Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG

[Adrian Hunter]

On 11/04/24 10:56, Arnd Bergmann wrote:
> On Thu, Apr 11, 2024, at 09:16, Adrian Hunter wrote:
>> On 11/04/24 10:04, Arnd Bergmann wrote:
>>> On Wed, Apr 10, 2024, at 17:32, Adrian Hunter wrote:
>>>> BUG() does not return, and arch implementations of BUG() use unreachable()
>>>> or other non-returning code. However with !CONFIG_BUG, the default
>>>> implementation is often used instead, and that does not do that. x86 always
>>>> uses its own implementation, but powerpc with !CONFIG_BUG gives a build
>>>> error:
>>>>
>>>>   kernel/time/timekeeping.c: In function ‘timekeeping_debug_get_ns’:
>>>>   kernel/time/timekeeping.c:286:1: error: no return statement in function
>>>>   returning non-void [-Werror=return-type]
>>>>
>>>> Add unreachable() to default !CONFIG_BUG BUG() implementation.
>>>
>>> I'm a bit worried about this patch, since we have had problems
>>> with unreachable() inside of BUG() in the past, and as far as I
>>> can remember, the current version was the only one that
>>> actually did the right thing on all compilers.
>>>
>>> One problem with an unreachable() annotation here is that if
>>> a compiler misanalyses the endless loop, it can decide to
>>> throw out the entire code path leading up to it and just
>>> run into undefined behavior instead of printing a BUG()
>>> message.
>>>
>>> Do you know which compiler version show the warning above?
>>
>> Original report has a list
>>
> 
> It looks like it's all versions of gcc, though no versions
> of clang show the warnings. I did a few more tests and could
> not find any differences on actual code generation, but
> I'd still feel more comfortable changing the caller than
> the BUG() macro. It's trivial to add a 'return 0' there.

AFAICT every implementation of BUG() except this one has
unreachable() or equivalent, so that inconsistency seems
wrong.

Could add 'return 0', but I do notice other cases
where a function does not have a return value, such as
cpus_have_final_boot_cap(), so there is already an expectation
that that is OK.

> Another interesting observation is that clang-11 and earlier
> versions end up skipping the endless loop, both with and
> without the __builtin_unreachable, see
> https://godbolt.org/z/aqa9zqz8x

Adding volatile asm("") to the loop would probably fix that,
but it seems like a separate issue.

> 
> clang-12 and above do work like gcc, so I guess that is good.
> 
>      Arnd