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SubjectRe: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86_X32=y
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On September 14, 2017 10:31:55 PM PDT, Ingo Molnar <mingo@kernel.org> wrote:
>
>* Andy Lutomirski <luto@kernel.org> wrote:
>
>> >> > diff --git a/arch/x86/entry/entry_64.S
>b/arch/x86/entry/entry_64.S
>> >> > index 4916725..3bab6af 100644
>> >> > --- a/arch/x86/entry/entry_64.S
>> >> > +++ b/arch/x86/entry/entry_64.S
>> >> > @@ -185,12 +185,10 @@ entry_SYSCALL_64_fastpath:
>> >> > */
>> >> > TRACE_IRQS_ON
>> >> > ENABLE_INTERRUPTS(CLBR_NONE)
>> >> > -#if __SYSCALL_MASK == ~0
>> >> > - cmpq $__NR_syscall_max, %rax
>> >> > -#else
>> >> > - andl $__SYSCALL_MASK, %eax
>> >> > - cmpl $__NR_syscall_max, %eax
>> >> > +#if __SYSCALL_MASK != ~0
>> >> > + andq $__SYSCALL_MASK, %rax
>> >> > #endif
>> >> > + cmpq $__NR_syscall_max, %rax
>> >>
>> >> I don't know much about x32 userspace, but there's an argument
>that
>> >> the high bits *should* be masked off if the x32 bit is set.
>> >
>> > Why?
>>
>> Because it always worked that way.
>>
>> That being said, I'd be okay with applying your patch and seeing
>> whether anything breaks. Ingo?
>
>So I believe this was introduced with x32 as a 'fresh, modern syscall
>ABI'
>behavioral aspect, because we wanted to protect the overly complex
>syscall entry
>code from 'weird' input values. IIRC there was an old bug where we'd
>overflow the
>syscall table in certain circumstances ...
>
>But our new, redesigned entry code is a lot less complex, a lot more
>readable and
>a lot more maintainable (not to mention a lot more robust), so if
>invalid RAX
>values with high bits set get reliably turned into -ENOSYS or such then
>I'd not
>mind the patch per se either, as a general consistency improvement.
>
>Of course if something in x32 user-land breaks then this turns into an
>ABI and we
>have to reintroduce this aspect, as a quirk :-/
>
>It should also improve x32 syscall performance a tiny bit, right? So
>might be
>worth a try on various grounds.
>
>( Another future advantage would be that _maybe_ we could use the high
>RAX
>component as an extra (64-bit only) special argument of sorts. Not that
>I can
> think of any such use right now. )
>
>Thanks,
>
> Ingo

If the consensus is that we should change the x86-64 ABI then we should change the x32 ABI to match, though.
--
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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