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.
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/char/tpm/tpm.c

@@ -696,7 +696,6 @@ int __tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
 
 	cmd.header.in = pcrread_header;
 	cmd.params.pcrread_in.pcr_idx = cpu_to_be32(pcr_idx);
-	BUILD_BUG_ON(cmd.header.in.length > READ_PCR_RESULT_SIZE);
 	rc = transmit_cmd(chip, &cmd, cmd.header.in.length,
 			  "attempting to read a pcr value");
 
@@ -760,7 +759,6 @@ int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash)
 		return -ENODEV;
 
 	cmd.header.in = pcrextend_header;
-	BUILD_BUG_ON(be32_to_cpu(cmd.header.in.length) > EXTEND_PCR_SIZE);
 	cmd.params.pcrextend_in.pcr_idx = cpu_to_be32(pcr_idx);
 	memcpy(cmd.params.pcrextend_in.hash, hash, TPM_DIGEST_SIZE);
 	rc = transmit_cmd(chip, &cmd, cmd.header.in.length,

drivers/net/niu.c

@@ -5615,7 +5615,7 @@ static void niu_init_tx_mac(struct niu *np)
 	/* The XMAC_MIN register only accepts values for TX min which
 	 * have the low 3 bits cleared.
 	 */
-	BUILD_BUG_ON(min & 0x7);
+	BUG_ON(min & 0x7);
 
 	if (np->flags & NIU_FLAGS_XMAC)
 		niu_init_tx_xmac(np, min, max);

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);

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__)

include/linux/kmemcheck.h

@@ -140,7 +140,7 @@ static inline void kmemcheck_mark_initialized_pages(struct page *p,
 	do if (ptr) {							\
 		int _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)

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);