ANDROID: mnt: Fix next_descendent

next_descendent did not properly handle the case
where the initial mount had no slaves. In this case,
we would look for the next slave, but since don't
have a master, the check for wrapping around to the
start of the list will always fail. Instead, we check
for this case, and ensure that we end the iteration
when we come back to the root.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 62094374
Change-Id: I43dfcee041aa3730cb4b9a1161418974ef84812e

fs/pnode.c

@@ -504,9 +504,14 @@ static struct mount *next_descendent(struct mount *root, struct mount *cur)
 	if (!IS_MNT_NEW(cur) && !list_empty(&cur->mnt_slave_list))
 		return first_slave(cur);
 	do {
-		if (cur->mnt_slave.next != &cur->mnt_master->mnt_slave_list)
-			return next_slave(cur);
-		cur = cur->mnt_master;
+		struct mount *master = cur->mnt_master;
+
+		if (!master || cur->mnt_slave.next != &master->mnt_slave_list) {
+			struct mount *next = next_slave(cur);
+
+			return (next == root) ? NULL : next;
+		}
+		cur = master;
 	} while (cur != root);
 	return NULL;
 }