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

danielfsantos
2012-10-24T16:42:14Z

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
+ * function with the error attribute (should always creates an error 4.3+). 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 { \
- ((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

--
1.7.3.4