socketcan J1939过滤器在python中使用

Question

在Python语言中，我尝试使用linux内核文档中提到的J1939过滤：https://www.kernel.org/doc/html/latest/networking/j1939.html

以下代码在setsockopt()行(设置过滤器)失败：

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()

首先，内核文档提到使用SOL_CAN_J1939作为第一个参数。但是，socket.SOL_CAN_J1939不存在于套接字包中。看一下这个位置的代码，我就能理解这个int值应该是107：http://socket-can.996257.n3.nabble.com/RFC-v3-0-6-CAN-add-SAE-J1939-protocol-td7571.html

至于setsockopt()的第三个参数，我打包了过滤器以匹配j1939_filter结构(如前面链接的代码中所述的26个字节)。这类似于在can.interfaces.socketcan.utils中对原始罐头所做的操作。

我做错了什么导致setsockopt()失败？

Answer 1

第一个问题是struct.pack格式(can_filter_fmt)错误。我首先假设内核j1939_filter结构大小是成员的总和。这是错误的，因为编译器添加了填充。这可以作为x添加到struct.pack格式中，例如2Q2I2B6x。请参阅Why isn't sizeof for a struct equal to the sum of sizeof of each member?

第二个问题是，can_filter_fmt不是打包为2Q2B2I，而是2Q2I2B6x ( addr成员在中间)。

至于SOL_CAN_J1939，我是正确的，需要在文件中创建，因为它还不在包中。

最终代码如下：

#!/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()

谢谢。

Answer 2

要让J1939与SocketCAN协同工作，您需要两件事：

1. 内核5.4+
2. can-j1939内核模块已启用

can-1939测试：

如果你安装了can-utils，在sudo modprobe -J1939之后，你得到的是致命错误，或者如果你从can-utils启动testj1939，你得到的错误是协议不受支持，那么这意味着你的内核中没有启用can-j1939，你需要手动编译它。

以下是我在Debian 10内核中启用can-j1939的说明：

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