[PATCH v3] landlock: Expand restrict flags example for ABI version 8

[Mickaël Salaün]

On Sat, Feb 28, 2026 at 10:36:59PM +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 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 | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> index 13134bccdd39d78ddce3daf454f32dda162ce91b..b71ac7aa308260b8141e5d35248fb68cec6dcba9 100644
> --- a/Documentation/userspace-api/landlock.rst
> +++ b/Documentation/userspace-api/landlock.rst
> @@ -196,13 +196,33 @@ 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!
> +         * Don't copy-paste this just yet! This example impacts enforcement
> +         * and can potentially decrease protection if misused.

What could be the use case when that would be an issue? What would be
the consequence wrt just tampering with a sibling thread?

> +         *
> +         * Below ABI v8, a Landlock policy can only be enforced for the calling
> +         * thread and its children. This behavior remains a default for ABI v8,
> +         * but the flag ``LANDLOCK_RESTRICT_SELF_TSYNC`` can now be used to
> +         * enforce policies across all threads of the calling process. If an
> +         * application's Landlock integration was designed under the assumption
> +         * that the flag is used (such as when children threads are responsible
> +         * for enforcing and/or overriding policies of parents and siblings),
> +         * removing said flag can decrease protection for older Linux versions.

In this case, the application *must* check the ABI version and exit with
an error if it is not supported.  That's the same use case as programs
wishing to sandbox themselves at least with a specific set of
restrictions.

> +         */
> +        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>
> 
>