BUILD_BUG_ON(): fix it and a couple of bogus uses of it

gcc permitting variable length arrays makes the current construct used for
BUILD_BUG_ON() useless, as that doesn't produce any diagnostic if the
controlling expression isn't really constant.  Instead, this patch makes
it so that a bit field gets used here.  Consequently, those uses where the
condition isn't really constant now also need fixing.

Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases
MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even if
the expression is compile time constant (__builtin_constant_p() yields
true), the array is still deemed of variable length by gcc, and hence the
whole expression doesn't have the intended effect.

[akpm@linux-foundation.org: make arch/sparc/include/asm/vio.h compile]
[akpm@linux-foundation.org: more nonsensical assertions in tpm.c..]
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
Cc: Mimi Zohar <zohar@us.ibm.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 9 insertions, 6 deletions

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index f53e9b8..557bdad 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -220,7 +220,7 @@ static inline enum zone_type gfp_zone(gfp_t flags)
 					 ((1 << ZONES_SHIFT) - 1);
 
 	if (__builtin_constant_p(bit))
-		BUILD_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
+		MAYBE_BUILD_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
 	else {
 #ifdef CONFIG_DEBUG_VM
 		BUG_ON((GFP_ZONE_BAD >> bit) & 1);
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 55723af..63dcaec 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -678,13 +678,17 @@ struct sysinfo {
 };
 
 /* Force a compilation error if condition is true */
-#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
+
+/* Force a compilation error if condition is constant and true */
+#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
 
 /* 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
    e.g. in a structure initializer (or where-ever else comma expressions
    aren't permitted). */
-#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
+#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
+#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
 
 /* Trap pasters of __FUNCTION__ at compile-time */
 #define __FUNCTION__ (__func__)
diff --git a/include/linux/kmemcheck.h b/include/linux/kmemcheck.h
index 136cdcd..e880d4cf9 100644
--- a/include/linux/kmemcheck.h
+++ b/include/linux/kmemcheck.h
@@ -152,7 +152,7 @@ static inline bool kmemcheck_is_obj_initialized(unsigned long addr, size_t size)
 									\
 		_n = (long) &((ptr)->name##_end)			\
 			- (long) &((ptr)->name##_begin);		\
-		BUILD_BUG_ON(_n < 0);					\
+		MAYBE_BUILD_BUG_ON(_n < 0);				\
 									\
 		kmemcheck_mark_initialized(&((ptr)->name##_begin), _n);	\
 	} while (0)
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index e547e3c..0093dd7 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -109,8 +109,7 @@ static inline bool virtio_has_feature(const struct virtio_device *vdev,
 				      unsigned int fbit)
 {
 	/* Did you forget to fix assumptions on max features? */
-	if (__builtin_constant_p(fbit))
-		BUILD_BUG_ON(fbit >= 32);
+	MAYBE_BUILD_BUG_ON(fbit >= 32);
 
 	if (fbit < VIRTIO_TRANSPORT_F_START)
 		virtio_check_driver_offered_feature(vdev, fbit);