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

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Date:	Wed, 5 Mar 2008 21:23:19 +0100
From:	Aurelien Jarno <aurelien@...el32.net>
To:	Joe Buck <Joe.Buck@...opsys.COM>
Cc:	"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

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.

I also tested Hurd, and it causes a kernel crash.

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32@...ian.org         | aurelien@...el32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net
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