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

["H. Peter Anvin" <hpa at zytor dot com>]

Chris Lattner wrote:
> > > > Richard Guenther wrote:
> > > > > We didn't yet run into this issue and build openSUSE with 4.3 since 
> > > > > more
> > > > > than
> > > > > three month.
> > > >
> > > > Well, how often do you take a trap inside an overlapping memmove()?
> > >
> > > How hard is it to change the kernel signal entry path from "pushf" to
> > > "pushf;cld"?  Problem solved, no?
> >
> > The problem is with old kernels, which by definition stay unfixed.
>
> My impression was that the problem occurs in GCC compiled code in the 
> kernel itself, not in user space:

That's wrong.

The issue is that the kernel is entered (due to a trap, interrupt or 
whatever) and the state is saved.  The kernel decides to revector 
userspace to a signal handler.  The kernel modifies the userspace state 
to do so, but doesn't set DF=0.

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.

	-hpa