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SubjectRe: i386/x86_64 segment register issuses (Re: PATCH: Fix x86 segment register access)
On Tue, Mar 29, 2005 at 06:44:18PM -0800, Linus Torvalds wrote:
>
>
> On Tue, 29 Mar 2005, H. J. Lu wrote:
> >
> > > the smaller and faster version do not want to just rely on gas
> > > automatically getting it right, especially since gas has historically been
> > > very very bad at getting things right.
> >
> > We are fixing those issues in assembler. If people run into problems
> > like that with gas, they can report them. They will be fixed.
>
> It's fine if gas fixes things. It's not fine if gas breaks things that
> used to work, for no really good reason.
>
> > > What is the advantage of not allowing "movl %ds,mem"? Really? Especially
> > > since I suspect the kernel is pretty much the only one who does this, and
> > > the kernel really does do it on purpose. The kernel explicitly wants the
> > > 32-bit version, knowing that the upper bits are undefined.
> > >
> >
> > Kernel has
> >
> > unsigned gsindex;
> > asm volatile("movl %%gs,%0" : "=g" (gsindex));
>
> Ok, that's a real x86-64 bug, it seems. Andi, please fix, preferably by
> just making the "g" be a "r".
>
> However, your argument isn't very valid, since:
>
> > The new assembler will make sure that it won't happen.
>
> Not true, since the suggestion was just to change all segment "movl"
> things to "mov", at which point the same old bug is still there, and the
> assembler didn't really help us at all.

The new assembler won't accept

movl %gs,128(%rsp)

It makes it harder to generate binary code user doesn't tend. FWIW,
what I suggested are in

http://sourceware.org/ml/binutils/2005-03/msg00873.html

Thera are things like

- asm volatile("movl %%fs,%0" : "=g" (fsindex));
+ asm volatile("movl %%fs,%0" : "=r" (fsindex));

>
> See the problem? You're not actually protecting anything. The change just
> makes it _harder_ to make sizes explicit, and suddenly we have to trust an
> assembler to be clever about sizes, when that assembler historically has
> definitely _not_ been very clever about them at all.
>

There is no such an instruction of "movl %ds,(%eax)". The old assembler
accepts it and turns it into "movw %ds,(%eax)". It won't catch problems
like

unsigned fsindex;
asm volatile("movl %%fs,%0" : "=m" (fsindex));

The "movw %ds,(%eax)" bug was fixed in binutils 2.15.94.0.1. Gas no
longer generates 0x66 for it. If you find gas preventing you from doing
what the hardware supports, I will be happy to fix it.


H.J.
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