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

On Mon, Sep 12, 2022 at 09:15:06PM +0200, Mickaël Salaün wrote:

On 08/09/2022 21:58, Günther Noack wrote:

quoted

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 <redacted>
---
  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).

Done.

quoted

+    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.

--