the grammatical subject of the upstairs verb (both of them c-command the missing subject). The object of a preposition in the higher dause, however, is never allowed to be a controller basically because it is embedded "too deeply" in the sentence's tree structure -command the missing subject. That's why Mary was told by John to leave has Mary as the controller. Ot is also why, incidentally, the sentence Mary was promised by John to leave is unintelligible -it would require a prepositional phrase to be the controller, which is ruled out by the Structural Principle.)