Access another process's mount namespace

Solution 1:

You'll need to define "user friendly" if you want a guaranteed satisfactory answer, but depending on your needs, /proc/<PID>/root to get the entire filesystem hierarchy as seen by a specific process, and /proc/<PID>/mounts (and /proc/<PID>/mountinfo) for what's mounted where, might get you where you need to go. If you only have a file descriptor for a namespace, you're in a far less pleasant position, because I'm not aware of any way you can go from file descriptor to a PID that's already in the namespace, but if that's where you're coming from, you left the "user friendly" reservation a long time ago, and contortions are to be expected.