landlock: Minor comments improvements

This patch contains some small comment changes.  The first three
comments for ruleset.c, I sort of made along the way while working on /
trying to understand Landlock, and the one from ruleset.h was from the
hashtable patch but extracted here.  In fs.c, one comment which I found
would have been helpful to me when reading this.

Signed-off-by: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/20250602134150.67189-1-m@maowtm.org
Link: https://lore.kernel.org/r/20297185fd71ffbb5ce4fec14b38e5444c719c96.1748379182.git.m@maowtm.org
[mic: Squash patches with updated description, cosmetic fixes]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

security/landlock/fs.c

@@ -714,7 +714,8 @@ static void test_is_eacces_with_write(struct kunit *const test)
  * is_access_to_paths_allowed - Check accesses for requests with a common path
  *
  * @domain: Domain to check against.
- * @path: File hierarchy to walk through.
+ * @path: File hierarchy to walk through.  For refer checks, this would be
+ *     the common mountpoint.
  * @access_request_parent1: Accesses to check, once @layer_masks_parent1 is
  *     equal to @layer_masks_parent2 (if any).  This is tied to the unique
  *     requested path for most actions, or the source in case of a refer action

security/landlock/ruleset.c

@@ -83,6 +83,10 @@ static void build_check_rule(void)
 		.num_layers = ~0,
 	};
 
+	/*
+	 * Checks that .num_layers is large enough for at least
+	 * LANDLOCK_MAX_NUM_LAYERS layers.
+	 */
 	BUILD_BUG_ON(rule.num_layers < LANDLOCK_MAX_NUM_LAYERS);
 }
 
@@ -290,6 +294,10 @@ static void build_check_layer(void)
 		.access = ~0,
 	};
 
+	/*
+	 * Checks that .level and .access are large enough to contain their expected
+	 * maximum values.
+	 */
 	BUILD_BUG_ON(layer.level < LANDLOCK_MAX_NUM_LAYERS);
 	BUILD_BUG_ON(layer.access < LANDLOCK_MASK_ACCESS_FS);
 }
@@ -644,8 +652,8 @@ bool landlock_unmask_layers(const struct landlock_rule *const rule,
 		bool is_empty;
 
 		/*
-		 * Records in @layer_masks which layer grants access to each
-		 * requested access.
+		 * Records in @layer_masks which layer grants access to each requested
+		 * access: bit cleared if the related layer grants access.
 		 */
 		is_empty = true;
 		for_each_set_bit(access_bit, &access_req, masks_array_size) {

security/landlock/ruleset.h

@@ -27,7 +27,7 @@ struct landlock_hierarchy;
  */
 struct landlock_layer {
 	/**
-	 * @level: Position of this layer in the layer stack.
+	 * @level: Position of this layer in the layer stack.  Starts from 1.
 	 */
 	u16 level;
 	/**