[PATCH 07/21] TTY: ldisc, wait for idle ldisc in release

From: Jiri Slaby
Date: Thu Oct 18 2012 - 16:29:31 EST

We reintroduced tty_ldisc_wait_idle in 100eeae2c5c (TTY: restore
tty_ldisc_wait_idle) and used in set_ldisc. Then we added it also to
the hangup path in 92f6fa09bd453 (TTY: ldisc, do not close until there
are readers). And we noted that there is one more path:
~ Before 65b770468e98 tty_ldisc_wait_idle was called also from
~ tty_ldisc_release. It is called from tty_release, so I don't think
~ we need to restore that one.

Well, I was wrong. There might still be holders of an ldisc
reference. Not from userspace, but drivers. If they take a reference
and a user closes the device immediately after that, we have a
problem. ldisc is halted and closed by TTY, but the driver still may
call some ldisc's operation and cause a crash.

So restore the tty_ldisc_wait_idle call also to the third location
where it was before 65b770468e98 (tty-ldisc: turn ldisc user count
into a proper refcount). Now we should be safe with respect to the
ldisc reference counting as all* tty_ldisc_close paths are safely
called with reference count of one.

* Not the one in tty_ldisc_setup's fail path. But that is called
before the first open finishes. So userspace does not see it yet.
Even thought the driver is given the TTY already via ->install, it
should not take a reference to the ldisc yet. If some driver is to
do this, we should put one tty_ldisc_wait_idle also in the setup.

Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
drivers/tty/tty_ldisc.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 0f2a2c5..47e3968 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -897,6 +897,11 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)

static void tty_ldisc_kill(struct tty_struct *tty)
+ /* There cannot be users from userspace now. But there still might be
+ * drivers holding a reference via tty_ldisc_ref. Do not steal them the
+ * ldisc until they are done. */
+ tty_ldisc_wait_idle(tty, MAX_SCHEDULE_TIMEOUT);
* Now kill off the ldisc

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