[PATCH v6 5/5] landlock: Document Landlock's file truncation support

[Günther Noack]

On Mon, Sep 12, 2022 at 09:15:06PM +0200, Mickaël Salaün wrote:
> 
> 
> On 08/09/2022 21:58, Günther Noack wrote:
> > Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.
> > 
> > Adapt the backwards compatibility example and discussion to remove the
> > truncation flag where needed.
> > 
> > Point out potential surprising behaviour related to truncate.
> > 
> > Signed-off-by: Günther Noack <gnoack3000 at gmail.com>
> > ---
> >   Documentation/userspace-api/landlock.rst | 62 +++++++++++++++++++++---
> >   1 file changed, 54 insertions(+), 8 deletions(-)
> > 
> > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> > index b8ea59493964..57802fd1e09b 100644
> > --- a/Documentation/userspace-api/landlock.rst
> > +++ b/Documentation/userspace-api/landlock.rst
> > @@ -8,7 +8,7 @@ Landlock: unprivileged access control
> >   =====================================
> >   :Author: Mickaël Salaün
> > -:Date: May 2022
> > +:Date: September 2022
> >   The goal of Landlock is to enable to restrict ambient rights (e.g. global
> >   filesystem access) for a set of processes.  Because Landlock is a stackable
> > @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights.
> >               LANDLOCK_ACCESS_FS_MAKE_FIFO |
> >               LANDLOCK_ACCESS_FS_MAKE_BLOCK |
> >               LANDLOCK_ACCESS_FS_MAKE_SYM |
> > -            LANDLOCK_ACCESS_FS_REFER,
> > +            LANDLOCK_ACCESS_FS_REFER |
> > +            LANDLOCK_ACCESS_FS_TRUNCATE,
> >       };
> >   Because we may not know on which kernel version an application will be
> > @@ -69,16 +70,26 @@ should try to protect users as much as possible whatever the kernel they are
> >   using.  To avoid binary enforcement (i.e. either all security features or
> >   none), we can leverage a dedicated Landlock command to get the current version
> >   of the Landlock ABI and adapt the handled accesses.  Let's check if we should
> > -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported
> > -starting with the second version of the ABI.
> > +remove the `LANDLOCK_ACCESS_FS_REFER` or `LANDLOCK_ACCESS_FS_TRUNCATE` access
> > +rights, which are only supported starting with the second and third version of
> > +the ABI.
> >   .. code-block:: c
> >       int abi;
> >       abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
> > -    if (abi < 2) {
> > -        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
> > +    switch (abi) {
> > +    case -1:
> > +            perror("The running kernel does not enable to use Landlock");
> > +            return 1;
> 
> I think it would be easier to understand to explicitly check for abi < 0
> in a dedicated block as in the sample, instead of case -1, and return 0
> (instead of 1) with a comment to inform that Landlock is not handled but
> it is OK (expected error).

Done.

> 
> 
> > +    case 1:
> > +            /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
> > +            ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
> > +            __attribute__((fallthrough));
> > +    case 2:
> > +            /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
> > +            ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
> >       }
> >   This enables to create an inclusive ruleset that will contain our rules.

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