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

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Message-ID: <47D00BCA.7060900@zytor.com>
Date:	Thu, 06 Mar 2008 07:20:42 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jakub Jelinek <jakub@...hat.com>
CC:	Andi Kleen <andi@...stfloor.org>,
	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

Jakub Jelinek wrote:
> 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.

If so, that's a much worse bug.

>> 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.

You can still take a signal inside memmove() itself, of course.

	-hpa
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