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

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Message-Id: <08B92B44-CCA9-4B83-B9CC-F1601D44B73F@amacapital.net>
Date:   Tue, 10 Dec 2019 07:29:52 -0800
From:   Andy Lutomirski <luto@...capital.net>
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 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.

Is this code even reachable?

> SYM_CODE_END(ignore_sysret)
> #endif
>