> one major use of union mounts is to let users modify "read-only" directory
> trees.  If /usr/src were owner root, mode 700, and I union mounted
> /home/tls/mysrc on top, if the shadow directory permissions followed the
> bottom layer's permissions, I'd have a useless union mount: I could not
> create any files even in my top layer!

Ah.  Probably needs to be an option - because another use of union mounts
is for the sysadmin to layer them over a set of baseline files to capture
the changes to them, but arbitrarily munging the owners and permissions
of arbitrary system files and directories is not likely to be a winning
idea.  An option to select a "system" or "user" style union mount--or even
just selecting the behavior based on whether the mount was done by root
or a regular user--would probably be the way to go.

-Chap