o Remove comment indicating policy permits loop-back debugging, but

semantics don't: in practice, both policy and semantics permit
  loop-back debugging operations, only it's just a subset of debugging
  operations (i.e., a proc can open its own /dev/mem), and that's at a
  higher layer.

sys/kern/kern_prot.c

@@ -1158,7 +1158,6 @@ p_candebug(struct proc *p1, struct proc *p2, int *privused)
 	if (privused != NULL)
 		*privused = 0;
 
-	/* XXX it is authorized, but semantics don't permit it */
 	if (p1 == p2)
 		return (0);