tracing/filter: Check the created pred tree

Since the filter walks a tree to determine if a match is made or not,
if the tree was incorrectly created, it could cause an infinite loop.

Add a check to walk the entire tree before assigning it as a filter
to make sure the tree is correct.

Cc: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events_filter.c

@@ -1358,6 +1358,68 @@ static int count_preds(struct filter_parse_state *ps)
 	return n_preds;
 }
 
+/*
+ * The tree is walked at filtering of an event. If the tree is not correctly
+ * built, it may cause an infinite loop. Check here that the tree does
+ * indeed terminate.
+ */
+static int check_pred_tree(struct event_filter *filter,
+			   struct filter_pred *root)
+{
+	struct filter_pred *preds;
+	struct filter_pred *pred;
+	enum move_type move = MOVE_DOWN;
+	int count = 0;
+	int done = 0;
+	int max;
+
+	/*
+	 * The max that we can hit a node is three times.
+	 * Once going down, once coming up from left, and
+	 * once coming up from right. This is more than enough
+	 * since leafs are only hit a single time.
+	 */
+	max = 3 * filter->n_preds;
+
+	preds = filter->preds;
+	if  (!preds)
+		return -EINVAL;
+	pred = root;
+
+	do {
+		if (WARN_ON(count++ > max))
+			return -EINVAL;
+
+		switch (move) {
+		case MOVE_DOWN:
+			if (pred->left != FILTER_PRED_INVALID) {
+				pred = &preds[pred->left];
+				continue;
+			}
+			/* A leaf at the root is just a leaf in the tree */
+			if (pred == root)
+				break;
+			pred = get_pred_parent(pred, preds,
+					       pred->parent, &move);
+			continue;
+		case MOVE_UP_FROM_LEFT:
+			pred = &preds[pred->right];
+			move = MOVE_DOWN;
+			continue;
+		case MOVE_UP_FROM_RIGHT:
+			if (pred == root)
+				break;
+			pred = get_pred_parent(pred, preds,
+					       pred->parent, &move);
+			continue;
+		}
+		done = 1;
+	} while (!done);
+
+	/* We are fine. */
+	return 0;
+}
+
 static int replace_preds(struct ftrace_event_call *call,
 			 struct event_filter *filter,
 			 struct filter_parse_state *ps,
@@ -1366,6 +1428,7 @@ static int replace_preds(struct ftrace_event_call *call,
 {
 	char *operand1 = NULL, *operand2 = NULL;
 	struct filter_pred *pred;
+	struct filter_pred *root;
 	struct postfix_elt *elt;
 	struct pred_stack stack = { }; /* init to NULL */
 	int err;
@@ -1442,7 +1505,7 @@ add_pred:
 		if (!pred)
 			return -EINVAL;
 		/* This item is where we start from in matching */
-		filter->root = pred;
+		root = pred;
 		/* Make sure the stack is empty */
 		pred = __pop_pred_stack(&stack);
 		if (WARN_ON(pred)) {
@@ -1450,6 +1513,13 @@ add_pred:
 			filter->root = NULL;
 			goto fail;
 		}
+		err = check_pred_tree(filter, root);
+		if (err)
+			goto fail;
+
+		/* We don't set root until we know it works */
+		barrier();
+		filter->root = root;
 	}
 
 	err = 0;