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Re: [PATCH] signals: work around random wakeups in sigsuspend()

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On 01/25, Andrew Morton wrote:
>
> On Mon, 25 Jan 2016 10:21:46 -0500 Sasha Levin <sasha.levin@oracle.com> wrote:
>
> > A random wakeup can get us out of sigsuspend() without TIF_SIGPENDING
> > being set.
> >
> > Avoid that by making sure we were signaled, like sys_pause() does.
>
> What we're lacking here is any description of the end-user-visible
> effects of the bug.

The warning in dmesg and -ERESTARTNOHAND which we should never return to user
space, although I bet nobody checks the error code returned by sigsuspend().

Plus, of course, sys_sigsuspend() can return while it should not.

> Enough for people to be able to decide (and to
> recognize!) whether their kernel needs this patch.</stdrefrain>

I don't think this problem is really serious, plus it is very unlikely. The
spurious return from sigsuspend() should not really hurt.

And, ironically, there is another more serious "reverse" problem ;) sigsuspend()
orany other user of -ERESTARTNOHAND can "miss" the signal, in a sense that the
kernel can wrongly restart this syscall after return from signal handler. This
is not trivial to fix..

Oleg.

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Thread

[PATCH] signals: work around random wakeups in sigsuspend() Sasha Levin <sasha.levin@oracle.com> - 2016-01-25 17:00 +0100
  Re: [PATCH] signals: work around random wakeups in sigsuspend() Oleg Nesterov <oleg@redhat.com> - 2016-01-25 20:10 +0100
    Re: [PATCH] signals: work around random wakeups in sigsuspend() Peter Zijlstra <peterz@infradead.org> - 2016-01-25 20:40 +0100
  Re: [PATCH] signals: work around random wakeups in sigsuspend() Andrew Morton <akpm@linux-foundation.org> - 2016-01-25 22:40 +0100
    Re: [PATCH] signals: work around random wakeups in sigsuspend() Oleg Nesterov <oleg@redhat.com> - 2016-01-26 22:20 +0100
      Re: [PATCH] signals: work around random wakeups in sigsuspend() Peter Zijlstra <peterz@infradead.org> - 2016-01-27 09:50 +0100
        Re: [PATCH] signals: work around random wakeups in sigsuspend() Oleg Nesterov <oleg@redhat.com> - 2016-01-27 17:50 +0100
          Re: [PATCH] signals: work around random wakeups in sigsuspend() Peter Zijlstra <peterz@infradead.org> - 2016-01-27 19:00 +0100
          Re: [PATCH] signals: work around random wakeups in sigsuspend() Andrew Morton <akpm@linux-foundation.org> - 2016-01-27 19:40 +0100
            Re: [PATCH] signals: work around random wakeups in sigsuspend() Oleg Nesterov <oleg@redhat.com> - 2016-01-27 22:10 +0100
        Re: [PATCH] signals: work around random wakeups in sigsuspend() Sasha Levin <sasha.levin@oracle.com> - 2016-01-27 18:30 +0100
  Re: [PATCH] signals: work around random wakeups in sigsuspend() Ingo Molnar <mingo@kernel.org> - 2016-01-26 07:50 +0100
    Re: [PATCH] signals: work around random wakeups in sigsuspend() Oleg Nesterov <oleg@redhat.com> - 2016-01-26 16:10 +0100

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