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    Subject[PATCH] fix BUILD_BUG_ON() 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.

    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);



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