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

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Message-ID: <CAK8P3a14uKvDZ4OevR5z2+AJervkepDcPjGWwstTo5antbQyXA@mail.gmail.com>
Date:   Fri, 18 Jun 2021 10:20:30 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Tom Rix <trix@...hat.com>
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 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.

      Arnd