mount -o union doesn't allow changes to sub directories?

[Lawrence Stewart]

Hi again,

If I run something like:

mount -o union /dev/md0 /etc

where md0 is a memory backed fs device, and /etc is fully populated but
mounted RO off a CD, I can touch a new file in /etc, but can't in any
subdir of etc (fails with "Read-only filesystem" reported).

As far as I can tell this is a bug, but wanted to check if I'm
misunderstanding how the "-o union" mount option is supposed to work.

Cheers,
Lawrence