[PATCH v2] 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 | 41 ++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 15 deletions(-)

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index f3223f96469..f6bccd87aa0 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -130,21 +130,32 @@ 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 always considered
+ *   handled by any ruleset in such a way that reparenting a file hierarchy is
+ *   always denied by default.  If left unspecified during the creation of a
+ *   ruleset, linking and renaming files between different directories will be
+ *   forbidden, which is also the case when using the first Landlock ABI.
+ *
+ *   In addition to permitting this access right, the following constraints
+ *   must hold for the access rights on the source and destination directory:
+ *
+ *   * The destination directory may not grant any access rights which are
+ *     forbidden for the source file. Otherwise, the operation results in an
+ *     ``EXDEV`` error.
+ *
+ *   * When linking or renaming, the ``LANDLOCK_ACCESS_FS_MAKE_*`` right for
+ *     the respective file type is required in the destination directory.
+ *     Otherwise, the operation results in an ``EACCES`` error.
+ *
+ *   * When renaming, the ``LANDLOCK_ACCESS_FS_REMOVE_*`` right for the
+ *     respective file type is required in 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