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    Subject[PATCH v2] fs: O_* bit numbers uniqueness check
    On Wed, Jan 06, 2010 at 03:08:08PM +0800, Roland Dreier wrote:
    >
    > > + /* please add new bits here to ensure allocation uniqueness */
    > > + BUG_ON(20 != hweight32(
    > > + O_RDONLY | O_WRONLY | O_RDWR |
    >
    > I wonder if there's a way to make this BUILD_BUG_ON(), so the problem is
    > caught at compile time (a change like adding an O_ flag is often compile
    > tested only for obscure archs). One could create a compile-time
    > macro-ized version of hweight32() and use that, I guess, although I'm
    > not sure it's worth the ugliness.

    Yeah, I chose the boot time check because of hweight32()..

    > Failing that maybe this should be WARN_ON()? I'd be annoyed if my arch
    > wouldn't boot because some strange new O_ flag happened to collide.

    Good suggestion! Here is the updated patch.

    Andrew, this patch relies on the newly introduced O_RANDOM and Eric's
    FMODE_NONOTIFY.

    Thanks,
    Fengguang
    ---
    fs: O_* bit numbers uniqueness check

    The O_* bit numbers are defined in 20+ arch/*, and hence can silently
    overlap. Add a boot time check to ensure the uniqueness as suggested
    by David Miller.

    v2: change to WARN_ON() as suggested by Roland

    CC: David Miller <davem@davemloft.net>
    CC: Stephen Rothwell <sfr@canb.auug.org.au>
    CC: Al Viro <viro@zeniv.linux.org.uk>
    CC: Christoph Hellwig <hch@infradead.org>
    CC: Eric Paris <eparis@redhat.com>
    CC: Roland Dreier <rdreier@cisco.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    fs/fcntl.c | 14 ++++++++++++--
    include/asm-generic/fcntl.h | 2 ++
    2 files changed, 14 insertions(+), 2 deletions(-)

    --- linux.orig/fs/fcntl.c 2010-01-06 14:41:26.000000000 +0800
    +++ linux/fs/fcntl.c 2010-01-06 15:15:11.000000000 +0800
    @@ -709,11 +709,21 @@ void kill_fasync(struct fasync_struct **
    }
    EXPORT_SYMBOL(kill_fasync);

    -static int __init fasync_init(void)
    +static int __init fcntl_init(void)
    {
    + /* please add new bits here to ensure allocation uniqueness */
    + WARN_ON(20 != hweight32(
    + O_RDONLY | O_WRONLY | O_RDWR |
    + O_CREAT | O_EXCL | O_NOCTTY |
    + O_TRUNC | O_APPEND | O_NONBLOCK |
    + O_SYNC | FASYNC | O_DIRECT |
    + O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW |
    + O_NOATIME | O_CLOEXEC | O_RANDOM |
    + FMODE_EXEC | FMODE_NONOTIFY));
    +
    fasync_cache = kmem_cache_create("fasync_cache",
    sizeof(struct fasync_struct), 0, SLAB_PANIC, NULL);
    return 0;
    }

    -module_init(fasync_init)
    +module_init(fcntl_init)
    --- linux.orig/include/asm-generic/fcntl.h 2010-01-06 14:45:34.000000000 +0800
    +++ linux/include/asm-generic/fcntl.h 2010-01-06 14:50:57.000000000 +0800
    @@ -4,6 +4,8 @@
    #include <linux/types.h>

    /*
    + * When introducing new O_* bits, please check its uniqueness in fcntl_init().
    + *
    * FMODE_EXEC is 0x20
    * FMODE_NONOTIFY is 0x1000000
    * These cannot be used by userspace O_* until internal and external open

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