Report bad file record frames as undecodable

pymodbus/pdu/file_message.py

@@ -6,7 +6,7 @@
 from dataclasses import dataclass
 
 from ..datastore import ModbusServerContext
-from ..exceptions import ModbusException
+from ..exceptions import ModbusException, ModbusIOException
 from .decoders import DecodePDU
 from .pdu import ModbusPDU
 
@@ -68,7 +68,9 @@ def x_decode(self, data: bytes) -> None:
         """Decode the incoming request."""
         self.records = []
         if (byte_count := int(data[0])) > len(data) - 1:
-            raise ModbusException(f"Invalid byte count: {byte_count}")
+            raise ModbusIOException(
+                f"Invalid byte count: {byte_count}", function_code=self.function_code
+            )
         for count in range(1, byte_count, 7):
             decoded = struct.unpack(">BHHH", data[count : count + 7])
             record = FileRecord(
@@ -176,7 +178,9 @@ def x_encode(self) -> bytes:
     def x_decode(self, data: bytes) -> None:
         """Decode the incoming request."""
         if (byte_count := int(data[0])) > len(data) - 1:
-            raise ModbusException(f"Invalid byte count: {byte_count}")
+            raise ModbusIOException(
+                f"Invalid byte count: {byte_count}", function_code=self.function_code
+            )
         count = 1
         self.records.clear()
         while count < byte_count:
@@ -323,7 +327,9 @@ def x_decode(self, data: bytes) -> None:
         self.values = []
         _, count = struct.unpack(">HH", data[0:4])
         if 4 + count * 2 > len(data):
-            raise ModbusException(f"Invalid fifo count: {count}")
+            raise ModbusIOException(
+                f"Invalid fifo count: {count}", function_code=self.function_code
+            )
         for index in range(0, count):
             idx = 4 + index * 2
             self.values.append(struct.unpack(">H", data[idx : idx + 2])[0])

test/framer/test_framer.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from pymodbus.exceptions import ModbusIOException
 from pymodbus.framer import (
     FramerAscii,
     FramerBase,
@@ -956,6 +957,17 @@ def test_framer_encode(self, test_framer, msg, mockPDU):
             actual = test_framer.buildFrame(message)
             assert msg == actual
 
+    def test_bad_file_record_request_for_framer_socket(self):
+        """Test a bad file record request keeps the ids the server reply needs."""
+        framer = FramerSocket(DecodePDU(True))
+        # transaction 1, protocol 0, length 3, device 1, read file record, byte count 16
+        data = b"\x00\x01\x00\x00\x00\x03\x01\x14\x10"
+        with pytest.raises(ModbusIOException) as exc:
+            framer.handleFrame(data, 0, 0)
+        assert exc.value.transaction_id == 1
+        assert exc.value.dev_id == 1
+        assert exc.value.used_len == len(data)
+
     def test_invalid_protocol_id_for_framer_socket(self):
         """Test that ModbusSocketFramer rejects an invalid Protocol ID."""
         framer = FramerSocket(DecodePDU(False))

test/pdu/test_decoders.py

@@ -139,6 +139,19 @@ def test_decode_bad_frame(self, decoder, frame):
         with pytest.raises(ModbusIOException):
             decoder.decode(frame)
 
+    @pytest.mark.parametrize(
+        ("decoder", "frame"),
+        [
+            (server, b"\x14\x10"),  # read file record, byte count past the frame
+            (server, b"\x15\x10"),  # write file record, byte count past the frame
+            (client, b"\x18\x00\x06\x00\x05\x00\x01"),  # fifo count past the frame
+        ],
+    )
+    def test_decode_bad_file_record_frame(self, decoder, frame):
+        """Test bad file record frames fail like other undecodable frames."""
+        with pytest.raises(ModbusIOException):
+            decoder.decode(frame)
+
     def test_decode_unknown_sub(self):
         """Test for unknown sub code."""
         with pytest.raises(ModbusIOException):