[PATCH v3 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 where needed.

Point out potential surprising behaviour related to truncate.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 Documentation/userspace-api/landlock.rst | 41 ++++++++++++++++++++----
 1 file changed, 34 insertions(+), 7 deletions(-)

diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index d92e335380d4..9c3c9fa65958 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
@@ -69,16 +70,24 @@ 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 < 2) {
-        ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
+    switch (abi) {
+    case -1:
+            perror("Landlock is not supported with the running kernel");
+            return 1;
+    case 1:
+            ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
+            __attribute__((fallthrough));
+    case 2:
+            ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
     }
 
 This enables to create an inclusive ruleset that will contain our rules.
@@ -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
@@ -251,6 +260,24 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
 process, a sandboxed process should have a subset of the target process rules,
 which means the tracee must be in a sub-domain of the tracer.
 
+Truncating files
+----------------
+
+The operations covered by `LANDLOCK_ACCESS_FS_WRITE_FILE` and
+`LANDLOCK_ACCESS_FS_TRUNCATE` both change the contents of a file and
+sometimes overlap in non-intuitive ways. It is recommended to always
+specify both of these together.
+
+A particularly surprising example is :manpage:`creat(2)`. The name
+suggests that this system call requires the rights to create and write
+files. However, it also requires the truncate right if an existing
+file under the same name is already present.
+
+It should also be noted that truncating files does not necessarily
+require the `LANDLOCK_ACCESS_FS_WRITE_FILE` right. Apart from the
+obvious :manpage:`truncate(2)` system call, this can also be done
+through :manpage:`open(2)` with the flags `O_RDONLY` and `O_TRUNC`.
+
 Compatibility
 =============
 
-- 
2.37.1