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    Subject[PATCH 2/2] debugfs: Make automount point inodes permanently empty
    Date
    Starting with 4.1 the tracing subsystem has its own filesystem
    which is automounted in the tracing subdirectory of debugfs.
    Prior to this debugfs could be bind mounted in a cloned mount
    namespace, but if tracefs has been mounted under debugfs this
    now fails because there is a locked child mount. This creates
    a regression for container software which bind mounts debugfs
    to satisfy the assumption of some userspace software.

    In other pseudo filesystems such as proc and sysfs we're already
    creating mountpoints like this in such a way that no dirents can
    be created in the directories, allowing them to be exceptions to
    some MNT_LOCKED tests. In fact we're already do this for the
    tracefs mountpoint in sysfs.

    Do the same in debugfs_create_automount(), since the intention
    here is clearly to create a mountpoint. This fixes the regression,
    as locked child mounts on permanently empty directories do not
    cause a bind mount to fail.

    Cc: stable@vger.kernel.org # v4.1+
    Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
    ---
    fs/debugfs/inode.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
    index bece948b363d..8580831ed237 100644
    --- a/fs/debugfs/inode.c
    +++ b/fs/debugfs/inode.c
    @@ -457,7 +457,7 @@ struct dentry *debugfs_create_automount(const char *name,
    if (unlikely(!inode))
    return failed_creating(dentry);

    - inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
    + make_empty_dir_inode(inode);
    inode->i_flags |= S_AUTOMOUNT;
    inode->i_private = data;
    dentry->d_fsdata = (void *)f;
    --
    1.9.1
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