[PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

[Ingo Molnar]

constant expressions get detected at build time via:

  kernel/sched.c: In function âtest':
  kernel/sched.c:9187: error: size of array âtype name' is negative
  make[1]: *** [kernel/sched.o] Error 1

but non-constant expressions (for example BUILD_BUG_ON(variable)) simply
get discarded by the compiler - turning BUILD_BUG_ON() into a dangerous
construct.

So add another layer at the link level to detect such mishaps:

  kernel/built-in.o: In function `test':
  : undefined reference to `__BUILD_BUG_ON_non_constant'

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 include/linux/kernel.h |   18 ++++++++++++++++--
 1 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 2651f80..36c841e 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -467,8 +467,22 @@ struct sysinfo {
 	char _f[20-2*sizeof(long)-sizeof(int)];	/* Padding: libc5 uses this.. */
 };
 
-/* Force a compilation error if condition is true */
-#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+/*
+ * Force a compilation error if condition is true [array index becomes
+ * negative], and a linker error if condition is not constant [non-defined
+ * variable is used as an array index]:
+ *
+ * ( The linker trick relies on gcc optimizing out a multiplication with
+ *   constant zero - which should be reasonable enough. )
+ */
+extern unsigned int __BUILD_BUG_ON_non_constant;
+
+#define BUILD_BUG_ON(condition)					\
+do {								\
+	(void)sizeof(char[1 - 2*!!(condition)]);		\
+	if (!__builtin_constant_p(condition))			\
+		__BUILD_BUG_ON_non_constant++;			\
+} while (0)
 
 /* Force a compilation error if condition is true, but also produce a
    result (of value 0 and type size_t), so the expression can be used