chown-overlay fix ID-space mismatch in set and apply guards

[doanbaotrung]

chown-overlay: fix ID-space mismatch in set/apply guards
chown_overlay_set receives cur_uid/cur_gid from chown_result where
they were taken directly from host_st->st_uid/st_gid (the raw macOS
stat values). new_owner/new_group are guest-virtual UIDs/GIDs.
These two live in different ID namespaces, so comparing them:

next_uid = new_owner == -1 ? cur_uid : new_owner;
if (next_uid == cur_uid && next_gid == cur_gid) goto out;

produces wrong answers whenever host UID != GUEST_UID (e.g. host
user has UID 501, guest runs as GUEST_UID 1000), or whenever the
file already has an overlay entry that virtualises its owner to a
different value.

Two complementary fixes:

fs.c - chown_result:
Apply the existing overlay to a temporary stat copy before
extracting cur_uid/cur_gid. This makes the values passed to
chown_overlay_set reflect what the guest currently sees for the
file rather than the physical host owner.

chown-overlay.c - chown_overlay_set / chown_overlay_apply:
Initialise new overlay entries with the sentinel (uint32_t)-1
("no override for this field") instead of cur_uid/cur_gid.
Resolve sentinels to cur before the stale-entry removal check
so both sides of the comparison use guest-virtual IDs.
Skip sentinel fields in chown_overlay_apply so a partial chown
does not corrupt the untouched field with a stale value.

fix #136

---

Summary by cubic

Fixes ID-space mismatches in the chown overlay by evaluating guards in the guest-visible UID/GID space to avoid false no-ops and stale entries. Corrects ownership updates on macOS and with virtualized owners; fixes #136.

• Bug Fixes
  • fs.c: Apply the existing overlay to a temp stat to get guest-visible uid/gid before calling chown_overlay_set; return ENOMEM if overlay allocation fails to avoid reporting a state the guest can’t observe.
  • chown-overlay.c: Initialize new entries with sentinel (uint32_t)-1, resolve sentinels before no-op/stale-removal checks, and skip sentinel fields in apply so partial chown doesn’t overwrite untouched values.

^{Written for commit a124c8d. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed

<sub>Reply with feedback, questions, or to request a fix.

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[jserv]

Rebase latest main and resolve conflicts.

[doanbaotrung]

Rebased and fix conflicted