[PATCH v2 4/4] landlock: Document Landlock's file truncation support

[Günther Noack]

On Fri, Jul 29, 2022 at 12:47:46PM +0200, Mickaël Salaün wrote:
>
> On 12/07/2022 23:14, 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 if needed.
> >
> > Signed-off-by: Günther Noack <gnoack3000 at gmail.com>
> > Link: https://lore.kernel.org/all/20220707200612.132705-1-gnoack3000@gmail.com/
> > ---
> >   Documentation/userspace-api/landlock.rst | 19 ++++++++++++++-----
> >   1 file changed, 14 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> > index b86fd94ae797..41fa464cc8b8 100644
> > --- a/Documentation/userspace-api/landlock.rst
> > +++ b/Documentation/userspace-api/landlock.rst
> > @@ -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
> > @@ -69,14 +70,22 @@ 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` and `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 == -1) {
> > +        perror("Landlock is unsupported on this kernel");
>
> "Landlock is not supported with the running kernel"?

Done.

>
>
> > +        return 1;
> > +    }
> > +    if (abi < 3) {
> > +        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
> > +    }
>
> I guess we could use the same switch/case code as for the sample. I'm not
> sure what would be the less confusing for users though.

Done. (Both are mildly confusing, IMHO %-))

> [...]

--