Re: [PATCH] tty races

From: Jason Baron
Date: Tue Apr 26 2005 - 08:52:55 EST

On Mon, 25 Apr 2005, Andrew Morton wrote:

> Jason Baron <jbaron@xxxxxxxxxx> wrote:
> >
> > There are a couple of tty race conditions, which lead to inconsistent tty
> > reference counting and tty layer oopses.
> >
> > The first is a tty_open vs. tty_close race in drivers/char/
> > Basically, from the time that the tty->count is deemed to be 1 and that we
> > are going to free it to the time that TTY_CLOSING bit is set, needs to be
> > atomic with respect to the manipulation of tty->count in init_dev(). This
> > atomicity was previously guarded by the BKL. However, this is no longer
> > true with the addition of a down() call in the middle of the
> > release_dev()'s atomic path. So either the down() needs to be moved
> > outside the atomic patch or dropped. I would vote for simply dropping it
> > as i don't see why it is necessary.
> The release_dev() changes looks very fishy to me. It _removes_ locking.
> If that fixes the testcase then one of two things is happening:
> a) we have lock_kernel() coverage and the down()'s sleeping breaks the
> lock_kenrel() coverage or
> b) we don't have lock_kernel() coverage, but removing the down() just
> alters the timing and makes the race less probable.
> I think it's b). lock_kernel() coverage in there is very incomplete on the
> open() side.

The patch was written for case a. Indeed lock_kernel() may appear
incomplete on the open side, but it protects paths where we don't sleep.
So, the 'fast_track' path in 'init_dev', is protected against the
release_dev path from setting the 'tty_closing' local variable to the
setting of the TTY_CLOSING flag. Thus, i believe the dropping of the
down() is correct.

This was the previous locking model for open vs. close afaict, before the
down() was introduced in the release_dev path that was supposed to be
atomic with respect to init_dev().

> I think it would be better to _increase_ the tty_sem coverage in
> release_dev() and to make sure that all callers of init_dev() are using
> tty_sem (they are).
> One approach would be to require that all callers of release_dev() hold
> tty_sem, and make release_dev() drop and reacquire tty_sem in those cases
> where release_dev() needs to go to sleep when waiting for other threads of
> control to reelase the tty's resources.

Indeed, the situation would be improved if it was held around the
driver->close() routine. This routine does sometimes look at tty->count
value, see con_close(), where in fact the tty_sem is added to avoid just
this problem. However, it is incorrect as one can see in release_dev() the
schedule(), can cause the tty->count to change via tty_open(). However, i
think this is an extremely rare corner case, b/c con_close() keys off
tty->count of 1, which implies that this is the last close() and thus the
schedule for 'write_wait' would seem impossible, although AL Viro has
said that it is possible in this case. Thus, dropping the tty_sem and
reacquiring it, probably isn't good, b/c the driver->close() routines can
free resources based upon tty->count==1.

The patch was written as the least invasive and low risk way to fix a
nasty race condition, which has the potential to corrupt data. The oops in
vt_ioctl has also been seen on system boots with some frequency. The patch
imo, returns the the tty_open vs. tty_close paths to their original
locking assumptions which have been well tested.


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