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

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Date:	Wed, 05 Mar 2008 16:47:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Chris Lattner <clattner@...le.com>
CC:	Michael Matz <matz@...e.de>,
	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

Chris Lattner wrote:
>>>> 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:

That's wrong.

The issue is that the kernel is entered (due to a trap, interrupt or 
whatever) and the state is saved.  The kernel decides to revector 
userspace to a signal handler.  The kernel modifies the userspace state 
to do so, but doesn't set DF=0.

Upon return to userspace, the modified state kicks in.  Thus the signal 
handler is entered with DF from userspace at trap time, not DF=0.

So it's an asynchronous state leak from one piece of userspace to another.

	-hpa
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