linux-kernel - Re: RELEASE BLOCKER: Linux doesn't follow x86/x86-64 ABI wrt direction flag

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Message-ID: <20080306090109.GD24887@devserv.devel.redhat.com>
Date:	Thu, 6 Mar 2008 04:01:09 -0500
From:	Jakub Jelinek <jakub@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Joe Buck <Joe.Buck@...opsys.com>, Michael Matz <matz@...e.de>,
	Jan Hubicka <hubicka@....cz>,
	Aurelien Jarno <aurelien@...el32.net>,
	linux-kernel@...r.kernel.org, gcc@....gnu.org
Subject: Re: RELEASE BLOCKER: Linux doesn't follow x86/x86-64 ABI wrt direction  flag

On Thu, Mar 06, 2008 at 09:44:05AM +0100, Andi Kleen wrote:
> "H. Peter Anvin" <hpa@...or.com> writes:
> 
> > Richard Guenther wrote:
> > > We didn't yet run into this issue and build openSUSE with 4.3 since
> > > more than
> > > three month.
> > >
> > 
> > Well, how often do you take a trap inside an overlapping memmove()?
> 
> That was the state with older gcc, but with newer gcc it does not necessarily
> reset the flag before the next function call.
> 
> so e.g. if you have
> 
>         memmove(...)
>         for (... very long loop .... ) {
>                 /* no function calls */
>                 /* signals happen */
>         }
> 
> the signal could see the direction flag

memmove is supposed to (and does) do a cld insn after it finishes the
backward copying.

	Jakub
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