[PATCH v2 4/4] landlock: Document Landlock's file truncation support

["Günther Noack" <gnoack3000@gmail.com>]

Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.

Adapt the backwards compatibility example and discussion to remove the
truncation flag if needed.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/all/20220707200612.132705-1-gnoack3000@gmail.com/
---
 Documentation/userspace-api/landlock.rst | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index b86fd94ae797..41fa464cc8b8 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -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
@@ -69,14 +70,22 @@ 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` and `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 == -1) {
+        perror("Landlock is unsupported on this kernel");
+        return 1;
+    }
+    if (abi < 3) {
+        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
+    }
     if (abi < 2) {
         ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
     }
@@ -127,8 +136,8 @@ descriptor.
 
 It may also be required to create rules following the same logic as explained
 for the ruleset creation, by filtering access rights according to the Landlock
-ABI version.  In this example, this is not required because
-`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule.
+ABI version.  In this example, this is not required because all of the requested
+``allowed_access`` rights are already available in ABI 1.
 
 We now have a ruleset with one rule allowing read access to ``/usr`` while
 denying all other handled accesses for the filesystem.  The next step is to
-- 
2.37.0