Re: [PATCH v4 RESEND] x86/asm/entry/32, selftests: Add 'test_syscall_vdso' test

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Sep 14, 2015 11:00 PM, "Ingo Molnar" <mingo@kernel.org> wrote:
> >
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > > On Sep 14, 2015 1:15 AM, "Ingo Molnar" <mingo@kernel.org> wrote:
> > > >
> > > >
> > > > * Denys Vlasenko <dvlasenk@redhat.com> wrote:
> > > >
> > > > > >> +                       /* INT80 syscall entrypoint can be used by
> > > > > >> +                        * 64-bit programs too, unlike SYSCALL/SYSENTER.
> > > > > >> +                        * Therefore it must preserve R12+
> > > > > >> +                        * (they are callee-saved registers in 64-bit C ABI).
> > > > > >> +                        *
> > > > > >> +                        * This was probably historically not intended,
> > > > > >> +                        * but R8..11 are clobbered (cleared to 0).
> > > > > >> +                        * IOW: they are the only registers which aren't
> > > > > >> +                        * preserved across INT80 syscall.
> > > > > >> +                        */
> > > > > >> +                       if (*r64 == 0 && num <= 11)
> > > > > >> +                               continue;
> > > > > >
> > > > > > Ugh.  I'll change my big entry patchset to preserve these and maybe to
> > > > > > preserve all of the 64-bit regs.
> > > > >
> > > > > If you do that, this won't change the ABI: we don't _promise_
> > > > > to save them. If we accidentally do, that means nothing.
> > > >
> > > > Argh, that's dangerous nonsense! You _still_ don't seem to understand what the
> > > > Linux ABI means and how to change code that implements it...
> > >
> > > I think Denys might be taking about R8-R11 here.  If we change them
> > > from clobbered to saved, that's probably fine.  Certainly I have to
> > > save R12-R15 -- my v1 is just buggy there.  I was too deep in
> > > __kernel_vsyscall when I wrote that code, and I wasn't thinking about
> > > the raw int $0x80 entry variant.
> > >
> > > I'd be rather surprised if anything broke if we started preserving
> > > R8-R11 instead of zeroing them.
> >
> > Well, read the statement:
> >
> >   " If you do that, this won't change the ABI: we don't _promise_
> >     to save them. If we accidentally do, that means nothing. "
> >
> > of _course_ it means everything: if we preserve R8-R11 and any app learns to rely
> > on it, it becomes an ABI.
> 
> Right, it changes the ABI in a way that we can't undo, but it probably
> doesn't break the old ABI.

it's unknown: user-space code might have (unknowingly) started relying on the 
zeroing behavior. If that happened (and let's hope it didn't - but there's no 
guarantee), then the zeroing behavior is an ABI too.

> Certainly for v2, I'll try to preserve the old behavior exactly.  If
> we change it later to preserve all high regs, that'll be a separate
> patch.

Yeah, cleanly separating ABI-invariant patches from ABI-impacting ones (no matter 
how innocious the effect on the ABI looks) is a must.

Thanks,

	Ingo