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

[Günther Noack]

On Mon, Sep 12, 2022 at 07:47:11PM +0200, Mickaël Salaün wrote:
> On 12/09/2022 17:46, Günther Noack wrote:
> > On Fri, Sep 09, 2022 at 03:51:35PM +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;
> > > > +    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.
> > > > @@ -127,8 +138,8 @@ descriptor.
> > > >    It may also be required to create rules following the same logic as explained
> > > >    for the ruleset creation, by filtering access rights according to the Landlock
> > > > -ABI version.  In this example, this is not required because
> > > > -`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule.
> > > > +ABI version.  In this example, this is not required because all of the requested
> > > > +``allowed_access`` rights are already available in ABI 1.
> > > 
> > > This fix is correct, but it should not be part of this series. FYI, I have a
> > > patch almost ready to fix some documentation style issues. Please remove
> > > this hunk for the next series. I'll deal with the merge conflicts if any.
> > 
> > Can you please clarify what part of it should not be part of this
> > series?
> 
> My mistake, I guess I was reviewing something else… I was thinking about
> style changes, but it is not the case here. Using "``" is correct.
> 
> 
> > 
> > In this hunk, I've started using double backquote, but I've also
> > changed the meaning of the sentence slightly so that it is still
> > correct when the truncate right is introduced.
> > 
> > It is still correct that the backwards compatibility check is not
> > required because LANDLOCK_ACCESS_FS_REFER is not allowed by any rule.
> > But with the new truncate flag, LANDLOCK_ACCESS_FS_TRUNCATE may also
> > not be allowed by any rule so that we can skip this check.
> > 
> > Should I remove this hunk entirely?
> 
> Keep your changes, it's better like this.

Thanks, reverted that part then.

—Günther

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