linux-kernel - Re: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86

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Date:   Thu, 14 Sep 2017 22:47:49 -0700
From:   hpa@...or.com
To:     Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Eugene Syromyatnikov <evgsyr@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86_X32=y

On September 14, 2017 10:31:55 PM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* Andy Lutomirski <luto@...nel.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.
-- 
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