// SPDX-License-Identifier: GPL-2.0
#include <linux/types.h>
#include <linux/errno.h>
#include <linux/kmod.h>
#include <linux/sched.h>
#include <linux/interrupt.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/file.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/proc_fs.h>
#include <linux/module.h>
#include <linux/device.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/seq_file.h>
#include <linux/uaccess.h>
#include <linux/ratelimit.h>
#include "tty.h"
#undef LDISC_DEBUG_HANGUP
#ifdef LDISC_DEBUG_HANGUP
#define tty_ldisc_debug(tty, f, args...) tty_debug(tty, f, ##args)
#else
#define tty_ldisc_debug(tty, f, args...)
#endif
/* lockdep nested classes for tty->ldisc_sem */
enum {
LDISC_SEM_NORMAL,
LDISC_SEM_OTHER,
};
/*
* 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_RAW_SPINLOCK(tty_ldiscs_lock);
/* Line disc dispatch table */
static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
/**
* tty_register_ldisc - install a line discipline
* @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_ldiscs_lock to guard against ldisc races
*/
int tty_register_ldisc(struct tty_ldisc_ops *new_ldisc)
{
unsigned long flags;
if (new_ldisc->num < N_TTY || new_ldisc->num >= NR_LDISCS)
return -EINVAL;
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
tty_ldiscs[new_ldisc->num] = new_ldisc;
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
return 0;
}
EXPORT_SYMBOL(tty_register_ldisc);
/**
* tty_unregister_ldisc - unload a line discipline
* @ldisc: ldisc number
*
* Remove a line discipline from the kernel providing it is not currently in
* use.
*
* Locking: takes %tty_ldiscs_lock to guard against ldisc races
*/
void tty_unregister_ldisc(struct tty_ldisc_ops *ldisc)
{
unsigned long flags;
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
tty_ldiscs[ldisc->num] = NULL;
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
}
EXPORT_SYMBOL(tty_unregister_ldisc);
static struct tty_ldisc_ops *get_ldops(int disc)
{
unsigned long flags;
struct tty_ldisc_ops *ldops, *ret;
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
ret = ERR_PTR(-EINVAL);
ldops = tty_ldiscs[disc];
if (ldops) {
ret = ERR_PTR(-EAGAIN);
if (try_module_get(ldops->owner))
ret = ldops;
}
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
return ret;
}
static void put_ldops(struct tty_ldisc_ops *ldops)
{
unsigned long flags;
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
module_put(ldops->owner);
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
}
int tty_ldisc_autoload = IS_BUILTIN(CONFIG_LDISC_AUTOLOAD);
/**
* tty_ldisc_get - take a reference to an ldisc
* @tty: tty device
* @disc: ldisc number
*
* Takes a reference to a line discipline. Deals with refcounts and module
* locking counts. If the discipline is not available, its module loaded, if
* possible.
*
* Returns:
* * -%EINVAL if the discipline index is not [%N_TTY .. %NR_LDISCS] or if the
* discipline is not registered
* * -%EAGAIN if request_module() failed to load or register the discipline
* * -%ENOMEM if allocation failure
* * Otherwise, returns a pointer to the discipline and bumps the ref count
*
* Locking: takes %tty_ldiscs_lock to guard against ldisc races
*/
static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
{
struct tty_ldisc *ld;
struct tty_ldisc_ops *ldops;
if (disc < N_TTY || disc >= NR_LDISCS)
return ERR_PTR(-EINVAL);
/*
* Get the ldisc ops - we may need to request them to be loaded
* dynamically and try again.
*/
ldops = get_ldops(disc);
if (IS_ERR(ldops)) {
if (!capable(CAP_SYS_MODULE) && !tty_ldisc_autoload)
return ERR_PTR(-EPERM);
request_module("tty-ldisc-%d", disc);
ldops = get_ldops(disc);
if (IS_ERR(ldops))
return ERR_CAST(ldops);
}
/*
* There is no way to handle allocation failure of only 16 bytes.
* Let's simplify error handling and save more memory.
*/
ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL | __GFP_NOFAIL);
ld->ops = ldops;
ld->tty = tty;
return ld;
}
/**
* tty_ldisc_put - release the ldisc
* @ld: lisdsc to release
*
* Complement of tty_ldisc_get().
*/
static void tty_ldisc_put(struct tty_ldisc *ld)
{
if (WARN_ON_ONCE(!ld))
return;
put_ldops(ld->ops);
kfree(ld);
}
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_ops *ldops;
ldops = get_ldops(i);
if (IS_ERR(ldops))
return 0;
seq_printf(m, "%-10s %2d\n", ldops->name ? ldops->name : "???", i);
put_ldops(ldops);
return 0;
}
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,
};
/**
* 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.
*
* Returns: %NULL if the tty has been hungup and not re-opened with a new file
* descriptor, otherwise valid ldisc reference
*
* Note 1: 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).
*
* Note 2: a file_operations routine (read/poll/write) should use this function
* to wait for any ldisc lifetime events to finish.
*/
struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
struct tty_ldisc *ld;
ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
ld = tty->ldisc;
if (!ld)
ldsem_up_read(&tty->ldisc_sem);
return ld;
}
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.
*/
struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
{
struct tty_ldisc *ld = NULL;
if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
ld = tty->ldisc;
if (!ld)
ldsem_up_read(&tty->ldisc_sem);
}
return ld;
}
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.
*/
void tty_ldisc_deref(struct tty_ldisc *ld)
{
ldsem_up_read(&ld->tty->ldisc_sem);
}
EXPORT_SYMBOL_GPL(tty_ldisc_deref);
static inline int
__tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
{
return ldsem_down_write(&tty->ldisc_sem, timeout);
}
static inline int
__tty_ldisc_lock_nested(struct tty_struct *tty, unsigned long timeout)
{
return ldsem_down_write_nested(&tty->ldisc_sem,
LDISC_SEM_OTHER, timeout);
}
static inline void __tty_ldisc_unlock(struct tty_struct *tty)
{
ldsem_up_write(&tty->ldisc_sem);
}
int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
{
int ret;
/* Kindly asking blocked readers to release the read side */
set_bit(TTY_LDISC_CHANGING, &tty->flags);
wake_up_interruptible_all(&tty->read_wait);
wake_up_interruptible_all(&tty->write_wait);
ret = __tty_ldisc_lock(tty, timeout);
if (!ret)
return -EBUSY;
set_bit(TTY_LDISC_HALTED, &tty->flags);
return 0;
}
void tty_ldisc_unlock(struct tty_struct *tty)
{
clear_bit(TTY_LDISC_HALTED, &tty->flags);
/* Can be cleared here - ldisc_unlock will wake up writers firstly */
clear_bit(TTY_LDISC_CHANGING, &tty->flags);
__tty_ldisc_unlock(tty);
}
static int
tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
unsigned long timeout)
{
int ret;
if (tty < tty2) {
ret = __tty_ldisc_lock(tty, timeout);
if (ret) {
ret = __tty_ldisc_lock_nested(tty2, timeout);
if (!ret)
__tty_ldisc_unlock(tty);
}
} else {
/* if this is possible, it has lots of implications */
WARN_ON_ONCE(tty == tty2);
if (tty2 && tty != tty2) {
ret = __tty_ldisc_lock(tty2, timeout);
if (ret) {
ret = __tty_ldisc_lock_nested(tty, timeout);
if (!ret)
__tty_ldisc_unlock(tty2);
}
} else
ret = __tty_ldisc_lock(tty, timeout);
}
if (!ret)
return -EBUSY;
set_bit(TTY_LDISC_HALTED, &tty->flags);
if (tty2)
set_bit(TTY_LDISC_HALTED, &tty2->flags);
return 0;
}
static void tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
{
tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
}
static void tty_ldisc_unlock_pair(struct tty_struct *tty,
struct tty_struct *tty2)
{
__tty_ldisc_unlock(tty);
if (tty2)
__tty_ldisc_unlock(tty2);
}
/**
* tty_ldisc_flush - flush line discipline queue
* @tty: tty to flush ldisc for
*
* Flush the line discipline queue (if any) and the tty flip buffers for this
* @tty.
*/
void tty_ldisc_flush(struct tty_struct *tty)
{
struct tty_ldisc *ld = tty_ldisc_ref(tty);
tty_buffer_flush(tty, ld);
if (ld)
tty_ldisc_deref(ld);
}
EXPORT_SYMBOL_GPL(tty_ldisc_flush);
/**
* tty_set_termios_ldisc - set ldisc field
* @tty: tty structure
* @disc: 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.
*
* The line discipline-related tty_struct fields are reset to prevent the ldisc
* driver from re-using stale information for the new ldisc instance.
*
* Locking: takes termios_rwsem
*/
static void tty_set_termios_ldisc(struct tty_struct *tty, int disc)
{
down_write(&tty->termios_rwsem);
tty->termios.c_line = disc;
up_write(&tty->termios_rwsem);
tty->disc_data = NULL;
tty->receive_room = 0;
}
/**
* tty_ldisc_open - open a line discipline
* @tty: tty we are opening the ldisc on
* @ld: discipline to open
*
* A helper opening method. Also a convenient debugging and check point.
*
* Locking: always called with BTM already held.
*/
static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
{
WARN_ON(test_and_set_bit(TTY_LDISC_OPEN, &tty->flags));
if (ld->ops->open) {
int ret;
/* BTM here locks versus a hangup event */
ret = ld->ops->open(tty);
if (ret)
clear_bit(TTY_LDISC_OPEN, &tty->flags);
tty_ldisc_debug(tty, "%p: opened\n", ld);
return ret;
}
return 0;
}
/**
* tty_ldisc_close - close a line discipline
* @tty: tty we are opening the ldisc on
* @ld: discipline to close
*
* A helper close method. Also a convenient debugging and check point.
*/
static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
{
lockdep_assert_held_write(&tty->ldisc_sem);
WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
clear_bit(TTY_LDISC_OPEN, &tty->flags);
if (ld->ops->close)
ld->ops->close(tty);
tty_ldisc_debug(tty, "%p: closed\n", ld);
}
/**
* tty_ldisc_failto - helper for ldisc failback
* @tty: tty to open the ldisc on
* @ld: ldisc we are trying to fail back to
*
* Helper to try and recover a tty when switching back to the old ldisc fails
* and we need something attached.
*/
static int tty_ldisc_failto(struct tty_struct *tty, int ld)
{
struct tty_ldisc *disc = tty_ldisc_get(tty, ld);
int r;
lockdep_assert_held_write(&tty->ldisc_sem);
if (IS_ERR(disc))
return PTR_ERR(disc);
tty->ldisc = disc;
tty_set_termios_ldisc(tty, ld);
r = tty_ldisc_open(tty, disc);
if (r < 0)
tty_ldisc_put(disc);
return r;
}
/**
* 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)
{
/* There is an outstanding reference here so this is safe */
if (tty_ldisc_failto(tty, old->ops->num) < 0) {
const char *name = tty_name(tty);
pr_warn("Falling back ldisc for %s.\n", name);
/*
* The traditional behaviour is to fall back to N_TTY, we
* want to avoid falling back to N_NULL unless we have no
* choice to avoid the risk of breaking anything
*/
if (tty_ldisc_failto(tty, N_TTY) < 0 &&
tty_ldisc_failto(tty, N_NULL) < 0)
panic("Couldn't open N_NULL ldisc for %s.", name);
}
}
/**
* tty_set_ldisc - set line discipline
* @tty: the terminal to set
* @disc: the line discipline number
*
* Set the discipline of a tty line. Must be called from a process context. The
* ldisc change logic has to protect itself against any overlapping ldisc
* change (including on the other end of pty pairs), the close of one side of a
* tty/pty pair, and eventually hangup.
*/
int tty_set_ldisc(struct tty_struct *tty, int disc)
{
int retval;
struct tty_ldisc *old_ldisc, *new_ldisc;
new_ldisc = tty_ldisc_get(tty, disc);
if (IS_ERR(new_ldisc))
return PTR_ERR(new_ldisc);
tty_lock(tty);
retval = tty_ldisc_lock(tty, 5 * HZ);
if (retval)
goto err;
if (!tty->ldisc) {
retval = -EIO;
goto out;
}
/* Check the no-op case */
if (tty->ldisc->ops->num == disc)
goto out;
if (test_bit(TTY_HUPPED, &tty->flags)) {
/* We were raced by hangup */
retval = -EIO;
goto out;
}
if (tty->ops->ldisc_ok) {
retval = tty->ops->ldisc_ok(tty, disc);
if (retval)
goto out;
}
old_ldisc = tty->ldisc;
/* Shutdown the old discipline. */
tty_ldisc_close(tty, old_ldisc);
/* Now set up the new line discipline. */
tty->ldisc = new_ldisc;
tty_set_termios_ldisc(tty, disc);
retval = tty_ldisc_open(tty, new_ldisc);
if (retval < 0) {
/* Back to the old one or N_TTY if we can't */
tty_ldisc_put(new_ldisc);
tty_ldisc_restore(tty, old_ldisc);
}
if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
down_read(&tty->termios_rwsem);
tty->ops->set_ldisc(tty);
up_read(&tty->termios_rwsem);
}
/*
* At this point we hold a reference to the new ldisc and a
* reference to the old ldisc, or we hold two references to
* the old ldisc (if it was restored as part of error cleanup
* above). In either case, releasing a single reference from
* the old ldisc is correct.
*/
new_ldisc = old_ldisc;
out:
tty_ldisc_unlock(tty);
/*
* Restart the work queue in case no characters kick it off. Safe if
* already running
*/
tty_buffer_restart_work(tty->port);
err:
tty_ldisc_put(new_ldisc); /* drop the extra reference */
tty_unlock(tty);
return retval;
}
EXPORT_SYMBOL_GPL(tty_set_ldisc);
/**
* tty_ldisc_kill - teardown ldisc
* @tty: tty being released
*
* Perform final close of the ldisc and reset @tty->ldisc
*/
static void tty_ldisc_kill(struct tty_struct *tty)
{
lockdep_assert_held_write(&tty->ldisc_sem);
if (!tty->ldisc)
return;
/*
* Now kill off the ldisc
*/
tty_ldisc_close(tty, tty->ldisc);
tty_ldisc_put(tty->ldisc);
/* Force an oops if we mess this up */
tty->ldisc = NULL;
}
/**
* 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)
{
down_write(&tty->termios_rwsem);
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);
up_write(&tty->termios_rwsem);
}
/**
* tty_ldisc_reinit - reinitialise the tty ldisc
* @tty: tty to reinit
* @disc: line discipline to reinitialize
*
* Completely reinitialize the line discipline state, by closing the current
* instance, if there is one, and opening a new instance. If an error occurs
* opening the new non-%N_TTY instance, the instance is dropped and @tty->ldisc
* reset to %NULL. The caller can then retry with %N_TTY instead.
*
* Returns: 0 if successful, otherwise error code < 0
*/
int tty_ldisc_reinit(struct tty_struct *tty, int disc)
{
struct tty_ldisc *ld;
int retval;
lockdep_assert_held_write(&tty->ldisc_sem);
ld = tty_ldisc_get(tty, disc);
if (IS_ERR(ld)) {
BUG_ON(disc == N_TTY);
return PTR_ERR(ld);
}
if (tty->ldisc) {
tty_ldisc_close(tty, tty->ldisc);
tty_ldisc_put(tty->ldisc);
}
/* switch the line discipline */
tty->ldisc = ld;
tty_set_termios_ldisc(tty, disc);
retval = tty_ldisc_open(tty, tty->ldisc);
if (retval) {
tty_ldisc_put(tty->ldisc);
tty->ldisc = NULL;
}
return retval;
}
/**
* tty_ldisc_hangup - hangup ldisc reset
* @tty: tty being hung up
* @reinit: whether to re-initialise the tty
*
* Some tty devices reset their termios when they receive a hangup event. In
* that situation we must also switch back to %N_TTY properly before we reset
* the termios data.
*
* Locking: We can take the ldisc mutex as the rest of the code is careful to
* allow for this.
*
* In the pty pair case this occurs in the close() path of the tty itself so we
* must be careful about locking rules.
*/
void tty_ldisc_hangup(struct tty_struct *tty, bool reinit)
{
struct tty_ldisc *ld;
tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);
ld = tty_ldisc_ref(tty);
if (ld != NULL) {
if (ld->ops->flush_buffer)
ld->ops->flush_buffer(tty);
tty_driver_flush_buffer(tty);
if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
ld->ops->write_wakeup)
ld->ops->write_wakeup(tty);
if (ld->ops->hangup)
ld->ops->hangup(tty);
tty_ldisc_deref(ld);
}
wake_up_interruptible_poll(&tty->write_wait, EPOLLOUT);
wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);
/*
* Shutdown the current line discipline, and reset it to
* N_TTY if need be.
*
* Avoid racing set_ldisc or tty_ldisc_release
*/
tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
tty_reset_termios(tty);
if (tty->ldisc) {
if (reinit) {
if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0 &&
tty_ldisc_reinit(tty, N_TTY) < 0)
WARN_ON(tty_ldisc_reinit(tty, N_NULL) < 0);
} else
tty_ldisc_kill(tty);
}
tty_ldisc_unlock(tty);
}
/**
* 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
* disciplines and bind them to the @tty. This has no locking issues as the
* device isn't yet active.
*/
int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
{
int retval = tty_ldisc_open(tty, tty->ldisc);
if (retval)
return retval;
if (o_tty) {
/*
* Called without o_tty->ldisc_sem held, as o_tty has been
* just allocated and no one has a reference to it.
*/
retval = tty_ldisc_open(o_tty, o_tty->ldisc);
if (retval) {
tty_ldisc_close(tty, tty->ldisc);
return retval;
}
}
return 0;
}
/**
* tty_ldisc_release - release line discipline
* @tty: tty being shut down (or one end of pty pair)
*
* Called during the final close of a tty or a pty pair in order to shut down
* the line discpline layer. On exit, each tty's ldisc is %NULL.
*/
void tty_ldisc_release(struct tty_struct *tty)
{
struct tty_struct *o_tty = tty->link;
/*
* Shutdown this line discipline. As this is the final close,
* it does not race with the set_ldisc code path.
*/
tty_ldisc_lock_pair(tty, o_tty);
tty_ldisc_kill(tty);
if (o_tty)
tty_ldisc_kill(o_tty);
tty_ldisc_unlock_pair(tty, o_tty);
/*
* And the memory resources remaining (buffers, termios) will be
* disposed of when the kref hits zero
*/
tty_ldisc_debug(tty, "released\n");
}
/**
* 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.
*/
int tty_ldisc_init(struct tty_struct *tty)
{
struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
if (IS_ERR(ld))
return PTR_ERR(ld);
tty->ldisc = ld;
return 0;
}
/**
* tty_ldisc_deinit - ldisc cleanup for new tty
* @tty: tty that was allocated recently
*
* The tty structure must not be completely set up (tty_ldisc_setup()) when
* this call is made.
*/
void tty_ldisc_deinit(struct tty_struct *tty)
{
/* no ldisc_sem, tty is being destroyed */
if (tty->ldisc)
tty_ldisc_put(tty->ldisc);
tty->ldisc = NULL;
}
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