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

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Date:	Thu, 6 Mar 2008 10:45:20 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Aurelien Jarno <aurelien@...el32.net>
Cc:	Joe Buck <Joe.Buck@...opsys.COM>, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, gcc@....gnu.org
Subject: Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

Aurelien Jarno writes:
 > On Wed, Mar 05, 2008 at 11:58:34AM -0800, Joe Buck wrote:
 > > 
 > > Aurelien Jarno wrote:
 > > > >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.
 > > > >...
 > > > >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.
 > > 
 > > On Wed, Mar 05, 2008 at 08:00:42AM -0800, H. Peter Anvin wrote:
 > > > Linux should definitely follow the ABI.  This is a bug, and a pretty 
 > > > serious such.
 > > 
 > > Unfortunately, there are a lot of kernels out there already with this
 > > problem, and the symptoms are likely to be subtle.  So even if it is true
 > > that it is the kernel that is "in the wrong", I think we still are going
 > > to need to give users a workaround from the gcc side as well.
 > > 
 > > So I think gcc at least needs an *option* to revert to the old behavior,
 > > and there's a good argument to make it the default for now, at least for
 > > x86/x86-64 on Linux.
 > 
 > And for other kernels. I tested OpenBSD 4.1, FreeBSD 6.3, NetBSD 4.0,
 > they have the same behaviour as Linux, that is they don't clear DF
 > before calling the signal handler.

FWIW, Solaris 10 (both 32- and 64-bit) gets it right.
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