landlock: Document FS access right for pathname UNIX sockets

Add LANDLOCK_ACCESS_FS_RESOLVE_UNIX to the example code, and explain it
in the section about previous limitations.  The bulk of the interesting
flag documentation lives in the kernel header and is included in the
Sphinx rendering.

Cc: Justin Suess <utilityemal77@gmail.com>
Cc: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260327164838.38231-13-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: gnoack

Documentation/userspace-api/landlock.rst

@@ -77,7 +77,8 @@ to be explicit about the denied-by-default access rights.
             LANDLOCK_ACCESS_FS_MAKE_SYM |
             LANDLOCK_ACCESS_FS_REFER |
             LANDLOCK_ACCESS_FS_TRUNCATE |
-            LANDLOCK_ACCESS_FS_IOCTL_DEV,
+            LANDLOCK_ACCESS_FS_IOCTL_DEV |
+            LANDLOCK_ACCESS_FS_RESOLVE_UNIX,
         .handled_access_net =
             LANDLOCK_ACCESS_NET_BIND_TCP |
             LANDLOCK_ACCESS_NET_CONNECT_TCP,
@@ -127,6 +128,10 @@ version, and only use the available subset of access rights:
         /* Removes LANDLOCK_SCOPE_* for ABI < 6 */
         ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
                                  LANDLOCK_SCOPE_SIGNAL);
+        __attribute__((fallthrough));
+    case 6 ... 8:
+        /* Removes LANDLOCK_ACCESS_FS_RESOLVE_UNIX for ABI < 9 */
+        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_RESOLVE_UNIX;
     }
 
 This enables the creation of an inclusive ruleset that will contain our rules.
@@ -700,6 +705,13 @@ enforce Landlock rulesets across all threads of the calling process
 using the ``LANDLOCK_RESTRICT_SELF_TSYNC`` flag passed to
 sys_landlock_restrict_self().
 
+Pathname UNIX sockets (ABI < 9)
+-------------------------------
+
+Starting with the Landlock ABI version 9, it is possible to restrict
+connections to pathname UNIX domain sockets (:manpage:`unix(7)`) using
+the new ``LANDLOCK_ACCESS_FS_RESOLVE_UNIX`` right.
+
 .. _kernel_support:
 
 Kernel support