Ex Post Facto is after the fact; this waiver should be approved by the Senate before the SASC confirms GEN Mattis.
You are of course quite correct. I had a brain cramp, for which I humbly apologize. I meant to write Bill of Attainder, which I thought was a law only affecting one person, but I see now that it refers to a legislative act declaring a particular person guilty of a crime (usually treason).