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

[Oleg Nesterov]

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.

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.

Oleg.
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