Re: [PATCH v3 06/10] bug.h: Make BUILD_BUG_ON generate compile-time error

Borislav Petkov
2012-10-25T09:33:44Z

On Wed, Oct 24, 2012 at 11:33:57AM -0500, danielfsantos@att.net wrote:
> Negative sized arrays wont create a compile-time error in some cases
> starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced
> the error function attribute that will. This patch modifies
> BUILD_BUG_ON to behave like BUILD_BUG already does, using the error
> function attribute so that you don't have to build the entire kernel to
> discover that you have a problem, and then enjoy trying to track it down
> from a link-time error.
>
> Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
> ---
> include/linux/bug.h | 24 ++++++++++++++----------
> 1 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/bug.h b/include/linux/bug.h
> index a03c3ef..3d4b564 100644
> --- a/include/linux/bug.h
> +++ b/include/linux/bug.h
> @@ -43,24 +43,28 @@ struct pt_regs;
> * @condition: the condition which the compiler should know is false.
> *
> * If you have some code which relies on certain constants being equal, or
> - * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
> + * 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 (eg. not arguments
> - * to inline functions). So as a fallback we use the optimizer; if it can't
> - * prove the condition is false, it will cause a link error on the undefined
> - * "__build_bug_on_failed". This error message can be harder to track down
> - * though, hence the two different methods.
> + * 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 pre-gcc-4.4) and then call an undefined

.... always create an error on gcc versions older than 4.4)

> + * function with the error attribute (should always creates an error 4.3+). If

..... always create an error on gcc 4.3 and later)

> + * 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
> -extern int __build_bug_on_failed;
> -#define BUILD_BUG_ON(condition) \
> - do { \
> - ((void)sizeof(char[1 - 2*!!(condition)])); \
> - if (condition) __build_bug_on_failed = 1; \
> +#define BUILD_BUG_ON(condition) \
> + do { \
> + extern void __build_bug_on_failed(void) \
> + __compiletime_error("BUILD_BUG_ON failed"); \
> + ((void)sizeof(char[1 - 2*!!(condition)])); \
> + if (condition) \
> + __build_bug_on_failed(); \
> } while(0)
> #endif

With the changes above:

Acked-by: Borislav Petkov <bp@alien8.de>

Thanks.

--
Regards/Gruss,
Boris.