linux-kernel - Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an indirect jump

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Message-ID: <9dabd1fc-7445-a305-6632-04f64e012648@suse.com>
Date:   Sat, 8 Jul 2023 16:18:29 +0200
From:   Petr Pavlu <petr.pavlu@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        samitolvanen@...gle.com, x86@...nel.org,
        linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an
 indirect jump

On 7/6/23 13:34, Peter Zijlstra wrote:
> On Thu, Jul 06, 2023 at 06:00:14PM +0900, Masami Hiramatsu wrote:
>> On Thu, 6 Jul 2023 09:17:05 +0200
>> Peter Zijlstra <peterz@...radead.org> wrote:
>>
>>> On Thu, Jul 06, 2023 at 09:47:23AM +0900, Masami Hiramatsu wrote:
>>>
>>>>>> If I understand correctly, all indirect jump will be replaced with JMP_NOSPEC.
>>>>>> If you read the insn_jump_into_range, I onlu jecks the jump code, not call.
>>>>>> So the functions only have indirect call still allow optprobe.
>>>>>
>>>>> With the introduction of kCFI JMP_NOSPEC is no longer an equivalent to a
>>>>> C indirect jump.
>>>>
>>>> If I understand correctly, kCFI is enabled by CFI_CLANG, and clang is not
>>>> using jump-tables by default, so we can focus on gcc. In that case
>>>> current check still work, correct?
>>>
>>> IIRC clang can use jump tables, but like GCC needs RETPOLINE=n and
>>> IBT=n, so effectively nobody has them.
>>
>> So if it requires RETPOLINE=n, current __indirect_thunk_start/end checking
>> is not required, right? (that code is embraced with "#ifdef CONFIG_RETPOLINE")
> 
> Correct.

Thank you both for the explanation.

If I understand correctly, it means this second patch can be dropped and
I can instead replace it with a removal of the mentioned check. That
will also void the main motivation for the first patch but that one
should be still at least useful to make the LTO_CLANG=y build lay out
the code in the same way as with other configurations.

I will post an updated series with these changes.

-- Petr