[PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it

Jan Beulich
2009-08-18T15:59:28Z

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>

---
drivers/char/tpm/tpm.c | 2 --
drivers/net/niu.c | 2 +-
include/linux/gfp.h | 2 +-
include/linux/kernel.h | 8 ++++++--
include/linux/kmemcheck.h | 2 +-
include/linux/virtio_config.h | 3 +--
6 files changed, 10 insertions(+), 9 deletions(-)

--- linux-2.6.31-rc6/drivers/char/tpm/tpm.c 2009-08-18 15:31:17.000000000 +0200
+++ 2.6.31-rc6-build-bug-on/drivers/char/tpm/tpm.c 2009-08-17 15:21:11.000000000 +0200
@@ -696,7 +696,6 @@ int __tpm_pcr_read(struct tpm_chip *chip

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
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,
--- linux-2.6.31-rc6/drivers/net/niu.c 2009-08-18 15:31:34.000000000 +0200
+++ 2.6.31-rc6-build-bug-on/drivers/net/niu.c 2009-08-17 15:21:11.000000000 +0200
@@ -5615,7 +5615,7 @@ static void niu_init_tx_mac(struct niu *
/* 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);
--- linux-2.6.31-rc6/include/linux/gfp.h 2009-08-18 15:31:54.000000000 +0200
+++ 2.6.31-rc6-build-bug-on/include/linux/gfp.h 2009-08-17 15:21:11.000000000 +0200
@@ -220,7 +220,7 @@ static inline enum zone_type gfp_zone(gf
((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);
--- linux-2.6.31-rc6/include/linux/kernel.h 2009-08-18 15:31:55.000000000 +0200
+++ 2.6.31-rc6-build-bug-on/include/linux/kernel.h 2009-08-17 15:21:11.000000000 +0200
@@ -675,13 +675,17 @@ 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__)
--- linux-2.6.31-rc6/include/linux/kmemcheck.h 2009-08-18 15:31:55.000000000 +0200
+++ 2.6.31-rc6-build-bug-on/include/linux/kmemcheck.h 2009-08-17 15:21:11.000000000 +0200
@@ -140,7 +140,7 @@ static inline void kmemcheck_mark_initia
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)
--- linux-2.6.31-rc6/include/linux/virtio_config.h 2009-08-18 15:31:55.000000000 +0200
+++ 2.6.31-rc6-build-bug-on/include/linux/virtio_config.h 2009-08-17 15:21:11.000000000 +0200
@@ -109,8 +109,7 @@ static inline bool virtio_has_feature(co
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);