linux-kernel - Re: [PATCH] compiler.h: fix error in BUILD_BUG

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Date:   Wed, 1 Apr 2020 02:31:37 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Santos <daniel.santos@...ox.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ian Abbott <abbotti@....co.uk>
Subject: Re: [PATCH] compiler.h: fix error in BUILD_BUG_ON() reporting

On Tue, Mar 31, 2020 at 8:28 PM Vegard Nossum <vegard.nossum@...cle.com> wrote:
>
> compiletime_assert() uses __LINE__ to create a unique function name.
> This means that if you have more than one BUILD_BUG_ON() in the same
> source line (which can happen if they appear e.g. in a macro), then
> the error message from the compiler might output the wrong condition.
>
> For this source file:
>
>         #include <linux/build_bug.h>
>
>         #define macro() \
>                 BUILD_BUG_ON(1); \
>                 BUILD_BUG_ON(0);
>
>         void foo()
>         {
>                 macro();
>         }
>
> gcc would output:
>
> ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_9’ declared with attribute error: BUILD_BUG_ON failed: 0
>   _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>
> However, it was not the BUILD_BUG_ON(0) that failed, so it should say 1
> instead of 0. With this patch, we use __COUNTER__ instead of __LINE__, so
> each BUILD_BUG_ON() gets a different function name and the correct
> condition is printed:
>
> ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_0’ declared with attribute error: BUILD_BUG_ON failed: 1
>   _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>
> Cc: Daniel Santos <daniel.santos@...ox.com>
> Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>


Reviewed-by: Masahiro Yamada <yamada.masahiro@...ionext.com>




> Cc: Ian Abbott <abbotti@....co.uk>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
> ---
>  include/linux/compiler.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 5e88e7e33abec..034b0a644efcc 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -347,7 +347,7 @@ static inline void *offset_to_ptr(const int *off)
>   * compiler has support to do so.
>   */
>  #define compiletime_assert(condition, msg) \
> -       _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
> +       _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>
>  #define compiletime_assert_atomic_type(t)                              \
>         compiletime_assert(__native_word(t),                            \
> --
> 2.16.1.72.g5be1f00a9.dirty
>


-- 
Best Regards
Masahiro Yamada