linux-kernel - Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET

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Date:   Thu, 12 Dec 2019 13:43:12 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET

On Tue, Dec 10, 2019 at 7:40 AM Jan Beulich <jbeulich@...e.com> wrote:
>
> On 10.12.2019 16:29, Andy Lutomirski wrote:
> >> On Dec 10, 2019, at 2:48 AM, Jan Beulich <JBeulich@...e.com> wrote:
> >>
> >> Omitting suffixes from instructions in AT&T mode is bad practice when
> >> operand size cannot be determined by the assembler from register
> >> operands, and is likely going to be warned about by upstream gas in the
> >> future. Add the missing suffix here.
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> >>
> >> --- a/arch/x86/entry/entry_64.S
> >> +++ b/arch/x86/entry/entry_64.S
> >> @@ -1728,7 +1728,7 @@ END(nmi)
> >> SYM_CODE_START(ignore_sysret)
> >>    UNWIND_HINT_EMPTY
> >>    mov    $-ENOSYS, %eax
> >> -    sysret
> >> +    sysretl
> >
> > Isn’t the default sysretq?  sysretl looks more correct, but that suggests
> > that your changelog is wrong.
>
> No, this is different from ret, and more like iret and lret.
>
> > Is this code even reachable?
>
> Yes afaict, supported by the comment ahead of the symbol. syscall_init()
> puts its address into MSR_CSTAR when !IA32_EMULATION.
>

What I meant was: can a program actually get itself into 32-bit mode
to execute a 32-bit SYSCALL instruction?

Anyway, the change itself is Acked-by: Andy Lutomirski <luto@...nel.org>

But let's please clarify the changelog:

ignore_sysret contains an unsuffixed 'sysret' instruction.  gas
correctly interprets this as sysretl, but leaving it up to gas to
guess when there is no register operand that implies a size is bad
practice, and upstream gas is likely to warn about this in the future.
Use 'sysretl' explicitly.  This does not change the assembled output.

--Andy