linux-kernel - Re: [PATCH net v3 2/2] selftests/net: test TCP reuseport socket selection

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Message-ID: <CAAVpQUBAk=339yyCnG+hDnHEu-O1+WPnAVqX=X9H22T=POQ3Bg@mail.gmail.com>
Date: Tue, 5 Aug 2025 12:00:51 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: edumazet@...gle.com, ncardwell@...gle.com, davem@...emloft.net, 
	dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 
	shuah@...nel.org, kraig@...gle.com, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net v3 2/2] selftests/net: test TCP reuseport socket selection

On Sat, Aug 2, 2025 at 2:24 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> The test script is provided by Kuniyuki in [1], which is used to test the
> selection of the TCP reuseport socket problem.
>
> Link: https://lore.kernel.org/netdev/20250801040757.1599996-1-kuniyu@google.com/ [1]
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
>  tools/testing/selftests/net/Makefile         |  1 +
>  tools/testing/selftests/net/tcp_reuseport.py | 36 ++++++++++++++++++++
>  2 files changed, 37 insertions(+)
>  create mode 100755 tools/testing/selftests/net/tcp_reuseport.py
>
> diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
> index b31a71f2b372..0f4c3eea9709 100644
> --- a/tools/testing/selftests/net/Makefile
> +++ b/tools/testing/selftests/net/Makefile
> @@ -117,6 +117,7 @@ TEST_GEN_FILES += tfo
>  TEST_PROGS += tfo_passive.sh
>  TEST_PROGS += broadcast_pmtu.sh
>  TEST_PROGS += ipv6_force_forwarding.sh
> +TEST_PROGS += tcp_reuseport.py
>
>  # YNL files, must be before "include ..lib.mk"
>  YNL_GEN_FILES := busy_poller netlink-dumps
> diff --git a/tools/testing/selftests/net/tcp_reuseport.py b/tools/testing/selftests/net/tcp_reuseport.py
> new file mode 100755
> index 000000000000..eaeb7096382e
> --- /dev/null
> +++ b/tools/testing/selftests/net/tcp_reuseport.py
> @@ -0,0 +1,36 @@
> +#!/usr/bin/env python3
> +# SPDX-License-Identifier: GPL-2.0
> +
> +import os
> +
> +from lib.py import ksft_run, ksft_exit
> +from socket import *
> +
> +def test_reuseport_select() -> None:
> +    s1 = socket()
> +    s1.setsockopt(SOL_SOCKET, SO_REUSEPORT, 1)
> +    s1.setsockopt(SOL_SOCKET, SO_BINDTODEVICE, b'lo')
> +    s1.listen()
> +    s1.setblocking(False)
> +
> +    s2 = socket()
> +    s2.setsockopt(SOL_SOCKET, SO_REUSEPORT, 1)
> +    s2.bind(s1.getsockname())
> +    s2.listen()
> +    s2.setblocking(False)
> +
> +    for i in range(3):
> +        c = socket()
> +        c.connect(s1.getsockname())
> +        try:
> +            print("SUCCESS: assigned properly:", s1.accept())
> +        except:
> +            print("FAIL: wrong assignment")
> +            os.sys.exit(1)

It seems you don't need to handle an exception with ksft.
You can see os.sys.exit(1) triggers another exception when
you run it without patch 1.

TAP version 13
1..1
# timeout set to 3600
# selftests: net: tcp_reuseport.py
# TAP version 13
# 1..1
# FAIL: wrong assignment
# # Exception| Traceback (most recent call last):
# # Exception|   File
"/root/linux/tools/testing/selftests/net/./tcp_reuseport.py", line 26,
in test_reuseport_select
# # Exception|     print("SUCCESS: assigned properly:", s1.accept())
# # Exception|                                          ~~~~~~~~~^^
# # Exception|   File "/usr/lib64/python3.13/socket.py", line 295, in accept
# # Exception|     fd, addr = self._accept()
# # Exception|                ~~~~~~~~~~~~^^
# # Exception| BlockingIOError: [Errno 11] Resource temporarily unavailable
# # Exception|
# # Exception| During handling of the above exception, another
exception occurred:
# # Exception|
# # Exception| Traceback (most recent call last):
# # Exception|   File
"/root/linux/tools/testing/selftests/net/lib/py/ksft.py", line 244, in
ksft_run
# # Exception|     case(*args)
# # Exception|     ~~~~^^^^^^^
# # Exception|   File
"/root/linux/tools/testing/selftests/net/./tcp_reuseport.py", line 29,
in test_reuseport_select
# # Exception|     os.sys.exit(1)
# # Exception|     ~~~~~~~~~~~^^^
# # Exception| SystemExit: 1
# not ok 1 tcp_reuseport.test_reuseport_select
# # Totals: pass:0 fail:1 xfail:0 xpass:0 skip:0 error:0
not ok 1 selftests: net: tcp_reuseport.py # exit=1


btw, I'd write an official uAPI selftest in plain C for socket as
python sometimes does a tricky thing and I don't trust it.

For example, this is...

from socket import *

s = socket()
s.listen(-1)

internally translated to:

socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, IPPROTO_IP) = 3
listen(3, 0)                            = 0