Re: SIGTRAP vs. sys_exit_group race

From: Oleg Nesterov
Date: Thu Oct 16 2008 - 12:56:40 EST

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.

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.


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