linux-kernel - Re: [PATCH 18/41] OpenRISC: Don't reimplement force

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Message-ID: <1313523202.3436.180.camel@mfleming-mobl1.ger.corp.intel.com>
Date:	Tue, 16 Aug 2011 20:33:22 +0100
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jonas Bonn <jonas@...thpole.se>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 18/41] OpenRISC: Don't reimplement force_sigsegv()

On Tue, 2011-08-16 at 18:49 +0200, Oleg Nesterov wrote:
> On 08/11, Matt Fleming wrote:
> >
> > Instead of open coding the sequence from force_sigsegv() just call
> > it. This also fixes a race because sa_handler was being modified
> > without holding ->sighand->siglock.
> >
> > --- a/arch/openrisc/kernel/signal.c
> > +++ b/arch/openrisc/kernel/signal.c
> > @@ -257,9 +257,7 @@ static void setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
> >  	return;
> >
> >  give_sigsegv:
> > -	if (sig == SIGSEGV)
> > -		ka->sa.sa_handler = SIG_DFL;
> > -	force_sig(SIGSEGV, current);
> > +	force_sigsegv(sig, current);
> >  }
> 
> Agreed, but...
> 
> I don't really understand the changelog, which race this patch fix?
> 
> Yes, we shouldn't change sa_handler lockless, this "breaks the rules"
> but I do not see any immediate problem. And since force_sigsegv() drops
> the lock after setting SIG_DFL we can "race" with the sub-thread anyway.

Argh, yeah this and the rest of patches that try to fixup 'ka' are
wrong, because like you say, we're operating on a copy on the stack, so
there's no race. I missed that :-(

I did notice that race in force_sigsegv() too, is it a real problem? It
certainly looked to me like it could be a real problem.

> Hmm. Looking more, I think that this patch is not the cleanup, but the
> bugfix. The current code is simply wrong, it plays with ka, and it points
> to the _copy_ of sighand->action[], so this code is simply pointless.

Yeah, I will make that more explicit in the changelog.

> Unless I missed something, could you fix the changelog and resend?

Nope you haven't missed anything, you're spot on. I will resend.

-- 
Matt Fleming, Intel Open Source Technology Center

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