Definition val_inject_opt (j: meminj) (v1 v2: option val) :=
  match v1, v2 with Some v1', Some v2' => val_inject j v1' v2'
  | None, None => True
  | _, _ => False
  end.

Lemma extends_perm: forall m1 m2 (Ext: Mem.extends m1 m2) b ofs k p, Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p.