linux-kernel - Re: [PATCH v3 2/3] build_bug.h: remove negative-array fallback for BUILD_BUG

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Date:   Mon, 19 Nov 2018 10:10:27 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Kees Cook <keescook@...omium.org>, josh@...htriplett.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] build_bug.h: remove negative-array fallback for BUILD_BUG_ON()

On Mon, Nov 19, 2018 at 2:32 AM Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
>
> The kernel can only be compiled with an optimization option (-O2, -Os,
> or the currently proposed -Og). Hence, __OPTIMIZE__ is always defined
> in the kernel source.
>
> The fallback for the -O0 case is just hypothetical and pointless.
> Moreover, commit 0bb95f80a38f ("Makefile: Globally enable VLA warning")
> enabled -Wvla warning. The use of variable length arrays is banned.
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Acked-by: Kees Cook <keescook@...omium.org>
> ---
>
> Changes in v3: None
> Changes in v2: None
>
>  include/linux/build_bug.h | 14 --------------
>  1 file changed, 14 deletions(-)
>
> diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
> index 43d1fd5..d415c64 100644
> --- a/include/linux/build_bug.h
> +++ b/include/linux/build_bug.h
> @@ -51,23 +51,9 @@
>   * If you have some code which relies on certain constants being equal, or
>   * some other compile-time-evaluated condition, you should use BUILD_BUG_ON to
>   * detect if someone changes it.
> - *
> - * The implementation uses gcc's reluctance to create a negative array, but gcc
> - * (as of 4.4) only emits that error for obvious cases (e.g. not arguments to
> - * inline functions).  Luckily, in 4.3 they added the "error" function
> - * attribute just for this type of case.  Thus, we use a negative sized array
> - * (should always create an error on gcc versions older than 4.4) and then call
> - * an undefined function with the error attribute (should always create an
> - * error on gcc 4.3 and later).  If for some reason, neither creates a
> - * compile-time error, we'll still have a link-time error, which is harder to
> - * track down.
>   */
> -#ifndef __OPTIMIZE__
> -#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> -#else
>  #define BUILD_BUG_ON(condition) \
>         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
> -#endif
>
>  /**
>   * BUILD_BUG - break compile if used.
> --
> 2.7.4
>

Yep seems fine.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested an arm64 defconfig with Clang + this patch. Then tested again
with a `BUILD_BUG_ON(4 != 5)` to verify this still breaks the build.
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>

-- 
Thanks,
~Nick Desaulniers