[PATCH v1 9/9] landlock: Extend documentation about limitations

[Mickaël Salaün]

Explain limitations for the maximum number of stacked ruleset, and the
memory usage restrictions.

Cc: James Morris <jmorris at namei.org>
Cc: Jann Horn <jannh at google.com>
Cc: Serge E. Hallyn <serge at hallyn.com>
Signed-off-by: Mickaël Salaün <mic at digikod.net>
---
 Documentation/userspace-api/landlock.rst | 17 +++++++++++++++++
 security/landlock/syscall.c              |  2 ++
 2 files changed, 19 insertions(+)

diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index 8f727de20479..7e83e5def1bc 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -186,6 +186,23 @@ Enforcing a ruleset
 Current limitations
 ===================
 
+Ruleset layers
+--------------
+
+There is a limit of 64 layers of stacked rulesets.  This can be an issue for a
+task willing to enforce a new ruleset in complement to its 64 inherited
+rulesets.  Once this limit is reached, sys_landlock_enforce_ruleset_current()
+returns E2BIG.  It is then strongly suggested to carefully build rulesets once
+in the life of a thread, especially for applications able to launch other
+applications which may also want to sandbox themselves (e.g. shells, container
+managers, etc.).
+
+Memory usage
+------------
+
+Kernel memory allocated to create rulesets is accounted and can be restricted
+by the :doc:`/admin-guide/cgroup-v1/memory`.
+
 File renaming and linking
 -------------------------
 
diff --git a/security/landlock/syscall.c b/security/landlock/syscall.c
index 543ae36cd339..045bcac79e17 100644
--- a/security/landlock/syscall.c
+++ b/security/landlock/syscall.c
@@ -361,6 +361,8 @@ SYSCALL_DEFINE4(landlock_add_rule,
  * - EPERM: @ruleset_fd has no read access to the underlying ruleset, or the
  *   current thread is not running with no_new_privs, or it doesn't have
  *   CAP_SYS_ADMIN in its namespace.
+ * - E2BIG: The maximum number of stacked rulesets is reached for the current
+ *   task.
  */
 SYSCALL_DEFINE2(landlock_enforce_ruleset_current,
 		const int, ruleset_fd, const __u32, flags)
-- 
2.29.2