Users and permissions: accounts, login, enforced file permissions, setuid, su, passwd, doas

The kernel now checks owner/group/other on open, create, unlink, rename, exec
and path search; honours set-user-id and set-group-id; restricts kill, mount,
reboot and mknod; keeps supplementary groups; and implements chown. devtmpfs
nodes carry owners and modes that chmod/chown keep. Terminals run login, the
image has root and schos, X starts as the ordinary user.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

docs/architecture/kernel.md

@@ -435,9 +435,8 @@ allocation over 8 MB mapped straight into a neighbouring live mapping.
 - **No page cache.** `mmap` of a file *reads the range in* at map time, and
   writable `MAP_SHARED` of a file is refused outright rather than faked. Demand
   paging covers anonymous memory only.
-- **Permissions are tracked but not enforced.** uid/gid are maintained honestly
-  so `getuid()`/`setuid()` stop lying, and file access does not consult them;
-  `setpriority` is the one place the uids are checked.
+- **Root is `euid == 0` and nothing finer.** No capabilities; file access, `kill`, `exec`
+  and the privileged calls are enforced as [users.md](users.md) says.
 - **`mprotect` is accepted and not enforced** (`sys_mprotect` returns 0). RELRO is not read-only,
   a `PROT_NONE` guard does not fault, and nothing has `NX` (`EFER.NXE` is never set), so every
   mapping is executable.