Re: Thread group exec race -> null pointer... HELP

[George Anzinger <george@mvista.com>]

Oleg Nesterov wrote:
> George Anzinger wrote:
>
> > Still rooting around in the above.  The test program is attached.  It
> > creates and arms a repeating timer and then clones a thread which does
> > an exec() call.
>
>
> This patch:
>
> 	http://marc.theaimsgroup.com/?l=linux-kernel&m=113138286512847
>
> was intended to fix exactly this problem (and the same test program was
> used to exploit the race and test the fix).
>
> So, it does not help? I can't reproduce the problem.

Yes, it does fix it.  Somehow I missed the posting of that patch.
> Note: I think you also need this patch:
>
> 	http://marc.theaimsgroup.com/?l=linux-kernel&m=113059955626598
>
> otherwise I beleive OOPS can happen while killing this program if you are
> running the kernel with this change applied:
>
> 	[PATCH] Call exit_itimers from do_exit, not __exit_signal
> 	http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=25f407f0b668f5e4ebd5d13e1fb4306ba6427ead
>
>
>
> > first instance of this, we see that the thread-group leader is dead
> > and the exec code at line ~718 is setting the old leaders group-leader
> > to him self.
>
>
> I think this code at line ~718
>
> 	leader->group_leader = leader;
>
> is noop, because leader->group_leader == leader here.
>
>
> > -               leader->group_leader = leader;
> > +               leader->group_leader = current;
>
>
> This can't help, without SIGEV_THREAD_ID we don't check ->group_leader,
> the signal goes to the thread group via timer->it_process, which is equal
> to the old leader.

The signal code returns <0 so posix-timers digs into up the 
group_leader and trys again.  Still, the patch fixes it all.

--
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/
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