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linux/drivers/char/tty_ldisc.c

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#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/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 < 0) {
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));
WARN_ON(tty->ldisc.refcount == 0);
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. Clear the LDISC
* changing flag to indicate any ldisc change is now over.
*
* Note: nobody should set the TTY_LDISC bit except via this function.
* Clearing directly is allowed.
*/
void tty_ldisc_enable(struct tty_struct *tty)
{
set_bit(TTY_LDISC, &tty->flags);
clear_bit(TTY_LDISC_CHANGING, &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.
*
* We must clear the TTY_LDISC bit here to avoid a livelock
* with a userspace app continually trying to use the tty in
* parallel to the change and re-referencing the tty.
*/
clear_bit(TTY_LDISC, &tty->flags);
if (o_tty)
clear_bit(TTY_LDISC, &o_tty->flags);
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_CHANGING, &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;
}
/*
* This flag is used to avoid two parallel ldisc changes. Once
* open and close are fine grained locked this may work better
* as a mutex shared with the open/close/hup paths
*/
set_bit(TTY_LDISC_CHANGING, &tty->flags);
if (o_tty)
set_bit(TTY_LDISC_CHANGING, &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);
}
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