linux-kernel - Re: [PATCH v3 06/10] bug.h: Make BUILD_BUG

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Date:	Thu, 25 Oct 2012 11:33:36 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	danielfsantos@....net
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christopher Li <sparse@...isli.org>,
	Daniel Santos <daniel.santos@...ox.com>,
	David Daney <david.daney@...ium.com>,
	David Howells <dhowells@...hat.com>,
	Joe Perches <joe@...ches.com>,
	Josh Triplett <josh@...htriplett.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-sparse@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Pavel Pisa <pisa@....felk.cvut.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3 06/10] bug.h: Make BUILD_BUG_ON generate compile-time
 error

On Wed, Oct 24, 2012 at 11:33:57AM -0500, danielfsantos@....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@...ox.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@...en8.de>

Thanks.

-- 
Regards/Gruss,
    Boris.
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