linux-kernel - Re: Fix signalfd interaction with thread-private signals

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Date:	Thu, 21 Jun 2007 12:25:09 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Fix signalfd interaction with thread-private signals

On 06/20, Linus Torvalds wrote:
> 
> On Wed, 20 Jun 2007, Oleg Nesterov wrote:
> > 
> > Also, suppose that some thread does
> > 
> > 	for (;;)
> > 		signal(SIGSEGV, SIG_IGN);
> > 
> > Now we have the same situation. do_sigaction() can steal SIGSEGV from
> > another thread.
> 
> Actually, that shouldn't be possible.
> 
> See "force_sig_info()". It does not allow blocking or ignoring forced 
> signals. We will reset such a signal handler to SIG_DFL, and unlock it.
> 
> So if you get a SIGSEGV while SIGSEGV's are blocked or ignored, the kernel 
> *will* kill you. No questions asked.

Yes, and no.

Yes, force_sig() unblocks and un-ignores the signal. However, unlike group-wide
signals, thread-specific signals do not convert themselves to SIGKILL on delivery.
The target thread should dequeue SIGSEGV and then it calls do_group_exit().
Before it does so, another thread doing signal(SIGSEGV, SIG_IGN) can steal
the signal.

Of course, this is unlikely, and the target thread will take page fault again.
The same for signalfd.

Oleg.

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