Re: [PATCH v4 6/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

Borislav Petkov
2012-10-30T16:19:42Z

On Sun, Oct 28, 2012 at 03:57:12PM -0500, danielfsantos@att.net wrote:
> Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3,
> creating compile-time errors required a little trickery.
> BUILD_BUG{,_ON} uses this attribute when available to generate
> compile-time errors, but also uses the negative-sized array trick for
> older compilers, resulting in two error messages in some cases. The
> reason it's "some" cases is that as of gcc 4.4, the negative-sized array
> will not create an error in some situations, like inline functions.
>
> This patch replaces the negative-sized array code with the new
> __compiletime_error_fallback() macro which expands to the same thing
> unless the the error attribute is available, in which case it expands to
> do{}while(0), resulting in exactly one compile-time error on all
> versions of gcc.
>
> Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
> ---
> include/linux/bug.h | 4 ++--
> include/linux/compiler.h | 7 +++++++
> 2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/bug.h b/include/linux/bug.h
> index 03259d7..da03dc1 100644
> --- a/include/linux/bug.h
> +++ b/include/linux/bug.h
> @@ -57,13 +57,13 @@ struct pt_regs;
> * track down.
> */
> #ifndef __OPTIMIZE__
> -#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> +#define BUILD_BUG_ON(condition) __compiletime_error_fallback(condition)
> #else
> #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)])); \
> + __compiletime_error_fallback(condition); \
> if (condition) \
> __build_bug_on_failed(); \

If we're defining a fallback, shouldn't it come second? I.e.:

if (condition)
__build_bug_on_failed();
__compiletime_error_fallback(condition);

Also, the error message from __build_bug_on_failed is much more
informative:

arch/x86/kernel/cpu/amd.c: In function ‘early_init_amd’:
arch/x86/kernel/cpu/amd.c:486:2: error: call to ‘__build_bug_on_failed’ declared with attribute error: BUILD_BUG_ON failed
make[1]: *** [arch/x86/kernel/cpu/amd.o] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [arch/x86/kernel/cpu/] Error 2

than

arch/x86/kernel/cpu/amd.c: In function ‘early_init_amd’:
arch/x86/kernel/cpu/amd.c:486:2: error: size of unnamed array is negative
make[1]: *** [arch/x86/kernel/cpu/amd.o] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [arch/x86/kernel/cpu/] Error 2

Finally, you need to do:

bool __cond = !!(condition);

and use __cond so that condition doesn't get evaluated multiple times
(possibly with side effects).

Thanks.

--
Regards/Gruss,
Boris.