linux-kernel - Re: [PATCH 1/1] bug: mark generic BUG() as unreachable

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Date:   Fri, 18 Jun 2021 06:43:13 -0700
From:   Tom Rix <trix@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] bug: mark generic BUG() as unreachable


On 6/18/21 1:20 AM, Arnd Bergmann wrote:
> On Thu, Jun 17, 2021 at 11:44 PM <trix@...hat.com> wrote:
>> From: Tom Rix <trix@...hat.com>
>>
>> This spurious error is reported for powerpc64, CONFIG_BUG=n
>>
>> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
>> index f152b9bb916fc..b250e06d7de26 100644
>> --- a/include/asm-generic/bug.h
>> +++ b/include/asm-generic/bug.h
>> @@ -177,7 +177,10 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
>>
>>   #else /* !CONFIG_BUG */
>>   #ifndef HAVE_ARCH_BUG
>> -#define BUG() do {} while (1)
>> +#define BUG() do {                                             \
>> +               do {} while (1);                                \
>> +               unreachable();                                  \
>> +       } while (0)
>>   #endif
> Please let's not go back to this version, we had good reasons to use
> the infinite loop,
> mostly to avoid undefined behavior that would lead to the compiler producing
> completely random output in code paths that lead to a BUG() statement. Those
> do cause other kinds of warnings from objtool and from other compilers.
>
> The obvious workaround here would be to add a return statement locally, but
> it may also help to figure out what exactly triggers the warning, as I don't see
> it in my randconfig builds and it may be that there is a bug elsewhere.
>
> I've tried a simple reproducer on https://godbolt.org/z/341P949bG that did not
> show this warning in any of the compilers I tried. Can you try to narrow down
> the exact compiler versions and commmand line options that produce the
> warning? https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/ has
> most of the supported gcc versions in case you need those.

Please follow the link in the cover letter to the original issue 
reported for fs/afs/dir + gcc ppc64 9.x / 10.3.1

Adding the return was the first, rejected solution.

Tom

>
>        Arnd
>