Invalid Argument Error After Moving Code Block to Function

Question

I have this code that works fine when it's defined altogether, but if I move a portion of the code to its own function, I get an OSError: [Errno 22] Invalid argument.

from ctypes import create_string_buffer, addressof
from struct import pack, unpack

import socket


SHOW_ERROR: bool = True


def bpf_jump(code, k, jt, jf):
    return pack('HBBI', code, jt, jf, k)

def bpf_stmt(code, k):
    return bpf_jump(code, k, 0, 0)

# Instruction classes
BPF_LD = 0x00
BPF_JMP = 0x05
BPF_RET = 0x06

# ld/ldx fields
BPF_H = 0x08
BPF_B = 0x10
BPF_ABS = 0x20

# alu/jmp fields
BPF_JEQ = 0x10
BPF_K = 0x00

# As defined in asm/socket.h
SO_ATTACH_FILTER = 26

# Ordering of the filters is backwards of what would be intuitive for
# performance reasons: the check that is most likely to fail is first.
filters_list = [
    # Must be UDP (check protocol field at byte offset 23)
    bpf_stmt(BPF_LD | BPF_B | BPF_ABS, 23),
    bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 0x11, 0, 3),
    # Must be IPv4 (check ethertype field at byte offset 12)
    bpf_stmt(BPF_LD | BPF_H | BPF_ABS, 12),
    bpf_jump(BPF_JMP | BPF_JEQ | BPF_K, 0x0800, 0, 1),
    bpf_stmt(BPF_RET | BPF_K, 0x0FFFFFFF), # pass
    bpf_stmt(BPF_RET | BPF_K, 0), # reject
]

str_buf = None
def create_filters_struct() -> bytes:
    filters = b''.join(filters_list)
    str_buf = create_string_buffer(filters)
    mem_addr_of_filters = addressof(str_buf)
    fprog = pack('HL', len(filters_list), mem_addr_of_filters)
    return fprog

if SHOW_ERROR is False:
    filters = b''.join(filters_list)
    b = create_string_buffer(filters)
    mem_addr_of_filters = addressof(b)
    fprog = pack('HL', len(filters_list), mem_addr_of_filters)
else:
    fprog = create_filters_struct()

net_if = "eth0"

sock = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.ntohs(0x0003))
sock.setsockopt(socket.SOL_SOCKET,
                SO_ATTACH_FILTER,
                fprog)

Here is the error traceback:

Traceback (most recent call last):
  File "error_demo.py", line 68, in <module>
    fprog)
OSError: [Errno 22] Invalid argument

How can I fix this?

Please add the entire error message to your question. p.s. I didn't downvote...just asking for more details. — Code-Apprentice, May 16 '23 at 20:54
Perhaps you can make this into a [mre] by including the lines necessary for your code to run (including imports and definitions), and omitting those that are not. Also, you should probably include some indication of what your code is intended to do. — khelwood, May 16 '23 at 21:04
`addressof(b)` doesn't seem like a safe thing to do in the version of the code where `b` is a local variable, that's going to vanish before the filter actually gets used. — jasonharper, May 16 '23 at 21:08
@khelwood I gave it some more thought and updated the post with a minimally reproducible example. — Jim Fell, May 16 '23 at 21:19
@jasonharper I updated the example to place the string buffer outside of the function; the error persists. — Jim Fell, May 16 '23 at 21:26
@JimFell If that's what your last edit was supposed to do, it didn't. The `str_buf` variable inside the function is a different variable from the one outside the function. — khelwood, May 16 '23 at 21:28
@khelwood Thanks. I normally program in C and forgot to declare the global within the function. The problem now seems to be fixed. — Jim Fell, May 16 '23 at 21:30
interesting tag. perhaps it should be spelled "Berkeley" though. — Christoph Rackwitz, May 16 '23 at 21:46
@ChristophRackwitz Yes, you are correct. I am able to add tags, but not edit. Go figure. I'm hoping the mod that approves will fix it, or I'll flag it for correction later. — Jim Fell, May 16 '23 at 22:14
eh, why not just abandon the typo tag and create the corrected tag? I just did that. the review tools don't even list the typo tag now. typos in new tags should be tolerated by the site. *if* it doesn't, the site would need fixing. — Christoph Rackwitz, May 16 '23 at 22:18

score 0 · Answer 1 · answered May 16 '23 at 21:02

The function appears to work just fine and only other thing that changed was tuple(self.__command.interfaces.items())[0][1] which is probably where the invalid argument comes from.

PS: People don't like the question because adding it to a function isn't the "only" thing that changed. It's also part of a class now with no info on that. We also didn't get the whole error.

khelwood · Answer 2 · 2023-05-16T21:43:37.660

In this code:

def create_filters_struct() -> bytes:
    filters = b''.join(filters_list)
    str_buf = create_string_buffer(filters)
    mem_addr_of_filters = addressof(str_buf)
    fprog = pack('HL', len(filters_list), mem_addr_of_filters)
    return fprog

As jasonharper points out, you are getting the address from a local variable. Local variables are dropped when the function ends, and their contents become eligible for garbage collection. So trying to make use of that address outside of the function will be unsuccessful.

Storing the string buffer in a global variable instead seems to solve the problem. That could be as simple as adding global str_buf into your function definition.

Invalid Argument Error After Moving Code Block to Function

2 Answers2