I'm trying to understand whether it's possible to map two users from a user namespace to two different users on the host.

The goal is to replicate the same permissions I have on my host inside a rootfs (ubuntu base, because i'm trying to build a container from scratch).
For example:

• Everything under / should belong to root.
• /home/user should belong to the regular user.

To achieve this, I was thinking of using UID mapping in a user namespace, something like:

UID in user namespace      ---> UID on host
      1000 (admin)         ->       0 (root)
      1001 (bob)           ->    1001 (bob)

Is this kind of mapping even possible?

Here’s what I’ve already tried:

• Running echo -e "1000 0 1\n1001 1001 1" > /proc/[PID]/uid_map to define the mapping, but I get an error.
• Trying to manually modify /proc/[PID]/uid_map using newuidmap for each user.

However, I’ve never been able to map more than one user, and I can’t seem to map UID 0 (root) at all.