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

Mikael Pettersson
2008-03-06T09:46:33Z

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.