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    Subject[PATCH] fix posix lock on NFS

    I found a problem on NFS client code. It enables a local user to
    crash the system.

    NFS client prevents mandatory lock, but there is a flaw on it; Locks
    are possibly left if the mode is changed while locking. And a recent
    changes on VFS makes it calls BUG().

    For example:
    fd = open("file_on_nfs", O_RDWR | O_CREAT, 0644);
    memset(&lck, 0, sizeof(lck));
    lck.l_type = F_WRLCK;
    fcntl(fd, F_SETLK, &lck); // get locked
    fchmod(fd, 02644); // change i_mode to -rw-r-Sr--
    close(fd); // "kernel BUG at fs/locks.c:1932!"

    The cause is:
    o The situation that locking succeeds but unlocking fails is
    possible, because of i_mode.
    o locks_remove_flock() calls BUG() if posix locks remain on an
    inode when closing. It was changed at 2.6.13-rc4.

    Here is a patch against 2.6.15-rc6. This permits unlocking even if
    the mandatory lock bits are set.

    Signed-off-by: ASANO Masahiro <>

    --- linux-2.6.15-rc6/fs/nfs/file.c.orig 2005-12-21 21:30:14.000000000 +0900
    +++ linux-2.6.15-rc6/fs/nfs/file.c 2005-12-21 21:42:16.000000000 +0900
    @@ -524,7 +524,8 @@ static int nfs_lock(struct file *filp, i
    return -EINVAL;

    /* No mandatory locks over NFS */
    - if ((inode->i_mode & (S_ISGID | S_IXGRP)) == S_ISGID)
    + if ((inode->i_mode & (S_ISGID | S_IXGRP)) == S_ISGID &&
    + fl->fl_type != F_UNLCK)
    return -ENOLCK;

    if (IS_GETLK(cmd))
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      Last update: 2005-12-22 05:27    [W:0.022 / U:4.336 seconds]
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