In order to fully support unpriv containers placed in directories owned
by the foreign UID range, below some unpriv user $HOME, we need to away
to actually create these hierarchies in the first place.
Let's add a method call for that. It takes a dir fd, then validates that
its ownership matches the client's identity, and then creates a subdir,
chown()ing it to the foreign UID range. It then returns an fd to the
result.
The result could then be passed to MountDirectory() in order to get a
mount which can then be populated by some code running in a dynamic
userns.
Lennart Poettering
0261fe571b