linux-kernel - [PATCH] compiler.h: fix error in BUILD_BUG

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Message-Id: <20200331112637.25047-1-vegard.nossum@oracle.com>
Date:   Tue, 31 Mar 2020 13:26:37 +0200
From:   Vegard Nossum <vegard.nossum@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Daniel Santos <daniel.santos@...ox.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Ian Abbott <abbotti@....co.uk>
Subject: [PATCH] compiler.h: fix error in BUILD_BUG_ON() reporting

compiletime_assert() uses __LINE__ to create a unique function name.
This means that if you have more than one BUILD_BUG_ON() in the same
source line (which can happen if they appear e.g. in a macro), then
the error message from the compiler might output the wrong condition.

For this source file:

	#include <linux/build_bug.h>

	#define macro() \
		BUILD_BUG_ON(1); \
		BUILD_BUG_ON(0);

	void foo()
	{
		macro();
	}

gcc would output:

./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_9’ declared with attribute error: BUILD_BUG_ON failed: 0
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)

However, it was not the BUILD_BUG_ON(0) that failed, so it should say 1
instead of 0. With this patch, we use __COUNTER__ instead of __LINE__, so
each BUILD_BUG_ON() gets a different function name and the correct
condition is printed:

./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_0’ declared with attribute error: BUILD_BUG_ON failed: 1
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)

Cc: Daniel Santos <daniel.santos@...ox.com>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc: Ian Abbott <abbotti@....co.uk>
Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
---
 include/linux/compiler.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 5e88e7e33abec..034b0a644efcc 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -347,7 +347,7 @@ static inline void *offset_to_ptr(const int *off)
  * compiler has support to do so.
  */
 #define compiletime_assert(condition, msg) \
-	_compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
+	_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
 
 #define compiletime_assert_atomic_type(t)				\
 	compiletime_assert(__native_word(t),				\
-- 
2.16.1.72.g5be1f00a9.dirty