netfilter: xt_hashlimit: fix race between htable_destroy and htable_gc

[linux-2.6/mini2440.git] / drivers / char / tty_ldisc.c

#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/fdtable.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/wait.h>
#include <linux/bitops.h>
#include <linux/delay.h>
#include <linux/seq_file.h>

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

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

#include <linux/kmod.h>
#include <linux/nsproxy.h>

/*
 *      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);
/* Line disc dispatch table */
static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];

/**
 *      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_ops *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;
        new_ldisc->num = disc;
        new_ldisc->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] = NULL;
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);

        return ret;
}
EXPORT_SYMBOL(tty_unregister_ldisc);


/**
 *      tty_ldisc_try_get       -       try and reference an ldisc
 *      @disc: ldisc number
 *      @ld: tty ldisc structure to complete
 *
 *      Attempt to open and lock a line discipline into place. Return
 *      the line discipline refcounted and assigned in ld. On an error
 *      report the error code back
 */

static int tty_ldisc_try_get(int disc, struct tty_ldisc *ld)
{
        unsigned long flags;
        struct tty_ldisc_ops *ldops;
        int err = -EINVAL;
        
        spin_lock_irqsave(&tty_ldisc_lock, flags);
        ld->ops = NULL;
        ldops = tty_ldiscs[disc];
        /* Check the entry is defined */
        if (ldops) {
                /* If the module is being unloaded we can't use it */
                if (!try_module_get(ldops->owner))
                        err = -EAGAIN;
                else {
                        /* lock it */
                        ldops->refcount++;
                        ld->ops = ldops;
                        err = 0;
                }
        }
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return err;
}

/**
 *      tty_ldisc_get           -       take a reference to an ldisc
 *      @disc: ldisc number
 *      @ld: tty line discipline structure to use
 *
 *      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
 */

static int tty_ldisc_get(int disc, struct tty_ldisc *ld)
{
        int err;

        if (disc < N_TTY || disc >= NR_LDISCS)
                return -EINVAL;
        err = tty_ldisc_try_get(disc, ld);
        if (err == -EAGAIN) {
                request_module("tty-ldisc-%d", disc);
                err = tty_ldisc_try_get(disc, ld);
        }
        return err;
}

/**
 *      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
 */

static void tty_ldisc_put(struct tty_ldisc_ops *ld)
{
        unsigned long flags;
        int disc = ld->num;

        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);
}

static void * tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
{
        return (*pos < NR_LDISCS) ? pos : NULL;
}

static void * tty_ldiscs_seq_next(struct seq_file *m, void *v, loff_t *pos)
{
        (*pos)++;
        return (*pos < NR_LDISCS) ? pos : NULL;
}

static void tty_ldiscs_seq_stop(struct seq_file *m, void *v)
{
}

static int tty_ldiscs_seq_show(struct seq_file *m, void *v)
{
        int i = *(loff_t *)v;
        struct tty_ldisc ld;
        
        if (tty_ldisc_get(i, &ld) < 0)
                return 0;
        seq_printf(m, "%-10s %2d\n", ld.ops->name ? ld.ops->name : "???", i);
        tty_ldisc_put(ld.ops);
        return 0;
}

static const struct seq_operations tty_ldiscs_seq_ops = {
        .start  = tty_ldiscs_seq_start,
        .next   = tty_ldiscs_seq_next,
        .stop   = tty_ldiscs_seq_stop,
        .show   = tty_ldiscs_seq_show,
};

static int proc_tty_ldiscs_open(struct inode *inode, struct file *file)
{
        return seq_open(file, &tty_ldiscs_seq_ops);
}

const struct file_operations tty_ldiscs_proc_fops = {
        .owner          = THIS_MODULE,
        .open           = proc_tty_ldiscs_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = seq_release,
};

/**
 *      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)
{
        ld->refcount = 0;
        tty->ldisc = *ld;
}

/**
 *      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 necessary wakeups for existing sleepers.
 *
 *      Note: nobody should set this bit except via this function. Clearing
 *      directly is allowed.
 */

void tty_ldisc_enable(struct tty_struct *tty)
{
        set_bit(TTY_LDISC, &tty->flags);
        wake_up(&tty_ldisc_wait);
}

/**
 *      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);
}


/**
 *      tty_ldisc_restore       -       helper for tty ldisc change
 *      @tty: tty to recover
 *      @old: previous ldisc
 *
 *      Restore the previous line discipline or N_TTY when a line discipline
 *      change fails due to an open error
 */

static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
{
        char buf[64];
        struct tty_ldisc new_ldisc;

        /* There is an outstanding reference here so this is safe */
        tty_ldisc_get(old->ops->num, old);
        tty_ldisc_assign(tty, old);
        tty_set_termios_ldisc(tty, old->ops->num);
        if (old->ops->open && (old->ops->open(tty) < 0)) {
                tty_ldisc_put(old->ops);
                /* This driver is always present */
                if (tty_ldisc_get(N_TTY, &new_ldisc) < 0)
                        panic("n_tty: get");
                tty_ldisc_assign(tty, &new_ldisc);
                tty_set_termios_ldisc(tty, N_TTY);
                if (new_ldisc.ops->open) {
                        int r = new_ldisc.ops->open(tty);
                                if (r < 0)
                                panic("Couldn't open N_TTY ldisc for "
                                      "%s --- error %d.",
                                      tty_name(tty, buf), r);
                }
        }
}

/**
 *      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
 */

int tty_set_ldisc(struct tty_struct *tty, int ldisc)
{
        int retval;
        struct tty_ldisc o_ldisc, new_ldisc;
        int work;
        unsigned long flags;
        struct tty_struct *o_tty;

restart:
        /* This is a bit ugly for now but means we can break the 'ldisc
           is part of the tty struct' assumption later */
        retval = tty_ldisc_get(ldisc, &new_ldisc);
        if (retval)
                return retval;

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

        tty_wait_until_sent(tty, 0);

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

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

        tty->receive_room = 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(o_ldisc.ops);
                        /*
                         * 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(o_tty->ldisc.ops);
                        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)) {
                struct tty_ldisc *ld;
                spin_unlock_irqrestore(&tty_ldisc_lock, flags);
                tty_ldisc_put(new_ldisc.ops);
                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
         * MUST NOT hold locks here.
         */
        flush_scheduled_work();
        /* Shutdown the current discipline. */
        if (o_ldisc.ops->close)
                (o_ldisc.ops->close)(tty);

        /* Now set up the new line discipline. */
        tty_ldisc_assign(tty, &new_ldisc);
        tty_set_termios_ldisc(tty, ldisc);
        if (new_ldisc.ops->open)
                retval = (new_ldisc.ops->open)(tty);
        if (retval < 0) {
                tty_ldisc_put(new_ldisc.ops);
                tty_ldisc_restore(tty, &o_ldisc);
        }
        /* 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.ops->num != o_ldisc.ops->num && tty->ops->set_ldisc)
                tty->ops->set_ldisc(tty);

        tty_ldisc_put(o_ldisc.ops);

        /*
         *      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;
}


/**
 *      tty_ldisc_setup                 -       open line discipline
 *      @tty: tty being shut down
 *      @o_tty: pair tty for pty/tty pairs
 *
 *      Called during the initial open of a tty/pty pair in order to set up the
 *      line discplines and bind them to the tty.
 */

int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
{
        struct tty_ldisc *ld = &tty->ldisc;
        int retval;

        if (ld->ops->open) {
                retval = (ld->ops->open)(tty);
                if (retval)
                        return retval;
        }
        if (o_tty && o_tty->ldisc.ops->open) {
                retval = (o_tty->ldisc.ops->open)(o_tty);
                if (retval) {
                        if (ld->ops->close)
                                (ld->ops->close)(tty);
                        return retval;
                }
                tty_ldisc_enable(o_tty);
        }
        tty_ldisc_enable(tty);
        return 0;
}

/**
 *      tty_ldisc_release               -       release line discipline
 *      @tty: tty being shut down
 *      @o_tty: pair tty for pty/tty pairs
 *
 *      Called during the final close of a tty/pty pair in order to shut down the
 *      line discpline layer.
 */

void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
{
        unsigned long flags;
        struct tty_ldisc ld;
        /*
         * 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.
         *
         * FIXME: this MUST get fixed for the new reflocking
         */
        if (tty->ldisc.ops->close)
                (tty->ldisc.ops->close)(tty);
        tty_ldisc_put(tty->ldisc.ops);

        /*
         *      Switch the line discipline back
         */
        WARN_ON(tty_ldisc_get(N_TTY, &ld));
        tty_ldisc_assign(tty, &ld);
        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.ops->close)
                        (o_tty->ldisc.ops->close)(o_tty);
                tty_ldisc_put(o_tty->ldisc.ops);
                WARN_ON(tty_ldisc_get(N_TTY, &ld));
                tty_ldisc_assign(o_tty, &ld);
                tty_set_termios_ldisc(o_tty, N_TTY);
        }
}

/**
 *      tty_ldisc_init          -       ldisc setup for new tty
 *      @tty: tty being allocated
 *
 *      Set up the line discipline objects for a newly allocated tty. Note that
 *      the tty structure is not completely set up when this call is made.
 */

void tty_ldisc_init(struct tty_struct *tty)
{
        struct tty_ldisc ld;
        if (tty_ldisc_get(N_TTY, &ld) < 0)
                panic("n_tty: init_tty");
        tty_ldisc_assign(tty, &ld);
}

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