Re: [PATCH 1/2] build_bug.h: remove negative-array fallback for BUILD_BUG_ON()

From: Kees Cook
Date: Fri Nov 16 2018 - 14:01:17 EST


On Fri, Nov 16, 2018 at 12:19 AM, Masahiro Yamada
<yamada.masahiro@xxxxxxxxxxxxx> 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.
>
> A fallback for -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@xxxxxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>
> 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
>



--
Kees Cook