landlock: Document good practices about filesystem policies

Explain how to set access rights per hierarchy in an efficient and safe
way, especially with the LANDLOCK_ACCESS_FS_REFER side effect (i.e.
partial ordering and constraints for access rights per hierarchy).

Reviewed-by: Paul Moore <[e-mail]>
Signed-off-by: Mickaël Salaün <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/userspace-api/landlock.rst

···
 
 Full working code can be found in `samples/landlock/sandboxer.c`_.
 
+Good practices
+--------------
+
+It is recommended setting access rights to file hierarchy leaves as much as
+possible.  For instance, it is better to be able to have ``~/doc/`` as a
+read-only hierarchy and ``~/tmp/`` as a read-write hierarchy, compared to
+``~/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy.
+Following this good practice leads to self-sufficient hierarchies that don't
+depend on their location (i.e. parent directories).  This is particularly
+relevant when we want to allow linking or renaming.  Indeed, having consistent
+access rights per directory enables to change the location of such directory
+without relying on the destination directory access rights (except those that
+are required for this operation, see `LANDLOCK_ACCESS_FS_REFER` documentation).
+Having self-sufficient hierarchies also helps to tighten the required access
+rights to the minimal set of data.  This also helps avoid sinkhole directories,
+i.e.  directories where data can be linked to but not linked from.  However,
+this depends on data organization, which might not be controlled by developers.
+In this case, granting read-write access to ``~/tmp/``, instead of write-only
+access, would potentially allow to move ``~/tmp/`` to a non-readable directory
+and still keep the ability to list the content of ``~/tmp/``.
+
 Layers of file path access rights
 ---------------------------------