[PATCH v4] landlock: Expand restrict flags example for ABI version 8

[Günther Noack]

Hello!

On Mon, Mar 23, 2026 at 07:56:21PM +0100, Mickaël Salaün wrote:
> Thanks!  I pushed your patch in next with a minor fix.
> 
> Günther, does it look good to you?
> 
> On Wed, Mar 04, 2026 at 07:13:04PM +0100, Panagiotis "Ivory" Vasilopoulos wrote:
> > Add LANDLOCK_RESTRICT_SELF_TSYNC to the backwards compatibility example
> > for restrict flags. This introduces completeness, similar to that of
> > the ruleset attributes example. However, as the new example can impact
> > enforcement in certain cases, an appropriate warning is also included.
> > 
> > Additionally, I modified the two comments of the example to make them
> > more consistent with the ruleset attributes example's.
> > 
> > Signed-off-by: Panagiotis 'Ivory' Vasilopoulos <git at n0toose.net>
> > Co-developed-by: Dan Cojocaru <dan at dcdev.ro>
> > Signed-off-by: Dan Cojocaru <dan at dcdev.ro>
> > ---
> > Changes in v4:
> > - Make warning somewhat more terse, merge comments.
> >   - Remove some sensationalization. ("Don't copy-paste this just yet!")
> >   - Apply Günther's suggestion (v3 "recycled" some phrases, was long)
> >     - ... but also retain some of the wording on ABI differences
> > - Provide a brief overview that contextualizes the example further:
> >   - Clarify the difference behind ABI < 8 & ABI v8, to avoid
> >     misunderstandings on which option is the default.
> >   - Make "linear reading" easier.
> > - Based on Mickaël's feedback: Avoid cans of worms w.r.t. use cases
> > - Link to v3: https://lore.kernel.org/r/20260228-landlock-docs-add-tsync-example-v3-1-140ab50f0524@n0toose.net
> > 
> > Changes in v3:
> > - Add __attribute__((fallthrough)) like in earlier example.
> > - Improve comment for LANDLOCK_RESTRICT_SELF_TSYNC (ABI < 8) example.
> > - Add relevant warning for ABI < 8 example based on Günther's feedback.
> > - Link to v2: https://lore.kernel.org/r/20260221-landlock-docs-add-tsync-example-v2-1-60990986bba5@n0toose.net
> > 
> > Changes in v2:
> > - Fix formatting error.
> > - Link to v1: https://lore.kernel.org/r/20260221-landlock-docs-add-tsync-example-v1-1-f89383809eb4@n0toose.net
> > ---
> >  Documentation/userspace-api/landlock.rst | 22 ++++++++++++++++++----
> >  1 file changed, 18 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> > index 13134bccdd39d78ddce3daf454f32dda162ce91b..64c7138a788d74f99da0a71428da392b3d873bf8 100644
> > --- a/Documentation/userspace-api/landlock.rst
> > +++ b/Documentation/userspace-api/landlock.rst
> > @@ -196,13 +196,27 @@ similar backwards compatibility check is needed for the restrict flags
> >  (see sys_landlock_restrict_self() documentation for available flags):
> >  
> >  .. code-block:: c
> > -
> > -    __u32 restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
> > -    if (abi < 7) {
> > -        /* Clear logging flags unsupported before ABI 7. */
> > +    __u32 restrict_flags =
> > +        LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON |
> > +        LANDLOCK_RESTRICT_SELF_TSYNC;
> > +    switch (abi) {
> > +    case 1 ... 6:
> > +        /* Clear logging flags unsupported for ABI < 7 */
> >          restrict_flags &= ~(LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF |
> >                              LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON |
> >                              LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF);
> > +        __attribute__((fallthrough));
> > +    case 7:
> > +        /*
> > +         * Removes multithreaded enforcement flag unsupported for ABI < 8
> > +         *
> > +         * WARNING: Without this flag, calling landlock_restrict_self(2) is
> > +         * only equivalent if the calling process is single-threaded. Below
> > +         * ABI v8 (and as of ABI v8, when not using this flag), a Landlock
> > +         * policy would only be enforced for the calling thread and its
> > +         * children (and not for all threads, including parents and siblings).
> > +         */
> > +        restrict_flags &= ~LANDLOCK_RESTRICT_SELF_TSYNC;
> >      }
> >  
> >  The next step is to restrict the current thread from gaining more privileges
> > 
> > ---
> > base-commit: ceb977bfe9e8715e6cd3a4785c7aab8ea5cd2b77
> > change-id: 20260221-landlock-docs-add-tsync-example-e8fd5c64a366
> > 
> > Best regards,
> > -- 
> > Panagiotis "Ivory" Vasilopoulos <git at n0toose.net>
> > 
> > 

Apologies for the delay, this must have slipped through the cracks.
Thanks for bringing it up again.  Yes, this looks good.

Signed-off-by: Günther Noack <gnoack at google.com>

—Günther