linux-kernel - [PATCH 3.2 008/221] kernel.h: add BUILD

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Date:	Tue, 05 May 2015 02:16:39 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "David Howells" <dhowells@...hat.com>,
	"DM" <dm.n9107@...il.com>, "David Rientjes" <rientjes@...gle.com>,
	"David Daney" <david.daney@...ium.com>,
	"Ralf Baechle" <ralf@...ux-mips.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.2 008/221] kernel.h: add BUILD_BUG() macro

3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: David Daney <david.daney@...ium.com>

commit 1399ff86f2a2bbacbbe68fa00c5f8c752b344723 upstream.

We can place this in definitions that we expect the compiler to remove by
dead code elimination.  If this assertion fails, we get a nice error
message at build time.

The GCC function attribute error("message") was added in version 4.3, so
we define a new macro __linktime_error(message) to expand to this for
GCC-4.3 and later.  This will give us an error diagnostic from the
compiler on the line that fails.  For other compilers
__linktime_error(message) expands to nothing, and we have to be content
with a link time error, but at least we will still get a build error.

BUILD_BUG() expands to the undefined function __build_bug_failed() and
will fail at link time if the compiler ever emits code for it.  On GCC-4.3
and later, attribute((error())) is used so that the failure will be noted
at compile time instead.

Signed-off-by: David Daney <david.daney@...ium.com>
Acked-by: David Rientjes <rientjes@...gle.com>
Cc: DM <dm.n9107@...il.com>
Cc: Ralf Baechle <ralf@...ux-mips.org>
Acked-by: David Howells <dhowells@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -29,6 +29,8 @@
    the kernel context */
 #define __cold			__attribute__((__cold__))
 
+#define __linktime_error(message) __attribute__((__error__(message)))
+
 /*
  * GCC 'asm goto' miscompiles certain code sequences:
  *
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -297,7 +297,9 @@ void ftrace_likely_update(struct ftrace_
 #ifndef __compiletime_error
 # define __compiletime_error(message)
 #endif
-
+#ifndef __linktime_error
+# define __linktime_error(message)
+#endif
 /*
  * Prevent the compiler from merging or refetching accesses.  The compiler
  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -679,6 +679,7 @@ static inline void ftrace_dump(enum ftra
 #define BUILD_BUG_ON_ZERO(e) (0)
 #define BUILD_BUG_ON_NULL(e) ((void*)0)
 #define BUILD_BUG_ON(condition)
+#define BUILD_BUG() (0)
 #else /* __CHECKER__ */
 
 /* Force a compilation error if a constant expression is not a power of 2 */
@@ -717,6 +718,21 @@ extern int __build_bug_on_failed;
 		if (condition) __build_bug_on_failed = 1;	\
 	} while(0)
 #endif
+
+/**
+ * BUILD_BUG - break compile if used.
+ *
+ * If you have some code that you expect the compiler to eliminate at
+ * build time, you should use BUILD_BUG to detect if it is
+ * unexpectedly used.
+ */
+#define BUILD_BUG()						\
+	do {							\
+		extern void __build_bug_failed(void)		\
+			__linktime_error("BUILD_BUG failed");	\
+		__build_bug_failed();				\
+	} while (0)
+
 #endif	/* __CHECKER__ */
 
 /* Trap pasters of __FUNCTION__ at compile-time */

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