[systemd-devel] Users and system namespaces

[David Timothy Strauss]

To join a namespace, you'll need a file descriptor for the namespace
so you can run setns() [1]. It's possible to share a file descriptor
by keeping it open while forking (which is how socket activation
works) or passing it over a Unix domain socket [2].

I know this doesn't really answer your question, but I thought the
resources might be helpful.

[1] http://man7.org/linux/man-pages/man2/setns.2.html
[2] http://stackoverflow.com/a/2358843