socketcan J1939濾波器在python中的使用

Question

在 Python 中，我正在嘗試使用 linux kernel 文檔中提到的 J1939 過濾： https://www.kernel.org/doc/html/latest.networking/j1939.88747064

以下代碼在 setsockopt() 行（設置過濾器）失敗：

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，kernel 文檔提到使用 SOL_CAN_J1939 作為第一個參數。 但是 socket.SOL_CAN_J1939 不存在於套接字 package 中。因此查看此位置的代碼我能夠理解此 int 值應為 107： http://socket-can.996257.n3.nabble.com/RFC -v3-0-6-CAN-添加-SAE-J1939-協議-td7571.html

至於 setsockopt() 第三個參數，我打包了過濾器以匹配 j1939_filter 結構（26 字節，如上一個鏈接的代碼中所述）。 這類似於在 can.interfaces.socketcan.utils 中為原始 CAN 所做的。

我做錯了什么導致 setsockopt() 失敗？

Answer 1

第一個問題是 struct.pack 格式 (can_filter_fmt) 錯誤。 我首先假設 kernel j1939_filter 結構大小是成員的總和。 這是錯誤的，因為編譯器添加了填充。 這可以添加到 struct.pack 格式為 x，例如 2Q2I2B6x。 請參閱為什么結構的 sizeof 不等於每個成員的 sizeof 之和？

第二個問題是 can_filter_fmt 沒有打包為 2Q2B2I，而是打包為 2Q2I2B6x（addr 成員在中間）。

至於 SOL_CAN_J1939 我是正確的，需要在文件中創建，因為它還沒有在 package 中。

最終代碼如下：

#!/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()

謝謝你。

Answer 2

要使 J1939 與 SocketCAN 一起工作，您需要兩件事：

1. kernel 5.4+
2. 啟用 can-j1939 kernel 模塊

---

can-1939 測試：

如果你安裝 can-utils 並且在 sudo modprobe can-j1939 之后你得到的都是致命錯誤，或者如果你從 can-utils 啟動 testj1939 並且你得到協議不支持的錯誤，那么這意味着 can-j1939 沒有啟用你的 kernel 你需要手動編譯它。

---

以下是我在 Debian 10 kernel 中啟用 can-j1939 的說明：

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