d_path() and overlay fs

[Karel Zak <kzak@redhat.com>]

 Hi,

 kernel-4.0.0-0.rc4.git1.3.fc23 (I have nothing more recent right now,
 sorry it the problem is already fixed).
 
 It seems like d_path() kernel function returns nonsenses for overlay fs.

 Example:

   # mount -t overlay overlay -olowerdir=/mnt/low,upperdir=/mnt/up/data,workdir=/mnt/up/work /mnt/merged
   # dd if=/dev/zero of=/mnt/merged/image count=1 bs=1MiB

   # losetup -f /mnt/merged/image

   # cat /sys/block/loop0/loop/backing_file 
   /image


 ...should be /mnt/merged/image.

 drivers/block/loop.c: loop_attr_backing_file_show() uses d_path()

 It seems like serious problem because kernel uses d_path() (and
 friends) on many places to generate path strings for userspace in
 /proc and /sys and people already use overlay fs in containers.
 
 Another example:
 
   # cat /proc/swaps 
   Filename               Type        Size    Used    Priority
   /dev/sda6              partition   8191996 258308  -1
   /image                 file        1020    0       -2


    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com