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

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Message-ID: <20070621182340.GA92@tv-sign.ru>
Date:	Thu, 21 Jun 2007 22:23:40 +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, 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().
> 
> No it couldn't.
> 
> Why? Because the target thread is the one that *caused* the SIGSEGV in the 
> first place. It's not going to dequeue *anything*. It's either going to 
> take the SIGSEGV,

Hmm, can't understand.

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.

> or it's going to get another SIGSEGV and now it's no
> longer masked/handled and it's going to die.

Yes sure. As I said,

	> and the target thread will take page fault again.

My point was that it is _possible_ to steal a thread-local SIGSEGV even without
signalfd, nothing bad should happen.

Oleg.

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