[RFC PATCH v2 41/41] Documentation: uid_r: describe SMB and NFSv4

[Jan Koester]

ksmbd's login extension and SIDs for UUID and GUID users, NFSv4 owner
strings, and what AUTH_SYS and the SMB3 POSIX extensions can't carry.

Assisted-by: Claude:claude-opus-5-5

Signed-off-by: Jan Koester <jan.koester at tuxist.de>
---
 Documentation/userspace-api/uid_r.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/userspace-api/uid_r.rst b/Documentation/userspace-api/uid_r.rst
index c080485455..d16d4317f5 100644
--- a/Documentation/userspace-api/uid_r.rst
+++ b/Documentation/userspace-api/uid_r.rst
@@ -159,11 +159,28 @@ RPCSEC_GSS upcalls
 	user as ``__kernel_uid_r_t``, then the usual fields. The binary (v0)
 	upcall can't carry such users and fails with ``EACCES``.
 
+SMB (ksmbd)
+	The login response extension (``struct ksmbd_login_response_ext``)
+	carries the user's ids as ``__kernel_uid_r_t`` with
+	``KSMBD_LOGIN_EXT_ID_R`` and the supplementary groups as
+	``__kernel_gid_r_t`` with ``KSMBD_LOGIN_EXT_GROUPS_R``. Such users and
+	groups appear in security descriptors as
+	``S-1-22-<kind>-w0-w1-w2-w3``: kind 3 and 4 for a UUID user and group,
+	5 and 6 for a GUID user and group, w0 to w3 the 16 bytes of the id as
+	big endian words.
+
+NFSv4
+	Owners and ACL principals that are UUIDs or GUIDs travel as their text
+	form, on the server and the client, with or without idmapd.
+
 Limitations
 ===========
 
 - Quotas don't account UUID and GUID owners; quota files hold 32 bit ids.
 - RPCSEC_GSS needs a gssd that knows ``uid_r=`` for UUID and GUID users.
+- NFS with AUTH_SYS sends a 32 bit uid: a UUID or GUID user on a client
+  reaches the server as the overflow id.
+- The SMB3 POSIX extensions' fixed size SID fields show the overflow id.
 - UUID and GUID users can't create unprivileged ublk devices.
 - Audit filter rules, SafeSetID policies and IMA policy rules name
   classic ids only.
-- 
2.56.0