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

[Borislav Petkov <bp@alien8.de>]

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.
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