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

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Message-ID: <47CEC3AA.6080709@zytor.com>
Date:	Wed, 05 Mar 2008 08:00:42 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Aurelien Jarno <aurelien@...el32.net>,
	linux-kernel@...r.kernel.org, gcc@....gnu.org
Subject: Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

Aurelien Jarno wrote:
> Hi all,
> 
> Since version 4.3, gcc changed its behaviour concerning the x86/x86-64 
> ABI and the direction flag, that is it now assumes that the direction 
> flag is cleared at the entry of a function and it doesn't clear once 
> more if needed.
> 
> This causes some problems with the Linux kernel which does not clear
> the direction flag when entering a signal handler. The small code below
> (for x86-64) demonstrates that. 
> 
> If the signal handler is using code that need the direction flag cleared
> (for example bzero() or memset()), the code is incorrectly executed.
> 
> I guess this has to be fixed on the kernel side, but also gcc-4.3 could
> revert back to the old behaviour, that is clearing the direction flag
> when entering a routine that touches it until most people are running a
> fixed kernel.
> 

Linux should definitely follow the ABI.  This is a bug, and a pretty 
serious such.

	-hpa
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