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Message-ID: <47CF3F09.4080606@zytor.com>
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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