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

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Date:	Wed, 5 Mar 2008 16:36:41 -0800
From:	Chris Lattner <clattner@...le.com>
To:	Michael Matz <matz@...e.de>
Cc:	"H.Peter Anvin" <hpa@...or.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Joe Buck <Joe.Buck@...opsys.com>, 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

>>> 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()?
>>
>> How hard is it to change the kernel signal entry path from "pushf" to
>> "pushf;cld"?  Problem solved, no?
>
> The problem is with old kernels, which by definition stay unfixed.

My impression was that the problem occurs in GCC compiled code in the  
kernel itself, not in user space:

1. User space has direction flag set.
2. signal occurs
3. kernel code is entered
4. kernel code does string operation <boom>

Fixing this instance of the problem by changing GCC requires (at  
least) recompiling the kernel.

Changing the ABI for this seems like a pretty crazy solution to a very  
minor and easily fixable kernel bug.  Distros have control over what  
kernels they ship, they have absolute power to ensure this doesn't  
affect their users when running default kernels - without changing the  
compiler.

-Chris
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