grafana restored and then died with "attempt to write a readonly database",
repeatedly. Its database is recorded in the snapshot as 231543:1002 and landed
as 1002:1002 — the owner was lost, so grafana, running as 231543, could not
write it at mode 0640.
Restore runs as the backup user with no CAP_CHOWN, so it reinstates ownership
inside a user namespace. The prefix was
unshare --map-root-user --map-users=SUB:SUB:N --map-groups=SUB:SUB:N
and unshare accepts ONE range per option, so the backup user's own GID was never
mapped — while app data is written as <container-uid>:<backup-user>. The group
half of every such chown referred to an unmapped id, lchown returned EINVAL, and
the file kept the restoring user's ownership. restic reports those as "ignoring
error ..." and still exits 0, so nothing failed: 1626 of one 13-app restore's
2086 failed chowns were grafana's, under a restore that reported success.
restic-userns-exec uses newuidmap/newgidmap, which write the multi-range maps
unshare cannot express:
uid: 0 <- caller inner root, or caps are dropped at exec
SUB.. <- SUB.. identity, so restic can name the stored uid
gid: caller <- caller identity: the group half of app-data chowns
SUB.. <- SUB.. identity
The caller's own UID is deliberately not identity-mapped — that slot is spent on
inner root — and a file stored as <caller>:<caller> lands owned by the caller
anyway, because that is who inner root is outside. So the one case this cannot
map is the one case needing no mapping. `unshare --map-auto --map-current-user`
is not a shortcut: it maps the subuid range to low inner ids while restic needs
identity. Tested.
Measured live, restoring grafana: failed chowns 1626 -> 12 (the 12 being the
caller's own files, correct), grafana.db back to 231543:1002, grafana up and
writing. Falls back to running the command plainly when there is no subuid
range, no newuidmap, or the namespace will not start.
scripts/dev/lp-userns-ownership-test pins all three ownership cases; verified
the old prefix fails it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
17316b7fe2