[PATCH v7 7/9] bug.h: Make BUILD_BUG_ON generate compile-time error

danielfsantos
2013-01-01T21:10:25Z

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: Daniel Santos <daniel.santos@pobox.com>
Acked-by: Borislav Petkov <bp@alien8.de>
---
include/linux/bug.h | 32 +++++++++++++++++++-------------
1 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/include/linux/bug.h b/include/linux/bug.h
index 0d75762..08d97e9 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -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); \
- ((void)sizeof(char[1 - 2 * __cond])); \
- if (__cond) __build_bug_on_failed = 1; \
+#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])); \
} while(0)
#endif

--
1.7.8.6