makes the disassembler more strict (#1381)

In #1375 instead of terminating the program on a knowledge base
conflict during instruction lifting, we decided to treat such instruction
as invalid and retract it and the whole path that led to it from the
set of valid instructions. It turned out that the retraction mechanism
wasn't quite complete and there were certain cases when an invalid
instruction was still reachable, which triggered conflicts downstream,
e.g., during the CFG reconstruction.

The problem mostly arises in the interworked code, where we have to
guess whether an instruction is in A32 or T32 mode using heuristics
such as byte patterns, which inevitably leads to conflicts. So the
first place where we have to enfore agreement is in the encoding
detection. Before this change, the information provided by the
knowledge base had precedence over the natural rules of encodings,
i.e., that the fall or regular jump can't change the encoding,
unless it is the encoding changing jump.

In addition, whenever we discover a fall or a jump to an already
disassembled instruction we have to check if the encodings agree and
discard it if they don't.

Finally, there were some missing cases, when the invalid code wasn't
retracted. First of all, it was possible when a jump destination was
invalid but the jump remained in the code set. And the dual problem,
when a basic block entry point was canceled not all incomming
destinations were canceled - only the path through which the block was
discovered. The last two issues were fixed and they affect even those
targets that do not use interworking, e.g., x86. Which is good, as
more code is discarded as invalid and gives us better CFG.

Author: ivg

lib/bap/bap_project.ml

@@ -242,6 +242,10 @@ module State = struct
     {self with disassembly}
 
   let partition self =
+    let self = {
+      self with
+      disassembly = Dis.forget_debt self.disassembly
+    } in
     Sub.update self.subroutines self.disassembly >>| fun subroutines ->
     {self with subroutines}