[PATCH] fix unchecked mutex_lock_interruptible (was Re: use mutex instead of semaphore in RocketPort driver)

[PATCH] fix unchecked mutex_lock_interruptible (was Re: use mutex instead of semaphore in RocketPort driver)

[Satyam Sharma]

On 5/23/07, Jiri Slaby <jirislaby@gmail.com> wrote:
> Simon Arlott napsal(a):
> > On 22/05/07 21:06, Matthias Kaehlcke wrote:
> >> would the following resolve the problem?
> >>
> >> if(mutex_lock_interruptible(&info->write_mtx))         return
> >> -ERESTARTSYS
> >>
> >> thanks for your comments

Hum. I remember suggesting the fix when this came few weeks back,
somehow the older version seems to have made it into Linus' tree.

> > No. At least one user of tty_operations/tty_driver's write function
> > doesn't check the return value so it would never be retried, mutex_lock
> > should be used instead.

Replacing mutex_lock_interruptible() with a mutex_lock() could be a
change in behaviour visible to userspace. Best to continue to use
_interruptible. Returning EINTR would be safe choice because if
Simon's right that this isn't retried, we could end up breaking
userspace that isn't quite ready to see ERESTARTSYS coming out of
a write(2), but ...

> Who? There are some drivers that returns ERESTARTSYS from write function.

Right, several others in drivers/char/ do return ERESTARTSYS. Hope
somebody picks this up -- no email listed in MAINTAINERS for this.

---

Check the return of mutex_lock_interruptible() in drivers/char/rocket.c
and return ERESTARTSYS if we were interrupted.

Signed-off-by: Satyam Sharma <satyam.sharma@gmail.com>

---

 drivers/char/rocket.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

---

diff -ruNp a/drivers/char/rocket.c b/drivers/char/rocket.c
--- a/drivers/char/rocket.c	2007-05-20 04:11:54.000000000 +0530
+++ b/drivers/char/rocket.c	2007-05-23 18:14:35.000000000 +0530
@@ -1702,7 +1702,8 @@ static int rp_write(struct tty_struct *t
 	if (count <= 0 || rocket_paranoia_check(info, "rp_write"))
 		return 0;

-	mutex_lock_interruptible(&info->write_mtx);
+	if (mutex_lock_interruptible(&info->write_mtx))
+		return -ERESTARTSYS;

 #ifdef ROCKET_DEBUG_WRITE
 	printk(KERN_INFO "rp_write %d chars...", count);

Re: [PATCH] fix unchecked mutex_lock_interruptible (was Re: use mutex instead of semaphore in RocketPort driver)

[Matthias Kaehlcke]

El Wed, May 23, 2007 at 06:25:49PM +0530 Satyam Sharma ha dit:

> On 5/23/07, Jiri Slaby <jirislaby@gmail.com> wrote:
> >Simon Arlott napsal(a):
> >> On 22/05/07 21:06, Matthias Kaehlcke wrote:
> >>> would the following resolve the problem?
> >>>
> >>> if(mutex_lock_interruptible(&info->write_mtx))         return
> >>> -ERESTARTSYS
> >>>
> >>> thanks for your comments
> 
> Hum. I remember suggesting the fix when this came few weeks back,
> somehow the older version seems to have made it into Linus' tree.

i also remembered your suggestion but thought it was a similar case in
another driver. i think it's my fault that the older version made it
into the Linus tree cause i sent the patch in a new thread instead of
responding to the initial post :(
 
> >> No. At least one user of tty_operations/tty_driver's write function
> >> doesn't check the return value so it would never be retried, mutex_lock
> >> should be used instead.
> 
> Replacing mutex_lock_interruptible() with a mutex_lock() could be a
> change in behaviour visible to userspace. Best to continue to use
> _interruptible. Returning EINTR would be safe choice because if
> Simon's right that this isn't retried, we could end up breaking
> userspace that isn't quite ready to see ERESTARTSYS coming out of
> a write(2), but ...

thanks for your fix!

-- 
Matthias Kaehlcke
Linux Application Developer
Barcelona

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