[Unionfs] Kernel oops with 2.6.16

[hooanon05 at yahoo.co.jp]

Mike Hearn:
> The 2.6.16-rc4-3-default kernel on OpenSUSE 10.1 doesn't work with the
> latest unionfs snapshot. Mounting it with the following command:
> 
>   mount -t unionfs -o dirs=/snapshots/snap1=rw:/=ro,delete=whiteout unionfs workspace/
> 
> Mar  4 00:45:47 linux kernel:  <4>unionfs: branches 0 and 1 overlap
	:::

First, your branches are 'overlapped' and unionfs doesn't allow such
branches. mount command is expected to exit with "Invalid argument" or
something.
Next, you found a bug of unionfs. It is an error handling bug. After
printing the error message (above), unionfs frees the memory, calls
dput() illegally and crashed.

And there is a little strange implementaion about checking overlapping
branches. The check is done at mount command only. So users can overlap
branches by unionctl command. I don't know this is an expected behaviour
or simply forgotten implementaion.

Junjiro Okajima