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Subject[PATCH] BUILD_BUG_ON() should not use array type
With gcc's extension of variable size arrays, using the size of an
array type for BUILD_BUG_ON() doesn't always work, since non-constant
conditions aren't being detected, and hence the intention of the
construct cannot be met anymore. It is therefore necessary to use a
construct where all compilers can be expected to only accept constant
expressions with certain values being invalid, and the only thing I
could think of are bit-fields.

Note that in the virtio_config.h case 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/net/niu.c | 2 +-
include/linux/kernel.h | 8 ++++++--
include/linux/virtio_config.h | 3 +--
3 files changed, 8 insertions(+), 5 deletions(-)

--- linux-2.6.27-rc5/drivers/net/niu.c 2008-08-21 14:37:31.000000000 +0200
+++ 2.6.27-rc5-build-bug-on/drivers/net/niu.c 2008-08-28 16:51:40.000000000 +0200
@@ -5144,7 +5144,7 @@ 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-2.6.27-rc5/include/linux/kernel.h 2008-08-21 14:37:35.000000000 +0200
+++ 2.6.27-rc5-build-bug-on/include/linux/kernel.h 2008-08-28 15:10:28.000000000 +0200
@@ -468,13 +468,17 @@ 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 *)BUILD_BUG_ON_ZERO(e))

/* Trap pasters of __FUNCTION__ at compile-time */
#define __FUNCTION__ (__func__)
--- linux-2.6.27-rc5/include/linux/virtio_config.h 2008-08-21 14:37:35.000000000 +0200
+++ 2.6.27-rc5-build-bug-on/include/linux/virtio_config.h 2008-08-28 15:08:47.000000000 +0200
@@ -96,8 +96,7 @@ 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);

virtio_check_driver_offered_feature(vdev, fbit);
return test_bit(fbit, vdev->features);




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