landlock: Annotate struct landlock_rule with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct landlock_rule.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230817210257.never.920-kees@kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: kees

security/landlock/ruleset.h

@@ -67,7 +67,7 @@ struct landlock_rule {
 	 * @layers: Stack of layers, from the latest to the newest, implemented
 	 * as a flexible array member (FAM).
 	 */
-	struct landlock_layer layers[];
+	struct landlock_layer layers[] __counted_by(num_layers);
 };
 
 /**