landlock: Explain file descriptor access rights

Starting with LANDLOCK_ACCESS_FS_TRUNCATE, it is worth explaining why we
choose to restrict access checks at open time. This new "File
descriptor access rights" section is complementary to the existing
"Inode access rights" section. Add a new guiding principle related to
this section.

Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221209193813.972012-1-mic@digikod.net
[mic: Include the latest Günther's suggestion, and fix spelling]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

landlock: Fix documentation style

It seems that all code should use double backquotes, which is also used
to convert "%" defines. Let's use an homogeneous style and remove all
use of simple backquotes (which should only be used for emphasis).

Cc: Günther Noack <gnoack3000@gmail.com>
Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220923154207.3311629-4-mic@digikod.net

landlock: Slightly improve documentation and fix spelling

Now that we have more than one ABI version, make limitation explanation
more consistent by replacing "ABI 1" with "ABI < 2". This also
indicates which ABIs support such past limitation.

Improve documentation consistency by not using contractions.

Fix spelling in fs.c .

Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20220923154207.3311629-3-mic@digikod.net

landlock: Add design choices documentation for filesystem access rights

Summarize the rationale of filesystem access rights according to the
file type.

Update the document date.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220506161102.525323-13-mic@digikod.net

docs: security: landlock.rst: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9174021ef2c87f395a4cc0895a4b2f7fd97db626.1623824363.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

landlock: Add user and kernel documentation

Add a first document describing userspace API: how to define and enforce
a Landlock security policy. This is explained with a simple example.
The Landlock system calls are described with their expected behavior and
current limitations.

Another document is dedicated to kernel developers, describing guiding
principles and some important kernel structures.

This documentation can be built with the Sphinx framework.

Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Vincent Dagonneau <vincent.dagonneau@ssi.gouv.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210422154123.13086-13-mic@digikod.net
Signed-off-by: James Morris <jamorris@linux.microsoft.com>