[PATCH v3] landlock: Clarify documentation for the LANDLOCK_ACCESS_FS_REFER right

[Günther Noack]

Clarify the "refer" documentation by splitting up a big paragraph of text.

- Call out specifically that the denial by default applies to ABI v1 as well.
- Turn the three additional constraints for link/rename operations
  into bullet points, to give it more structure.

Includes wording and semantics corrections by Mickaël Salaün.

Signed-off-by: Günther Noack <gnoack3000 at gmail.com>
---
 include/uapi/linux/landlock.h | 45 +++++++++++++++++++++++------------
 1 file changed, 30 insertions(+), 15 deletions(-)

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index f3223f96469..e549ad6360b 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -130,21 +130,36 @@ struct landlock_path_beneath_attr {
  * - %LANDLOCK_ACCESS_FS_MAKE_BLOCK: Create (or rename or link) a block device.
  * - %LANDLOCK_ACCESS_FS_MAKE_SYM: Create (or rename or link) a symbolic link.
  * - %LANDLOCK_ACCESS_FS_REFER: Link or rename a file from or to a different
- *   directory (i.e. reparent a file hierarchy).  This access right is
- *   available since the second version of the Landlock ABI.  This is also the
- *   only access right which is always considered handled by any ruleset in
- *   such a way that reparenting a file hierarchy is always denied by default.
- *   To avoid privilege escalation, it is not enough to add a rule with this
- *   access right.  When linking or renaming a file, the destination directory
- *   hierarchy must also always have the same or a superset of restrictions of
- *   the source hierarchy.  If it is not the case, or if the domain doesn't
- *   handle this access right, such actions are denied by default with errno
- *   set to ``EXDEV``.  Linking also requires a ``LANDLOCK_ACCESS_FS_MAKE_*``
- *   access right on the destination directory, and renaming also requires a
- *   ``LANDLOCK_ACCESS_FS_REMOVE_*`` access right on the source's (file or
- *   directory) parent.  Otherwise, such actions are denied with errno set to
- *   ``EACCES``.  The ``EACCES`` errno prevails over ``EXDEV`` to let user space
- *   efficiently deal with an unrecoverable error.
+ *   directory (i.e. reparent a file hierarchy).
+ *
+ *   This access right is available since the second version of the Landlock
+ *   ABI.  This is also the only access right which is implicitly handled by any
+ *   ruleset, even if the right is not specified at the time of creating the
+ *   ruleset.  So, by default, Landlock will deny linking and reparenting files
+ *   between different directories, and will only grant this right when it is
+ *   explicitly permitted for a directory by adding a rule.
+ *
+ *   When using the first Landlock ABI version, Landlock will always deny the
+ *   reparenting of files between different directories.
+ *
+ *   In addition to the source and destination directories having the
+ *   %LANDLOCK_ACCESS_FS_REFER access right, the attempted link or rename
+ *   operation must meet the following constraints:
+ *
+ *   * The reparented file may not gain more access rights in the destination
+ *     directory than it previously had in the source directory.  If this is
+ *     attempted, the operation results in an ``EXDEV`` error.
+ *
+ *   * When linking or renaming, the ``LANDLOCK_ACCESS_FS_MAKE_*`` right for the
+ *     respective file type must be granted for the destination directory.
+ *     Otherwise, the operation results in an ``EACCES`` error.
+ *
+ *   * When renaming, the ``LANDLOCK_ACCESS_FS_REMOVE_*`` right for the
+ *     respective file type must be granted for the source directory.  Otherwise,
+ *     the operation results in an ``EACCES`` error.
+ *
+ *   If multiple requirements are not met, the ``EACCES`` error code takes
+ *   precedence over ``EXDEV``.
  *
  * .. warning::
  *

base-commit: ceaa837f96adb69c0df0397937cd74991d5d821a
-- 
2.39.1