ldisc switching on a HUPped pty hangs the caller
PATCH 1/1 ldisc switching on a HUPped pty hangs the caller
Calling vhangup() and then switching line disciplines results in incomplete clean-up and a wedged process if it then calls ioctl() on the pty.
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I recently upgraded my gateway machine to 2.6.31. This caused ppp-over-ssh to stop working. Indeed, the PPP process got wedged into noninterruptible sleep, which this patch fixes.
(The comment, by the way, seems to be wrong. There was no race.)
The underlying problem, however, turns out to be the vhangup() syscall which the SSH server emits before exec'ing pppd. This causes the tty's HUPPED bit to get set, which in turn causes the above error.
Browbeating sshd into not issuing vhangup() (why does it do that, anyway? Presumably, using PTMX we can be sure that nobody is hogging the pty in the first place, right?) means that I now have a working ssh tunnel.
… until somebody forces me to switch to a 'real' VPN. :-P
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drivers/char/tty_ldisc.c +3 -4
@@ -621,9 +621,8 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)/* We were raced by the hangup method. It will have stomped the ldisc data and closed the ldisc down */ clear_bit (TTY_LDISC_CHANGING , &tty->flags); -mutex_unlock (&tty->ldisc_mutex); -tty_ldisc_put (new_ldisc); -return -EIO ; + retval = -EIO ; + goto out; }/* Shutdown the current discipline. */
@@ -652,7 +651,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)/* * Allow ldisc referencing to occur again */ - +out:tty_ldisc_enable (tty);if (o_tty)tty_ldisc_enable (o_tty);
Quoted 5 lines
I recently upgraded my gateway machine to 2.6.31. This caused ppp-over-ssh to stop working. Indeed, the PPP process got wedged into noninterruptible sleep, which this patch fixes.
(The comment, by the way, seems to be wrong. There was no race.)
Really ?
Think about
set_ldisc
hangup
close
open
set to N_TTY etc
Now what ???
Quoted 3 lines
The underlying problem, however, turns out to be the
vhangup()syscall which the SSH server emits before exec'ing pppd. This causes the tty's HUPPED bit to get set, which in turn causes the above error.
vhangup sets the huppet bit to make sure that anything touching the tty beyond that point goes away and dies.
Quoted 17 lines
drivers/char/tty_ldisc.c +2
-3@@ -621,9 +621,8 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)/* We were raced by the hangup method. It will have stomped the ldisc data and closed the ldisc down */ clear_bit (TTY_LDISC_CHANGING , &tty->flags); -mutex_unlock (&tty->ldisc_mutex); -tty_ldisc_put (new_ldisc); -return -EIO ; + retval = -EIO ; + goto out; }/* Shutdown the current discipline. */
drivers/char/tty_ldisc.c int tty_set_ldisc(struct tty_struct *tty, int ldisc) · line 657
Quoted 8 lines
drivers/char/tty_ldisc.c +1
-1@@ -652,7 +651,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)/* * Allow ldisc referencing to occur again */ - +out:tty_ldisc_enable (tty);if (o_tty)tty_ldisc_enable (o_tty);
And falls through to restart work queues and stuff that may not be safe to do
So: NAK
I agree there is a bug but you've swapped one bug for sevral nastier bugs.
As far as I can see from a quick look the real problem in your case is that we don't do enough work in the case where tty->driver->flags doesn't contain TTY_DRIVER_RESET_TERMIOS. We need to reinit the ldisc either way.
Quoted 4 lines · Alan Cox
Quoted 1 line
(The comment, by the way, seems to be wrong. There was no race.)
Really ?
Yes -- I didn't mean to imply that there was no possibility of a race, but that there need not be any race for creating a deadlock situation. A call to vhangup() is sufficient.
I'll test your fix later today. Rebooting a production gateway in the middle of the work day isn't a good idea. :-P
Give this a spin
tty: Fix the ldisc hangup race
From: Alan Cox <[email protected]>
This was noticed by Matthias Urlichs and he proposed a fix. This patch does the fixing a different way to avoid introducing several new race conditions into the code.
The problem case is TTY_DRIVER_RESET_TERMIOS = 0. In that case while we abort the ldisc change the hangup processing has not cleaned up and restarted the ldisc either.
We can't restart the ldisc stuff in the set_ldisc as we don't know what the hangup did and may touch stuff we shouldn't as we are no longer supposed to influence the tty at that point in case it has been re-opened before we get rescheduled.
Instead do it the simple way. Always re-init the ldisc on the hangup, but use TTY_DRIVER_RESET_TERMIOS to indicate that we should force N_TTY.
---
drivers/char/tty_ldisc.c | 50 ++++++++++++++++++++++++++++------------------ 1 files changed, 30 insertions(+), 20 deletions(-)
drivers/char/tty_ldisc.c +30 -20
@@ -706,12 +706,13 @@ static void tty_reset_termios(struct tty_struct *tty)/** * tty_ldisc_reinit - reinitialise the tty ldisc * @tty: tty to reinit +* @ldisc: line discipline to reinitialize * -* Switch the tty back to N_TTY line discipline and leave the -* ldisc state closed +* Switch the tty to a line discipline and leave the ldisc +* state closed */ -static void tty_ldisc_reinit (struct tty_struct *tty) +static void tty_ldisc_reinit (struct tty_struct *tty,int ldisc) {struct tty_ldisc *ld;
@@ -721,10 +722,10 @@ static void tty_ldisc_reinit(struct tty_struct *tty)/* * Switch the line discipline back */ - ld =tty_ldisc_get (N_TTY ); + ld =tty_ldisc_get (ldisc);BUG_ON (IS_ERR (ld));tty_ldisc_assign (tty, ld); -tty_set_termios_ldisc (tty,N_TTY ); +tty_set_termios_ldisc (tty, ldisc); }/**
@@ -745,6 +746,8 @@ static void tty_ldisc_reinit(struct tty_struct *tty)void tty_ldisc_hangup (struct tty_struct *tty) {struct tty_ldisc *ld; +int reset = tty->driver->flags &TTY_DRIVER_RESET_TERMIOS ; +int err =0 ;/* * FIXME! What are the locking issues here? This may me overdoing
@@ -772,25 +775,32 @@ void tty_ldisc_hangup(struct tty_struct *tty)wake_up_interruptible_poll (&tty->read_wait,POLLIN );/* * Shutdown the current line discipline, and reset it to -* N_TTY. +* N_TTY if need be. +* +* Avoid racing set_ldisc or tty_ldisc_release */ -if (tty->driver->flags &TTY_DRIVER_RESET_TERMIOS ) { -/* Avoid racing set_ldisc or tty_ldisc_release */ -mutex_lock (&tty->ldisc_mutex); -tty_ldisc_halt (tty); -if (tty->ldisc) {/* Not yet closed */ -/* Switch back to N_TTY */ -tty_ldisc_reinit (tty); -/* At this point we have a closed ldisc and we want to -reopen it. We could defer this to the next open but -it means auditing a lot of other paths so this is -a FIXME */ +mutex_lock (&tty->ldisc_mutex); +tty_ldisc_halt (tty); +/* At this point we have a closed ldisc and we want to +reopen it. We could defer this to the next open but +it means auditing a lot of other paths so this is +a FIXME */ +if (tty->ldisc) {/* Not yet closed */ +if (reset ==0 ) { +tty_ldisc_reinit (tty, tty->termios->c_line); + err =tty_ldisc_open (tty, tty->ldisc); + } +/* If the re-open fails or we reset then go to N_TTY. The +N_TTY open cannot fail */ +if (reset || err) { +tty_ldisc_reinit (tty,N_TTY );WARN_ON (tty_ldisc_open (tty, tty->ldisc)); -tty_ldisc_enable (tty); } -mutex_unlock (&tty->ldisc_mutex); -tty_reset_termios (tty); +tty_ldisc_enable (tty); } +mutex_unlock (&tty->ldisc_mutex); +if (reset) +tty_reset_termios (tty); }/**