Commit a3ccc497 authored by Daniel Santos's avatar Daniel Santos Committed by Linus Torvalds
Browse files

bug.h: make BUILD_BUG_ON generate compile-time error 



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.

Also, we are only including asm/bug.h and then expecting that
linux/compiler.h will eventually be included to define __linktime_error
(used in BUILD_BUG_ON).  This patch includes it directly for clarity and
to avoid the possibility of changes in <arch>/*/include/asm/bug.h being
changed or not including linux/compiler.h for some reason.

Signed-off-by: default avatarDaniel Santos <daniel.santos@pobox.com>
Acked-by: default avatarBorislav Petkov <bp@alien8.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joe Perches <joe@perches.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1d6a0d19

include/linux/bug.h

+19 −13
Original line number Diff line number Diff line
@@ -2,6 +2,7 @@ 
#define _LINUX_BUG_H



#include <asm/bug.h>

#include <linux/compiler.h>



enum bug_trap_type {

	BUG_TRAP_TYPE_NONE = 0,
@@ -43,25 +44,30 @@ 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.

 * 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

extern int __build_bug_on_failed;

#define BUILD_BUG_ON(condition)						\

	do {								\

		bool __cond = !!(condition);				\

		extern void __build_bug_on_failed(void)			\

			__compiletime_error("BUILD_BUG_ON failed");	\

		if (__cond)						\

			__build_bug_on_failed();			\

		((void)sizeof(char[1 - 2 * __cond]));			\

		if (__cond) __build_bug_on_failed = 1;		\

	} while (0)

#endif

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