What is the legal test for a motion to vacate, set aside or correct a federal conviction or sentence?

A federal prisoner who wishes to challenge the validity or constitutionality of his federal conviction or sentence must do so by moving the court that imposed the sentence to vacate, set aside, or correct the sentence under 28 U.S.C. 2255. Alaimalo v. United States, 645 F.3d 1042, 1046 (9th Cir. 2011). "The general rule is that a motion under 28 U.S.C. 2255 is the exclusive means by which a federal prisoner may test the legality of his detention, and that restrictions on the availability of a 2255 motion cannot be avoided through a petition under 28 U.S.C. 2241." Stephens v. Herrera, 464 F.3d 895, 897 (9th Cir. 2006) (citations omitted).