linux-kernel - Re: [BUG] signal: multithread program returns with wrong errno on receiving SIGSTOP

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Message-ID: <20070529192243.GA189@tv-sign.ru>
Date:	Tue, 29 May 2007 23:22:43 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [BUG] signal: multithread program returns with wrong errno on receiving SIGSTOP

On 05/29, Roland McGrath wrote:
>
> Your fix seems like the only way to go.  From skimming all the ERESTART*
> uses, I think that in all cases (except for n_tty.c:job_control before your
> patch), TIF_SIGPENDING is indeed set when a thread returns -ERESTART*.
> 
> But it makes me realize that there is a danger of leaking a -ERESTART*
> return code to userland when TIF_SIGPENDING gets cleared by another thread
> doing a recalc_sigpending_tsk.  Because of -ERESTART* I think we must make
> it a rule that no thread can clear another thread's TIF_SIGPENDING, only
> set it (unless it's known to be stopped in the signal code or something).
> >From our recent work on it, I think that do_sigaction is in fact the only
> place this can happen.  So that says we should err in the other direction
> from what I said before in do_sigaction, and not have it do recalc at all.

I think you are right.

But please note that cancel_freezing(p) is special. It is also called when
try_to_freeze_tasks() fails. So it should clear TIF_SIGPENDING if "p" is a
kernel thread, otherwise p may run with signal_pending() forever.

Unfortunately, it is not easy to detect the kernel thread, is_user_space()
is not reliable. Probably we should ignore this minor problem and do not
change cancel_freezing().

Oleg.

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