namespace_enter() tried to become root after joining a user namespace
but ignored failures, so if root wasn't mapped we'd just carry on with
our original UID/GID. Let's make that an error instead.
Dropping supplementary groups stays best effort, as setgroups() might
be denied (e.g. by the nsresourced BPF LSM) even when root is mapped.
Some callers do need to enter namespaces without root, so add
NAMESPACE_ENTER_KEEP_UID_GID_IF_ROOT_UNMAPPED for those. It's used by
systemd-analyze unit-shell and by the credential refresh in the user
manager. Everything else enters containers or namespaces we allocated
ourselves, where root is always mapped.
Daan De Meyer
cfd2f7c73e