landlock: Create find_rule() from unmask_layers()

This refactoring will be useful in a following commit.
Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-4-mic@digikod.net
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>

security/landlock/fs.c

@@ -183,23 +183,36 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
 
 /* Access-control management */
 
-static inline layer_mask_t
-unmask_layers(const struct landlock_ruleset *const domain,
-	      const struct path *const path, const access_mask_t access_request,
-	      layer_mask_t layer_mask)
+/*
+ * The lifetime of the returned rule is tied to @domain.
+ *
+ * Returns NULL if no rule is found or if @dentry is negative.
+ */
+static inline const struct landlock_rule *
+find_rule(const struct landlock_ruleset *const domain,
+	  const struct dentry *const dentry)
 {
 	const struct landlock_rule *rule;
 	const struct inode *inode;
-	size_t i;
 
-	if (d_is_negative(path->dentry))
-		/* Ignore nonexistent leafs. */
-		return layer_mask;
-	inode = d_backing_inode(path->dentry);
+	/* Ignores nonexistent leafs. */
+	if (d_is_negative(dentry))
+		return NULL;
+
+	inode = d_backing_inode(dentry);
 	rcu_read_lock();
 	rule = landlock_find_rule(
 		domain, rcu_dereference(landlock_inode(inode)->object));
 	rcu_read_unlock();
+	return rule;
+}
+
+static inline layer_mask_t unmask_layers(const struct landlock_rule *const rule,
+					 const access_mask_t access_request,
+					 layer_mask_t layer_mask)
+{
+	size_t layer_level;
+
 	if (!rule)
 		return layer_mask;
 
@@ -210,8 +223,9 @@ unmask_layers(const struct landlock_ruleset *const domain,
 	 * the remaining layers for each inode, from the first added layer to
 	 * the last one.
 	 */
-	for (i = 0; i < rule->num_layers; i++) {
-		const struct landlock_layer *const layer = &rule->layers[i];
+	for (layer_level = 0; layer_level < rule->num_layers; layer_level++) {
+		const struct landlock_layer *const layer =
+			&rule->layers[layer_level];
 		const layer_mask_t layer_bit = BIT_ULL(layer->level - 1);
 
 		/* Checks that the layer grants access to the full request. */
@@ -269,8 +283,9 @@ static int check_access_path(const struct landlock_ruleset *const domain,
 	while (true) {
 		struct dentry *parent_dentry;
 
-		layer_mask = unmask_layers(domain, &walker_path, access_request,
-					   layer_mask);
+		layer_mask =
+			unmask_layers(find_rule(domain, walker_path.dentry),
+				      access_request, layer_mask);
 		if (layer_mask == 0) {
 			/* Stops when a rule from each layer grants access. */
 			allowed = true;