How to switch user-space process back to the root network namespace?

Question

The problem is that /var/run/netns/ directory contains all the network namespaces, except the root namespace. So I am not sure, how to get the fd, that should be passed to the following kernel call:

syscall(__NR_setns, fd, CLONE_NEWNET)

Answer 1

AFAIU, you can use /proc/<pid>/ns/net.