Commit 892d1fa7 authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman

tty: Destroy ldisc instance on hangup

Currently, when the tty is hungup, the ldisc is re-instanced; ie., the
current instance is destroyed and a new instance is created. The purpose
of this design was to guarantee a valid, open ldisc for the lifetime of
the tty.

However, now that tty buffers are owned by and have lifetime equivalent
to the tty_port (since v3.10), any data received immediately after the
ldisc is re-instanced may cause continued driver i/o operations
concurrently with the driver's hangup() operation. For drivers that
shutdown h/w on hangup, this is unexpected and usually bad. For example,
the serial core may free the xmit buffer page concurrently with an
in-progress write() operation (triggered by echo).

With the existing stable and robust ldisc reference handling, the
cleaned-up tty_reopen(), the straggling unsafe ldisc use cleaned up, and
the preparation to properly handle a NULL tty->ldisc, the ldisc instance
can be destroyed and only re-instanced when the tty is re-opened.

If the tty was opened as /dev/console or /dev/tty0, the original behavior
of re-instancing the ldisc is retained (the 'reinit' parameter to
tty_ldisc_hangup() is true). This is required since those file descriptors
are never hungup.

This patch has neglible impact on userspace; the tty file_operations ptr
is changed to point to the hungup file operations _before_ the ldisc
instance is destroyed, so only racing file operations might now retrieve
a NULL ldisc reference (which is simply handled as if the hungup file
operation had been called instead -- see "tty: Prepare for destroying
line discipline on hangup").

This resolves a long-standing FIXME and several crash reports.
Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 7896f30d
......@@ -727,7 +727,7 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
while (refs--)
tty_kref_put(tty);
tty_ldisc_hangup(tty);
tty_ldisc_hangup(tty, cons_filp != NULL);
spin_lock_irq(&tty->ctrl_lock);
clear_bit(TTY_THROTTLED, &tty->flags);
......@@ -752,10 +752,9 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
} else if (tty->ops->hangup)
tty->ops->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.
* 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, which is now guaranteed.
*/
set_bit(TTY_HUPPED, &tty->flags);
tty_unlock(tty);
......@@ -1475,7 +1474,8 @@ static int tty_reopen(struct tty_struct *tty)
tty->count++;
WARN_ON(!tty->ldisc);
if (!tty->ldisc)
return tty_ldisc_reinit(tty, tty->termios.c_line);
return 0;
}
......
......@@ -257,6 +257,9 @@ const struct file_operations tty_ldiscs_proc_fops = {
* 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: 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
......@@ -642,14 +645,15 @@ static void tty_reset_termios(struct tty_struct *tty)
* @disc: line discipline to reinitialize
*
* Completely reinitialize the line discipline state, by closing the
* current instance 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.
* 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
*/
static int tty_ldisc_reinit(struct tty_struct *tty, int disc)
int tty_ldisc_reinit(struct tty_struct *tty, int disc)
{
struct tty_ldisc *ld;
int retval;
......@@ -693,11 +697,9 @@ static int tty_ldisc_reinit(struct tty_struct *tty, int disc)
* tty itself so we must be careful about locking rules.
*/
void tty_ldisc_hangup(struct tty_struct *tty)
void tty_ldisc_hangup(struct tty_struct *tty, bool reinit)
{
struct tty_ldisc *ld;
int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS;
int err = 0;
tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);
......@@ -725,25 +727,17 @@ void tty_ldisc_hangup(struct tty_struct *tty)
*/
tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
if (tty->ldisc) {
/* At this point we have a halted ldisc; we want to close it and
reopen a new ldisc. We could defer the reopen to the next
open but it means auditing a lot of other paths so this is
a FIXME */
if (reset == 0)
err = tty_ldisc_reinit(tty, tty->termios.c_line);
if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
tty_reset_termios(tty);
/* If the re-open fails or we reset then go to N_TTY. The
N_TTY open cannot fail */
if (reset || err < 0)
if (tty->ldisc) {
if (reinit) {
if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0)
tty_ldisc_reinit(tty, N_TTY);
} else
tty_ldisc_kill(tty);
}
tty_ldisc_unlock(tty);
if (reset)
tty_reset_termios(tty);
tty_ldisc_debug(tty, "%p: re-opened\n", tty->ldisc);
}
/**
......
......@@ -490,7 +490,8 @@ extern int tty_set_termios(struct tty_struct *tty, struct ktermios *kt);
extern struct tty_ldisc *tty_ldisc_ref(struct tty_struct *);
extern void tty_ldisc_deref(struct tty_ldisc *);
extern struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *);
extern void tty_ldisc_hangup(struct tty_struct *tty);
extern void tty_ldisc_hangup(struct tty_struct *tty, bool reset);
extern int tty_ldisc_reinit(struct tty_struct *tty, int disc);
extern const struct file_operations tty_ldiscs_proc_fops;
extern void tty_wakeup(struct tty_struct *tty);
......
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