socketcan J1939 filter use in python

Question

In Python, I am trying to use the J1939 filtering as mentionned in the linux kernel docs: https://www.kernel.org/doc/html/latest.networking/j1939.html

The following code fails at the setsockopt() line (setting up filters):

import socket
import struct

def pack_J1939_filters(can_filters):
    can_filter_fmt = "=" + "2Q2B2I" * len(can_filters)
    filter_data = []
    for can_filter in can_filters:
        name = can_filter['name']
        name_mask = can_filter['name_mask']
        addr = can_filter['addr']
        addr_mask = can_filter['addr_mask']
        pgn = can_filter['pgn']
        pgn_mask = can_filter['pgn_mask']

        filter_data.append(name)
        filter_data.append(name_mask)
        filter_data.append(addr)
        filter_data.append(addr_mask)
        filter_data.append(pgn)
        filter_data.append(pgn_mask)

    return struct.pack(can_filter_fmt, *filter_data)


s = socket.socket(socket.PF_CAN, socket.SOCK_DGRAM, socket.CAN_J1939) 

interface = "vcan0"

src_name = socket.J1939_NO_NAME
src_pgn = socket.J1939_NO_PGN 
src_addr = 0x81 
src_sck_addr = (interface, src_name, src_pgn, src_addr)
s.bind(src_sck_addr)

filters = [{"name": 0, "name_mask":0, "addr":0, "addr_mask":0, "pgn": 0, "pgn_mask": 0}]
packed_filters = pack_J1939_filters(filters)

# socket.SOL_CAN_J1939 does not seem to exist
SOL_CAN_BASE = 100
CAN_J1939 = 7
SOL_CAN_J1939 = SOL_CAN_BASE + CAN_J1939

s.setsockopt(SOL_CAN_J1939, socket.SO_J1939_FILTER , packed_filters) 

s.recvfrom(128)
s.close()

First, the kernel documentation mentions to use SOL_CAN_J1939 as the first argument. However socket.SOL_CAN_J1939 does not exist in the socket package. So looking at the code at this location I was able to understand that this int value should be 107: http://socket-can.996257.n3.nabble.com/RFC-v3-0-6-CAN-add-SAE-J1939-protocol-td7571.html

As for the setsockopt() third argument, I packed the filters to match the j1939_filter structure (26 bytes as described in the code from the previous link). This is similar to what is done in can.interfaces.socketcan.utils for raw CAN.

What am I doing wrong to cause setsockopt() to fail?

Answer 1

The first issue was with the struct.pack format (can_filter_fmt) being wrong. I first assumed that the kernel j1939_filter structure size was the sum of the members. This is wrong since the compiler adds padding. This can be added to the struct.pack format as x such as 2Q2I2B6x. Please see Why isn't sizeof for a struct equal to the sum of sizeof of each member?

The second issue was that can_filter_fmt is not packed as 2Q2B2I but as 2Q2I2B6x (the addr member is in the middle).

As for SOL_CAN_J1939 I was correct and needs to be created in file because it is not yet in the package.

The final code is the following:

#!/usr/bin/env python3

import socket
import struct

def pack_J1939_filters(can_filters=None):
    if can_filters is None:
        # Pass all messages
        can_filters = [{}]

    can_filter_fmt = "=" + "2Q2I2B6x" * len(can_filters)
    filter_data = []
    for can_filter in can_filters:
        if 'name' in can_filter:
            name = can_filter['name']
        else:
            name = 0

        if 'name_mask' in can_filter:
            name_mask = can_filter['name_mask']
        else:
            name_mask = 0

        if 'pgn' in can_filter:
            pgn = can_filter['pgn']
        else:
            pgn = 0

        if 'pgn_mask' in can_filter:
            pgn_mask = can_filter['pgn_mask']
        else:
            pgn_mask = 0
        
        if 'addr' in can_filter:
            addr = can_filter['addr']
        else:
            addr = 0

        if 'addr_mask' in can_filter:
            addr_mask = can_filter['addr_mask']
        else:
            addr_mask = 0

        filter_data.append(name)
        filter_data.append(name_mask)
        filter_data.append(pgn)
        filter_data.append(pgn_mask)
        filter_data.append(addr)
        filter_data.append(addr_mask)

    return struct.pack(can_filter_fmt, *filter_data)

def print_msg(data, sck_addr):
    print(f"SA:{hex(sck_addr[3])} PGN:{hex(sck_addr[2])}")

    for j in range(len(data)):
        if j % 8 == 0 and j != 0:
            print()

        if j % 8 == 0:
            print(f"bytes {j} to {j+7}: ", end="")

        print(f"{hex(data[j])} ", end="")

    print()
    print()
    

def main():
    s = socket.socket(socket.PF_CAN, socket.SOCK_DGRAM, socket.CAN_J1939) 

    # allows to receive broadcast messages
    s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
    
    interface = "vcan0"

    src_name = socket.J1939_NO_NAME
    src_pgn = socket.J1939_NO_PGN # always no PGN for source, unless filtering is needed
    src_addr = 0x81 # recvfrom() will not return destination specific messages for other addresses
    src_sck_addr = (interface, src_name, src_pgn, src_addr)
    s.bind(src_sck_addr)

    packed_filters = pack_J1939_filters()

    SOL_CAN_BASE = 100
    CAN_J1939 = 7
    SOL_CAN_J1939 = SOL_CAN_BASE + CAN_J1939

    s.setsockopt(SOL_CAN_J1939, socket.SO_J1939_FILTER , packed_filters) 

    (recv_data, recv_sck_addr) = s.recvfrom(128)
    print_msg(recv_data, recv_sck_addr)

    s.close()


if __name__ == "__main__":
    main()

Thank you.

Answer 2

For J1939 to work with SocketCAN you need two things:

1. kernel 5.4+
2. can-j1939 kernel module enabled

---

Testing for can-1939:

If you install can-utils and after sudo modprobe can-j1939 all you get is fatal error, or if you start testj1939 from can-utils and you get error that protocol is not supported, then it means that can-j1939 was not enabled in your kernel and you need to compile it manually.

---

Here are my instructions for enabling can-j1939 in Debian 10 kernel:

https://github.com/linux-can/can-utils/blob/master/can-j1939-install-kernel-module.md