX tcsendbreak() */   
                        return send_break(tty, arg ? arg*100 : 250);

                case TIOCMGET:
                        return tty_tiocmget(tty, file, p);

                case TIOCMSET:
                case TIOCMBIC:
                case TIOCMBIS:
                        return tty_tiocmset(tty, file, cmd, p);
        }
        if (tty->driver->ioctl) {
                retval = (tty->driver->ioctl)(tty, file, cmd, arg);
                if (retval != -ENOIOCTLCMD)
                        return retval;
        }
        ld = tty_ldisc_ref_wait(tty);
        retval = -EINVAL;
        if (ld->ioctl) {
                retval = ld->ioctl(tty, file, cmd, arg);
                if (retval == -ENOIOCTLCMD)
                        retval = -EINVAL;
        }
        tty_ldisc_deref(ld);
        return retval;
}


/*
 * This implements the "Secure Attention Key" ---  the idea is to
 * prevent trojan horses by killing all processes associated with this
 * tty when the user hits the "Secure Attention Key".  Required for
 * super-paranoid applications --- see the Orange Book for more details.
 * 
 * This code could be nicer; ideally it should send a HUP, wait a few
 * seconds, then send a INT, and then a KILL signal.  But you then
 * have to coordinate with the init process, since all processes associated
 * with the current tty must be dead before the new getty is allowed
 * to spawn.
 *
 * Now, if it would be correct ;-/ The current code has a nasty hole -
 * it doesn't catch files in flight. We may send the descriptor to ourselves
 * via AF_UNIX socket, close it and later fetch from socket. FIXME.
 *
 * Nasty bug: do_SAK is being called in interrupt context.  This can
 * deadlock.  We punt it up to process context.  AKPM - 16Mar2001
 */
void __do_SAK(struct tty_struct *tty)
{
#ifdef TTY_SOFT_SAK
        tty_hangup(tty);
#else
        struct task_struct *g, *p;
        struct pid *session;
        int             i;
        struct file     *filp;
        struct fdtable *fdt;
        
        if (!tty)
                return;
        session = tty->session;
        
        tty_ldisc_flush(tty);

        if (tty->driver->flush_buffer)
                tty->driver->flush_buffer(tty);
        
        read_lock(&tasklist_lock);
        /* Kill the entire session */
        do_each_pid_task(session, PIDTYPE_SID, p) {
                printk(KERN_NOTICE "SAK: killed process %d"
                        " (%s): process_session(p)==tty->session\n",
                        p->pid, p->comm);
                send_sig(SIGKILL, p, 1);
        } while_each_pid_task(session, PIDTYPE_SID, p);
        /* Now kill any processes that happen to have the
         * tty open.
         */
        do_each_thread(g, p) {
                if (p->signal->tty == tty) {
                        printk(KERN_NOTICE "SAK: killed process %d"
                            " (%s): process_session(p)==tty->session\n",
                            p->pid, p->comm);
                        send_sig(SIGKILL, p, 1);
                        continue;
                }
                task_lock(p);
                if (p->files) {
                        /*
                         * We don't take a ref to the file, so we must
                         * hold ->file_lock instead.
                         */
                        spin_lock(&p->files->file_lock);
                        fdt = files_fdtable(p->files);
                        for (i=0; i < fdt->max_fds; i++) {
                                filp = fcheck_files(p->files, i);
                                if (!filp)
                                        continue;
                                if (filp->f_op->read == tty_read &&
                                    filp->private_data == tty) {
                                        printk(KERN_NOTICE "SAK: killed process %d"
                                            " (%s): fd#%d opened to the tty\n",
                                            p->pid, p->comm, i);
                                        force_sig(SIGKILL, p);
                                        break;
                                }
                        }
                        spin_unlock(&p->files->file_lock);
                }
                task_unlock(p);
        } while_each_thread(g, p);
        read_unlock(&tasklist_lock);
#endif
}

static void do_SAK_work(struct work_struct *work)
{
        struct tty_struct *tty =
                container_of(work, struct tty_struct, SAK_work);
        __do_SAK(tty);
}

/*
 * The tq handling here is a little racy - tty->SAK_work may already be queued.
 * Fortunately we don't need to worry, because if ->SAK_work is already queued,
 * the values which we write to it will be identical to the values which it
 * already has. --akpm
 */
void do_SAK(struct tty_struct *tty)
{
        if (!tty)
                return;
        schedule_work(&tty->SAK_work);
}

EXPORT_SYMBOL(do_SAK);

/**
 *      flush_to_ldisc
 *      @work: tty structure passed from work queue.
 *
 *      This routine is called out of the software interrupt to flush data
 *      from the buffer chain to the line discipline.
 *
 *      Locking: holds tty->buf.lock to guard buffer list. Drops the lock
 *      while invoking the line discipline receive_buf method. The
 *      receive_buf method is single threaded for each tty instance.
 */
 
static void flush_to_ldisc(struct work_struct *work)
{
        struct tty_struct *tty =
                container_of(work, struct tty_struct, buf.work.work);
        unsigned long   flags;
        struct tty_ldisc *disc;
        struct tty_buffer *tbuf, *head;
        char *char_buf;
        unsigned char *flag_buf;

        disc = tty_ldisc_ref(tty);
        if (disc == NULL)       /*  !TTY_LDISC */
                return;

        spin_lock_irqsave(&tty->buf.lock, flags);
        head = tty->buf.head;
        if (head != NULL) {
                tty->buf.head = NULL;
                for (;;) {
                        int count = head->commit - head->read;
                        if (!count) {
                                if (head->next == NULL)
                                        break;
                                tbuf = head;
                                head = head->next;
                                tty_buffer_free(tty, tbuf);
                                continue;
                        }
                        if (!tty->receive_room) {
                                schedule_delayed_work(&tty->buf.work, 1);
                                break;
                        }
                        if (count > tty->receive_room)
                                count = tty->receive_room;
                        char_buf = head->char_buf_ptr + head->read;
                        flag_buf = head->flag_buf_ptr + head->read;
                        head->read += count;
                        spin_unlock_irqrestore(&tty->buf.lock, flags);
                        disc->receive_buf(tty, char_buf, flag_buf, count);
                        spin_lock_irqsave(&tty->buf.lock, flags);
                }
                tty->buf.head = head;
        }
        spin_unlock_irqrestore(&tty->buf.lock, flags);

        tty_ldisc_deref(disc);
}

/**
 *      tty_flip_buffer_push    -       terminal
 *      @tty: tty to push
 *
 *      Queue a push of the terminal flip buffers to the line discipline. This
 *      function must not be called from IRQ context if tty->low_latency is set.
 *
 *      In the event of the queue being busy for flipping the work will be
 *      held off and retried later.
 *
 *      Locking: tty buffer lock. Driver locks in low latency mode.
 */

void tty_flip_buffer_push(struct tty_struct *tty)
{
        unsigned long flags;
        spin_lock_irqsave(&tty->buf.lock, flags);
        if (tty->buf.tail != NULL)
                tty->buf.tail->commit = tty->buf.tail->used;
        spin_unlock_irqrestore(&tty->buf.lock, flags);

        if (tty->low_latency)
                flush_to_ldisc(&tty->buf.work.work);
        else
                schedule_delayed_work(&tty->buf.work, 1);
}

EXPORT_SYMBOL(tty_flip_buffer_push);


/**
 *      initialize_tty_struct
 *      @tty: tty to initialize
 *
 *      This subroutine initializes a tty structure that has been newly
 *      allocated.
 *
 *      Locking: none - tty in question must not be exposed at this point
 */

static void initialize_tty_struct(struct tty_struct *tty)
{
        memset(tty, 0, sizeof(struct tty_struct));
        tty->magic = TTY_MAGIC;
        tty_ldisc_assign(tty, tty_ldisc_get(N_TTY));
        tty->session = NULL;
        tty->pgrp = NULL;
        tty->overrun_time = jiffies;
        tty->buf.head = tty->buf.tail = NULL;
        tty_buffer_init(tty);
        INIT_DELAYED_WORK(&tty->buf.work, flush_to_ldisc);
        init_MUTEX(&tty->buf.pty_sem);
        mutex_init(&tty->termios_mutex);
        init_waitqueue_head(&tty->write_wait);
        init_waitqueue_head(&tty->read_wait);
        INIT_WORK(&tty->hangup_work, do_tty_hangup);
        mutex_init(&tty->atomic_read_lock);
        mutex_init(&tty->atomic_write_lock);
        spin_lock_init(&tty->read_lock);
        INIT_LIST_HEAD(&tty->tty_files);
        INIT_WORK(&tty->SAK_work, do_SAK_work);
}

/*
 * The default put_char routine if the driver did not define one.
 */

static void tty_default_put_char(struct tty_struct *tty, unsigned char ch)
{
        tty->driver->write(tty, &ch, 1);
}

static struct class *tty_class;

/**
 *      tty_register_device - register a tty device
 *      @driver: the tty driver that describes the tty device
 *      @index: the index in the tty driver for this tty device
 *      @device: a struct device that is associated with this tty device.
 *              This field is optional, if there is no known struct device
 *              for this tty device it can be set to NULL safely.
 *
 *      Returns a pointer to the struct device for this tty device
 *      (or ERR_PTR(-EFOO) on error).
 *
 *      This call is required to be made to register an individual tty device
 *      if the tty driver's flags have the TTY_DRIVER_DYNAMIC_DEV bit set.  If
 *      that bit is not set, this function should not be called by a tty
 *      driver.
 *
 *      Locking: ??
 */

struct device *tty_register_device(struct tty_driver *driver, unsigned index,
                                   struct device *device)
{
        char name[64];
        dev_t dev = MKDEV(driver->major, driver->minor_start) + index;

        if (index >= driver->num) {
                printk(KERN_ERR "Attempt to register invalid tty line number "
                       " (%d).\n", index);
                return ERR_PTR(-EINVAL);
        }

        if (driver->type == TTY_DRIVER_TYPE_PTY)
                pty_line_name(driver, index, name);
        else
                tty_line_name(driver, index, name);

        return device_create(tty_class, device, dev, name);
}

/**
 *      tty_unregister_device - unregister a tty device
 *      @driver: the tty driver that describes the tty device
 *      @index: the index in the tty driver for this tty device
 *
 *      If a tty device is registered with a call to tty_register_device() then
 *      this function must be called when the tty device is gone.
 *
 *      Locking: ??
 */

void tty_unregister_device(struct tty_driver *driver, unsigned index)
{
        device_destroy(tty_class, MKDEV(driver->major, driver->minor_start) + index);
}

EXPORT_SYMBOL(tty_register_device);
EXPORT_SYMBOL(tty_unregister_device);

struct tty_driver *alloc_tty_driver(int lines)
{
        struct tty_driver *driver;

        driver = kmalloc(sizeof(struct tty_driver), GFP_KERNEL);
        if (driver) {
                memset(driver, 0, sizeof(struct tty_driver));
                driver->magic = TTY_DRIVER_MAGIC;
                driver->num = lines;
                /* later we'll move allocation of tables here */
        }
        return driver;
}

void put_tty_driver(struct tty_driver *driver)
{
        kfree(driver);
}

void tty_set_operations(struct tty_driver *driver,
                        const struct tty_operations *op)
{
        driver->open = op->open;
        driver->close = op->close;
        driver->write = op->write;
        driver->put_char = op->put_char;
        driver->flush_chars = op->flush_chars;
        driver->write_room = op->write_room;
        driver->chars_in_buffer = op->chars_in_buffer;
        driver->ioctl = op->ioctl;
        driver->set_termios = op->set_termios;
        driver->throttle = op->throttle;
        driver->unthrottle = op->unthrottle;
        driver->stop = op->stop;
        driver->start = op->start;
        driver->hangup = op->hangup;
        driver->break_ctl = op->break_ctl;
        driver->flush_buffer = op->flush_buffer;
        driver->set_ldisc = op->set_ldisc;
        driver->wait_until_sent = op->wait_until_sent;
        driver->send_xchar = op->send_xchar;
        driver->read_proc = op->read_proc;
        driver->write_proc = op->write_proc;
        driver->tiocmget = op->tiocmget;
        driver->tiocmset = op->tiocmset;
}


EXPORT_SYMBOL(alloc_tty_driver);
EXPORT_SYMBOL(put_tty_driver);
EXPORT_SYMBOL(tty_set_operations);

/*
 * Called by a tty driver to register itself.
 */
int tty_register_driver(struct tty_driver *driver)
{
        int error;
        int i;
        dev_t dev;
        void **p = NULL;

        if (driver->flags & TTY_DRIVER_INSTALLED)
                return 0;

        if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM) && driver->num) {
                p = kzalloc(driver->num * 3 * sizeof(void *), GFP_KERNEL);
                if (!p)
                        return -ENOMEM;
        }

        if (!driver->major) {
                error = alloc_chrdev_region(&dev, driver->minor_start, driver->num,
                                                driver->name);
                if (!error) {
                        driver->major = MAJOR(dev);
                        driver->minor_start = MINOR(dev);
                }
        } else {
                dev = MKDEV(driver->major, driver->minor_start);
                error = register_chrdev_region(dev, driver->num, driver->name);
        }
        if (error < 0) {
                kfree(p);
                return error;
        }

        if (p) {
                driver->ttys = (struct tty_struct **)p;
                driver->termios = (struct ktermios **)(p + driver->num);
                driver->termios_locked = (struct ktermios **)(p + driver->num * 2);
        } else {
                driver->ttys = NULL;
                driver->termios = NULL;
                driver->termios_locked = NULL;
        }

        cdev_init(&driver->cdev, &tty_fops);
        driver->cdev.owner = driver->owner;
        error = cdev_add(&driver->cdev, dev, driver->num);
        if (error) {
                unregister_chrdev_region(dev, driver->num);
                driver->ttys = NULL;
                driver->termios = driver->termios_locked = NULL;
                kfree(p);
                return error;
        }

        if (!driver->put_char)
                driver->put_char = tty_default_put_char;
        
        mutex_lock(&tty_mutex);
        list_add(&driver->tty_drivers, &tty_drivers);
        mutex_unlock(&tty_mutex);
        
        if ( !(driver->flags & TTY_DRIVER_DYNAMIC_DEV) ) {
                for(i = 0; i < driver->num; i++)
                    tty_register_device(driver, i, NULL);
        }
        proc_tty_register_driver(driver);
        return 0;
}

EXPORT_SYMBOL(tty_register_driver);

/*
 * Called by a tty driver to unregister itself.
 */
int tty_unregister_driver(struct tty_driver *driver)
{
        int i;
        struct ktermios *tp;
        void *p;

        if (driver->refcount)
                return -EBUSY;

        unregister_chrdev_region(MKDEV(driver->major, driver->minor_start),
                                driver->num);
        mutex_lock(&tty_mutex);
        list_del(&driver->tty_drivers);
        mutex_unlock(&tty_mutex);

        /*
         * Free the termios and termios_locked structures because
         * we don't want to get memory leaks when modular tty
         * drivers are removed from the kernel.
         */
        for (i = 0; i < driver->num; i++) {
                tp = driver->termios[i];
                if (tp) {
                        driver->termios[i] = NULL;
                        kfree(tp);
                }
                tp = driver->termios_locked[i];
                if (tp) {
                        driver->termios_locked[i] = NULL;
                        kfree(tp);
                }
                if (!(driver->flags & TTY_DRIVER_DYNAMIC_DEV))
                        tty_unregister_device(driver, i);
        }
        p = driver->ttys;
        proc_tty_unregister_driver(driver);
        driver->ttys = NULL;
        driver->termios = driver->termios_locked = NULL;
        kfree(p);
        cdev_del(&driver->cdev);
        return 0;
}
EXPORT_SYMBOL(tty_unregister_driver);

dev_t tty_devnum(struct tty_struct *tty)
{
        return MKDEV(tty->driver->major, tty->driver->minor_start) + tty->index;
}
EXPORT_SYMBOL(tty_devnum);

void proc_clear_tty(struct task_struct *p)
{
        spin_lock_irq(&p->sighand->siglock);
        p->signal->tty = NULL;
        spin_unlock_irq(&p->sighand->siglock);
}

static void __proc_set_tty(struct task_struct *tsk, struct tty_struct *tty)
{
        if (tty) {
                /* We should not have a session or pgrp to here but.... */
                put_pid(tty->session);
                put_pid(tty->pgrp);
                tty->session = get_pid(task_session(tsk));
                tty->pgrp = get_pid(task_pgrp(tsk));
        }
        put_pid(tsk->signal->tty_old_pgrp);
        tsk->signal->tty = tty;
        tsk->signal->tty_old_pgrp = NULL;
}

static void proc_set_tty(struct task_struct *tsk, struct tty_struct *tty)
{
        spin_lock_irq(&tsk->sighand->siglock);
        __proc_set_tty(tsk, tty);
        spin_unlock_irq(&tsk->sighand->siglock);
}

struct tty_struct *get_current_tty(void)
{
        struct tty_struct *tty;
        WARN_ON_ONCE(!mutex_is_locked(&tty_mutex));
        tty = current->signal->tty;
        /*
         * session->tty can be changed/cleared from under us, make sure we
         * issue the load. The obtained pointer, when not NULL, is valid as
         * long as we hold tty_mutex.
         */
        barrier();
        return tty;
}
EXPORT_SYMBOL_GPL(get_current_tty);

/*
 * Initialize the console device. This is called *early*, so
 * we can't necessarily depend on lots of kernel help here.
 * Just do some early initializations, and do the complex setup
 * later.
 */
void __init console_init(void)
{
        initcall_t *call;

        /* Setup the default TTY line discipline. */
        (void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);

        /*
         * set up the console device so that later boot sequences can 
         * inform about problems etc..
         */
        call = __con_initcall_start;
        while (call < __con_initcall_end) {
                (*call)();
                call++;
        }
}

#ifdef CONFIG_VT
extern int vty_init(void);
#endif

static int __init tty_class_init(void)
{
        tty_class = class_create(THIS_MODULE, "tty");
        if (IS_ERR(tty_class))
                return PTR_ERR(tty_class);
        return 0;
}

postcore_initcall(tty_class_init);

/* 3/2004 jmc: why do these devices exist? */

static struct cdev tty_cdev, console_cdev;
#ifdef CONFIG_UNIX98_PTYS
static struct cdev ptmx_cdev;
#endif
#ifdef CONFIG_VT
static struct cdev vc0_cdev;
#endif

/*
 * Ok, now we can initialize the rest of the tty devices and can count
 * on memory allocations, interrupts etc..
 */
static int __init tty_init(void)
{
        cdev_init(&tty_cdev, &tty_fops);
        if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) ||
            register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0)
                panic("Couldn't register /dev/tty driver\n");
        device_create(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 0), "tty");

        cdev_init(&console_cdev, &console_fops);
        if (cdev_add(&console_cdev, MKDEV(TTYAUX_MAJOR, 1), 1) ||
            register_chrdev_region(MKDEV(TTYAUX_MAJOR, 1), 1, "/dev/console") < 0)
                panic("Couldn't register /dev/console driver\n");
        device_create(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 1), "console");

#ifdef CONFIG_UNIX98_PTYS
        cdev_init(&ptmx_cdev, &ptmx_fops);
        if (cdev_add(&ptmx_cdev, MKDEV(TTYAUX_MAJOR, 2), 1) ||
            register_chrdev_region(MKDEV(TTYAUX_MAJOR, 2), 1, "/dev/ptmx") < 0)
                panic("Couldn't register /dev/ptmx driver\n");
        device_create(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 2), "ptmx");
#endif

#ifdef CONFIG_VT
        cdev_init(&vc0_cdev, &console_fops);
        if (cdev_add(&vc0_cdev, MKDEV(TTY_MAJOR, 0), 1) ||
            register_chrdev_region(MKDEV(TTY_MAJOR, 0), 1, "/dev/vc/0") < 0)
                panic("Couldn't register /dev/tty0 driver\n");
        device_create(tty_class, NULL, MKDEV(TTY_MAJOR, 0), "tty0");

        vty_init();
#endif
        return 0;
}
module_init(tty_init);