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

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Date:   Tue, 10 Dec 2019 16:40:49 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Andy Lutomirski <luto@...capital.net>
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 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.

Jan