Re: [PATCH 1/1] signal: kill the obsolete SIGNAL_UNKILLABLE check in complete_signal()

[Oleg Nesterov <oleg@redhat.com>]

On 11/04, Andrew Morton wrote:
>
> On Wed, 4 Nov 2015 20:19:12 +0100 Oleg Nesterov <oleg@redhat.com> wrote:
>
> > This explains WARN_ON(!JOBCTL_STOP_PENDING) in task_participate_group_stop()
> > triggered by the test-case from Dmitry:
> >
> > 	int main()
> > 	{
> > 		int pid = 1;
> > 		ptrace(PTRACE_ATTACH, pid, 0, 0);
> > 		ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_EXITKILL);
> > 		sleep(1);
> > 		return 0;
> > 	}

...

> > The test-case above needs root and (correctly) crashes the kernel,

> I'm thinking this should be backported into -stable due to WARN_ONs and
> kernel crashes.

Ah, sorry for confusion. The kernel crash is fine/correct. Debugger kills
init process, the exiting init calls panic(). With or without this patch.
BTW, I always thought we should remove this panic(), but this is off-topic.

After this patch the test-case above still crashes the kernel, but without
warning ;)

> And as f008faff0e27 is from 2009, that means all
> kernels.

Yes, I think this change is safe for -stable. But the only visible problem
is WARN_ON_ONCE() in task_participate_group_stop(), so I am not sure...

Well. Actually there are more problems. zap_threads(), de_thread() can be
fooled by signal_group_exit() == F too. So a multi-threaded /sbin/init can
miss SIGKILL if it does execve(), or if it starts the coredump. But only if
SIGKILL was private (sent by tkill).

I do not see any serious problem this patch could fix.

Oleg.