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

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Message-ID: <47CF44E7.3020106@zytor.com>
Date:	Wed, 05 Mar 2008 17:12:07 -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:
>>
>> 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.
> 
> Fine, it can happen either way.  In either case, the distro vendor 
> should fix the the signal handler in the kernels they distribute.  If 
> you don't do that, you are still leaking information from one piece of 
> user space code to another, you're just papering over it in a horrible 
> way :)
> 
> GCC defines the direction flag to be clear before inline asm. Enforcing 
> the semantics you propose would require issuing a cld before every 
> inline asm, not just before every string operation.
> 

It's a kernel bug, and it needs to be fixed.  The discussion is about 
what to do in the meantime.

(And yes, you're absolutely right: between global subroutine entry and 
the first asm or string operation, you'd have to emit cld.)

	-hpa
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