linux-kernel - Re: [x86] BUG: unable to handle kernel paging request at 00740060

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Date:	Thu, 10 Oct 2013 08:22:38 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [x86] BUG: unable to handle kernel paging request at 00740060


* Jakub Jelinek <jakub@...hat.com> wrote:

> On Wed, Oct 09, 2013 at 09:02:31PM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 09, 2013 at 08:16:13PM +0200, Jakub Jelinek wrote:
> >
> > > Confirmed as gcc bug, filed http://gcc.gnu.org/PR58670 Seems all of 
> > > 4.[6-9] miscompile it.  Will have a look tomorrow unless somebody 
> > > beats me to it.  But historically, the case where asm goto labels 
> > > jump to fallthru basic block had numerous problems in the past.
> > 
> > That bug lists the component as middle end; this suggests x86_64 would 
> > be vulnerable too, can you confirm? So far we've only observed the 
> > wrong code on i386 targets, x86_64 targets appeared correct.
> 
> Any target, the testcase in the bugzilla aborts on x86_64 with -O2, and 
> even say on ppc64 (sure, one would have to rewrite the asm to have it 
> fail at runtime).

Please let us know once you know enough about the bug to suggest 
workarounds. Because it's a nice optimization even extra instruction(s) 
would be acceptable I suspect: we could perhaps put a NOP into a slowpath, 
with an (unused) goto to it, or something like that?

Thanks,

	Ingo
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