linux-kernel - [PATCH v7 7/9] bug.h: Make BUILD_BUG

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Date:	Tue,  1 Jan 2013 15:09:55 -0600
From:	danielfsantos@....net
To:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Christopher Li <sparse@...isli.org>,
	David Daney <david.daney@...ium.com>,
	David Rientjes <rientjes@...gle.com>,
	Joe Perches <joe@...ches.com>,
	Josh Triplett <josh@...htriplett.org>,
	linux-sparse@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Santos <daniel.santos@...ox.com>
Subject: [PATCH v7 7/9] 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: Daniel Santos <daniel.santos@...ox.com>
Acked-by: Borislav Petkov <bp@...en8.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

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