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

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Date:	Tue, 26 Jan 2016 07:44:25 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	oleg@...hat.com, peterz@...radead.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] signals: work around random wakeups in sigsuspend()


* Sasha Levin <sasha.levin@...cle.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.
> 
> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  kernel/signal.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 5da9180..3256c7e 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -3528,8 +3528,10 @@ static int sigsuspend(sigset_t *set)
>  	current->saved_sigmask = current->blocked;
>  	set_current_blocked(set);
>  
> -	__set_current_state(TASK_INTERRUPTIBLE);
> -	schedule();
> +	while (!signal_pending(current)) {
> +		__set_current_state(TASK_INTERRUPTIBLE);
> +		schedule();
> +	}
>  	set_restore_sigmask();
>  	return -ERESTARTNOHAND;
>  }

So this does not appear to be anything new, right?

I agree with the fix, but I'm somewhat worried about the potential ABI impact: 
does anything exist out there that has learned to rely on spurious returns from 
SyS_sigsuspend() or SyS_rt_sigsuspend() system calls? These are one of the most 
frequently used system calls in signal based event loops.

Thanks,

	Ingo