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

Mickaël Salaün
2026-03-23T19:01:44Z

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@n0toose.net>
> Co-developed-by: Dan Cojocaru <dan@dcdev.ro>
> Signed-off-by: Dan Cojocaru <dan@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@n0toose.net>
>
>