Correctly model tx.origin vs msg.sender

mythril/laser/ethereum/transaction/symbolic.py

@@ -83,6 +83,11 @@ def execute_message_call(laser_evm, callee_address: BitVec) -> None:
             continue
 
         next_transaction_id = get_next_transaction_id()
+
+        external_sender = symbol_factory.BitVecSym(
+            "sender_{}".format(next_transaction_id), 256
+        )
+
         transaction = MessageCallTransaction(
             world_state=open_world_state,
             identifier=next_transaction_id,
@@ -90,12 +95,8 @@ def execute_message_call(laser_evm, callee_address: BitVec) -> None:
                 "gas_price{}".format(next_transaction_id), 256
             ),
             gas_limit=8000000,  # block gas limit
-            origin=symbol_factory.BitVecSym(
+            origin=external_sender,
-                "origin{}".format(next_transaction_id), 256
+            caller=external_sender,
-            ),
-            caller=symbol_factory.BitVecSym(
-                "sender_{}".format(next_transaction_id), 256
-            ),
             callee_account=open_world_state[callee_address],
             call_data=SymbolicCalldata(next_transaction_id),
             call_value=symbol_factory.BitVecSym(