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

[Mickaël Salaün]

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;

I think it would be easier to understand to explicitly check for abi < 0 
in a dedicated block as in the sample, instead of case -1, and return 0 
(instead of 1) with a comment to inform that Landlock is not handled but 
it is OK (expected error).


> +    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.