Messages in this thread | | | Date | Wed, 16 Sep 2015 11:51:28 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v4 RESEND] x86/asm/entry/32, selftests: Add 'test_syscall_vdso' test |
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* 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
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