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

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Date:	Thu, 21 Jun 2007 22:58:56 +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 06/21, Linus Torvalds wrote:
> 
> On Thu, 21 Jun 2007, Oleg Nesterov wrote:
> > 
> > Yes, the target thread is the one that caused the SIGSEGV, it sends the signal
> > to itself. entry.S:ret_from_exception should notice this signal and _dequeue_
> > it, no? This signal could be stealed by signal(SIG_IGN) which runs after it
> > was delivered.
> 
> Right. But it will dequeue it by *taking* it.
> 
> IOW, this has absolutely nothing to do with signalfd.
> 
> That's all I mean.

Yes.

> > My point was that it is _possible_ to steal a thread-local SIGSEGV even without
> > signalfd, nothing bad should happen.
> 
> That makes no sense.
> 
> You don't "steal" it. You take it. It's what SIGSEGV (and _any_ signal) 
> has always been about. You get the signal, enter the signal handler, and 
> handle it. 
> 
> No "stealing". No signalfd, no *nothing*. Just normal signal behaviour.

_Another_ thread could steal SIGSEGV via read(signalfd) without Ben's patch.
This is what Ben and Davide are worried about. I think we should not worry,
we have the same situation if this "another" thread does

	for (;;)
		signal(SIGSEGV, SIG_IGN);

do_sigaction() does rm_from_queue_full().

Oleg.

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