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

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Date:   Fri, 15 Sep 2017 07:31:55 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.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

* 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