[bIackr0se]

Description

In a Report Slave ID response, byteCount counts the slave ID and the run indicator status byte (the Modbus application protocol specification, function 0x11; pymodbus and libmodbus encode it this way). ModbusPDU11ReportSlaveIdResponse uses byteCount as the slave ID length. As a result, building the response without an explicit byteCount raises TypeError, and a response from a device does not dissect.

from scapy.contrib.modbus import *
ModbusADUResponse(bytes.fromhex("00010000000fff110c41636d652d504c432d3031ff"))  # reply of a pymodbus 3.16.1 server
bytes(ModbusPDU11ReportSlaveIdResponse(slaveId=b"PLC-01", runIdicatorStatus=0xff))

Before:

<ModbusADUResponse  transId=0x1 protoId=0x0 len=15 unitId=0xff |<Raw  load=b'\x11\x0cAcme-PLC-01\xff' |>>
TypeError: While building field 'slaveId': '>' not supported between instances of 'NoneType' and 'int'

After:

<ModbusADUResponse  transId=0x1 protoId=0x0 len=15 unitId=0xff |<ModbusPDU11ReportSlaveIdResponse  funcCode=0x11 byteCount=12 slaveId=b'Acme-PLC-01' runIdicatorStatus=0xff |>>
b'\x11\x07PLC-01\xff'

The existing test raw(ModbusPDU11ReportSlaveIdResponse(byteCount=3, slaveId="ID")) == b'\x11\x03\x49\x44\x00' already uses this meaning of byteCount and still passes.

This message was written with the help of AI (Claude Opus 5.5).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.18%. Comparing base (bf1b540) to head (8e7743a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5249      +/-   ##
==========================================
+ Coverage   81.17%   81.18%   +0.01%     
==========================================
  Files         394      394              
  Lines       98770    98770              
==========================================
+ Hits        80178    80188      +10     
+ Misses      18592    18582      -10     

Files with missing lines	Coverage Δ
scapy/contrib/modbus.py	92.17% <ø> (ø)

... and 7 files with indirect coverage changes

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