gmane.linux.kernel

Jan Kiszka | 6 Oct 16:04

SIGTRAP vs. sys_exit_group race

Hi,

are there any news on these ideas?

http://marc.info/?l=linux-kernel&m=121540671602971

I've been caught by a race between a ptraced thread running on a
breakpoint, thus generating a SIGTRAP and another thread in this process
issuing sys_exit_group. The discussion above, specifically Oleg's
concerns, made me think that this is a generic issue of current
mainline.

I observed this on a heavily patched 2.6.26.5 kernel which comes, among
other things, with a higher probability for latencies/reschedules
between the queuing of SIGTRAP and the actual delivery. Right into this
window, the sys_exit_group comes. It informs gdb about the termination,
sends out SIGKILL to the other threads and turns the caller into a
zombie. Now the second thread has SIGKILL + SIGTRAP pending, and it
picks SIGTRAP for delivery. At this point gdb gets confused (maybe a bug
of its own?), sends SIGSTOP to the dead thread and waits for it to enter
the traced state (which it will never do) - deadlock of gdb, only
resolvable by killing the latter.

The patch below (rebased against latest git) resolves the issue for me,
but I'm definitely not sure about all its implications and if I'm not
papering over a different issue. Could you comment on my scenario? Is it
possible with mainline as well? Will Roland's approach resolve it?

Thanks,
Jan

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Oleg Nesterov | 16 Oct 18:57

Re: SIGTRAP vs. sys_exit_group race

Roland, what do you think?

On 10/06, Jan Kiszka wrote:
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1528,10 +1528,11 @@ static void ptrace_stop(int exit_code, i
>  		spin_unlock_irq(&current->sighand->siglock);
>  		arch_ptrace_stop(exit_code, info);
>  		spin_lock_irq(&current->sighand->siglock);
> -		if (sigkill_pending(current))
> -			return;
>  	}
>
> +	if (sigkill_pending(current))
> +		return;
> +

Personally, I think this change is good anyway. The tracee shouldn't
sleep in TASK_TRACED with the pending SIGKILL.

And the current code is confusing, imho. Why do we check sigkill_pending()
under arch_ptrace_stop_needed() ? Yes, it unlocks ->siglock and can sleep,
so SIGKILL can come in between. But it is quite possible that SIGKILL is
already pending when we enter ptrace_stop().

The only problem I can see this patch adds a user-visible change, even
if this change looks good to me. For example, if we send SIGKILL to
the thread group, the tracee will not send PTRACE_EVENT_EXIT.

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Roland McGrath | 3 Dec 05:47

Re: SIGTRAP vs. sys_exit_group race

> Roland, what do you think?
> 
> On 10/06, Jan Kiszka wrote:
> >
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -1528,10 +1528,11 @@ static void ptrace_stop(int exit_code, i
> >  		spin_unlock_irq(&current->sighand->siglock);
> >  		arch_ptrace_stop(exit_code, info);
> >  		spin_lock_irq(&current->sighand->siglock);
> > -		if (sigkill_pending(current))
> > -			return;
> >  	}
> >
> > +	if (sigkill_pending(current))
> > +		return;
> > +
> 
> Personally, I think this change is good anyway. The tracee shouldn't
> sleep in TASK_TRACED with the pending SIGKILL.

I think this is actually superfluous since TASK_WAKEKILL (2.6.24?).
It won't sleep in TASK_TRACED at all, because of signal_pending_state().

> I think we need further changes. If the thread group group was killed
> by some fatal signal (but not SIGKILL) the tracee will sleep with
> SIGNAL_GROUP_EXIT, this is not nice too. But imho the patch makes
> sense anyway.

When there is no (user-level) SIGKILL and no core dump synchronization, I

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Oleg Nesterov | 4 Dec 19:24

Re: SIGTRAP vs. sys_exit_group race

On 12/02, Roland McGrath wrote:
>
> > Roland, what do you think?
> >
> > On 10/06, Jan Kiszka wrote:
> > >
> > > --- a/kernel/signal.c
> > > +++ b/kernel/signal.c
> > > @@ -1528,10 +1528,11 @@ static void ptrace_stop(int exit_code, i
> > >  		spin_unlock_irq(&current->sighand->siglock);
> > >  		arch_ptrace_stop(exit_code, info);
> > >  		spin_lock_irq(&current->sighand->siglock);
> > > -		if (sigkill_pending(current))
> > > -			return;
> > >  	}
> > >
> > > +	if (sigkill_pending(current))
> > > +		return;
> > > +
> >
> > Personally, I think this change is good anyway. The tracee shouldn't
> > sleep in TASK_TRACED with the pending SIGKILL.
>
> I think this is actually superfluous since TASK_WAKEKILL (2.6.24?).
> It won't sleep in TASK_TRACED at all, because of signal_pending_state().

Yes. But what if the task was killed by the group-wide SIGKILL, and
already dequeued SIGKILL from ->pending ? (do_exit path).

> > I think we need further changes. If the thread group group was killed

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