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

[Mickaël Salaün]

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>
> 
>