Messages in this thread Patch in this message | | | Date | Fri, 29 Aug 2008 12:46:34 +0100 | From | "Jan Beulich" <> | Subject | [PATCH] BUILD_BUG_ON() should not use array type |
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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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