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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 14 Oct 2021 10:36:51 -0700
Message-ID: <CAKwvOdntTcMdXKCYLjPnNxgxMZTZWRFOsMEMp0MQSCoz30mOMg@mail.gmail.com>
Subject: Re: [PATCH] compiler_types: mark __compiletime_assert failure as __noreturn
To:     Miguel Ojeda <ojeda@kernel.org>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        Marc Zyngier <maz@kernel.org>
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On Thu, Oct 14, 2021 at 6:25 AM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> `__compiletime_assert` declares a fake `extern` function
> which appears (to the compiler) to be called when the test fails.
>
> Therefore, compilers may emit possibly-uninitialized warnings
> in some cases, even if it will be an error anyway (for compilers
> supporting the `error` attribute, e.g. GCC and Clang >= 14)
> or a link failure (for those that do not, e.g. Clang < 14).
>
> Annotating the fake function as `__noreturn` gives them
> the information they need to avoid the warning,
> e.g. see https://godbolt.org/z/x1v69jjYY.

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Link: https://lore.kernel.org/llvm/202110100514.3h9CI4s0-lkp@intel.com/
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
>  include/linux/compiler_types.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index b6ff83a714ca..ca1a66b8cd2f 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -298,7 +298,13 @@ struct ftrace_likely_data {
>  #ifdef __OPTIMIZE__
>  # define __compiletime_assert(condition, msg, prefix, suffix)          \
>         do {                                                            \
> -               extern void prefix ## suffix(void) __compiletime_error(msg); \
> +               /*                                                      \
> +                * __noreturn is needed to give the compiler enough     \
> +                * information to avoid certain possibly-uninitialized  \
> +                * warnings (regardless of the build failing).          \
> +                */                                                     \
> +               __noreturn extern void prefix ## suffix(void)           \
> +                       __compiletime_error(msg);                       \
>                 if (!(condition))                                       \
>                         prefix ## suffix();                             \
>         } while (0)
> --
> 2.33.1
>
>


-- 
Thanks,
~Nick Desaulniers