linux-kernel - Re: [PATCH 3/7] objtool: Add support for intra-function calls

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Date:   Fri, 3 Apr 2020 09:06:31 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Julien Thierry <jthierry@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH 3/7] objtool: Add support for intra-function calls


On 4/2/20 5:54 PM, Josh Poimboeuf wrote:
> On Thu, Apr 02, 2020 at 05:04:07PM +0200, Peter Zijlstra wrote:
>> On Thu, Apr 02, 2020 at 03:24:45PM +0200, Alexandre Chartre wrote:
>>> On 4/2/20 2:53 PM, Julien Thierry wrote:
>>>> On 4/2/20 9:22 AM, Alexandre Chartre wrote:
>>
>>>>> +Â Â Â  sec = find_section_by_name(file->elf,
>>>>> +Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  ".rela.discard.intra_function_call");
>>>>
>>>> I'm wondering, do we really need to annotate the intra_function_call
>>>> and group the in a section?
>>>>
>>>> Would it be a problem to consider all (static) call instructions with
>>>> a destination that is not the start offset of a symbol to be an
>>>> intra-function call (and set insn->intra_function_call and
>>>> insn->jump_dest accordingly)?
>>>
>>> Correct, we could automatically detect intra-function calls instead of
>>> having to annotate them. However, I choose to annotate them because I don't
>>> think that's not an expected construct in a "normal" code flow (at least
>>> on x86). So objtool would still issue a warning on intra-function calls
>>> by default, and you can annotate them to indicate if they are expected.
>>
>> I wondered the same thing when reading the patch. I'm confliected on
>> this. On the one hand auto-detecting this seems like an excellent idea.
>>
>> If/when the compiler generates them, they had better be okay too.
>>
>> Josh?
> 
> In general I prefer to keep it simple, and keep the annotations to a
> minimum.  And I don't think this warning has ever found anything useful.
> So I'd be inclined to say just allow them and automatically detect them.
> 
> However the fact that arm64 asm actually uses them worries me a bit.
> 
> So for me it kind of hinges on whether arm64 has a legitimate use case
> for them, or if the warning actually points to smelly code.
> 

Then what I can do is:
- by default, automatically detect and validate intra-function calls
- remove the INTRA_FUNCTION_CALL annotation
- add an objtool option to print a warning about intra-function calls
   (but still validate such calls).

This way by default intra-function calls are automatically detected and
validated. But you still have the option to print them out if you want
to check if there are any intra-function calls.

alex.