Re: [PATCH net v4 3/4] selftests: net: Test UDP length overflow with PMTU discover and big MTU
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2026-08-26 17:18:22
Alice Mikityanska wrote:
From: Alice Mikityanska <redacted> Two previous commits fixed overflow of UDP length when setsockopt IP(V6)_MTU_DISCOVER is set to IPV6_PMTUDISC_DO or IP(V6)_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU. This commit adds the selftests that replicate the described steps to reproduce for IPv6 and IPv4, and also one more test that ensures that sending UDP jumbograms over a raw socket is still possible after the fix. Signed-off-by: Alice Mikityanska <redacted>
Reviewed-by: Willem de Bruijn <willemb@google.com> Thanks for respinning for the code dedup between ipv4 and ipv6
quoted hunk ↗ jump to hunk
--- tools/testing/selftests/net/Makefile | 1 + tools/testing/selftests/net/cork_fragsize.py | 140 +++++++++++++++++++ 2 files changed, 141 insertions(+) create mode 100755 tools/testing/selftests/net/cork_fragsize.pydiff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile index 0f5c178bc224..c6d332c90a54 100644 --- a/tools/testing/selftests/net/Makefile +++ b/tools/testing/selftests/net/Makefile@@ -25,6 +25,7 @@ TEST_PROGS := \ cmsg_so_mark.sh \ cmsg_so_priority.sh \ cmsg_time.sh \ + cork_fragsize.py \ double_udp_encap.sh \ drop_monitor_tests.sh \ ecmp_rehash.sh \diff --git a/tools/testing/selftests/net/cork_fragsize.py b/tools/testing/selftests/net/cork_fragsize.py new file mode 100755 index 000000000000..c54af8f21e3b --- /dev/null +++ b/tools/testing/selftests/net/cork_fragsize.py@@ -0,0 +1,140 @@ +#!/usr/bin/env python3 +# SPDX-License-Identifier: GPL-2.0 + +# Test possible UDP length overflow in udp_send_skb/udp_v6_send_skb. + +import errno +import gzip +import os +import socket +import struct +import subprocess +from contextlib import contextmanager + +from lib.py import KsftSkipEx, ksft_eq, ksft_raises, ksft_true +from lib.py import KsftNamedVariant, ksft_exit, ksft_pr, ksft_run, ksft_variants +from lib.py import NetNS, NetNSEnter, ip + + +IP_MTU_DISCOVER = 10 +IP_PMTUDISC_PROBE = 3 +IPV6_MTU_DISCOVER = 23 +IPV6_PMTUDISC_DO = 2 +IPV6_PMTUDISC_PROBE = 3 +IPV6_TLV_JUMBO = 194 + + +def check_kernel_config(option: str) -> bool | None: + for filename, method in [ + ('/proc/config.gz', gzip.open), + (f'/boot/config-{os.uname().release}', open), + ]: + try: + with method(filename, 'rt') as config: + for line in config: + if line.rstrip() == f'{option}=y': + return True + return False + except OSError: + continue + return None + + +def assert_debug_kernel() -> None: + res = check_kernel_config('CONFIG_DEBUG_NET') + if res is None: + ksft_pr("WARN: Can't read kernel config; assuming debug kernel, and running the test") + elif not res: + raise KsftSkipEx('CONFIG_DEBUG_NET is not set') + + +def check_dmesg_clean(func: str) -> bool: + with subprocess.Popen(['dmesg'], stdout=subprocess.PIPE) as dmesg: + res = subprocess.run(['grep', '-q', f'WARNING:.*{func}'], stdin=dmesg.stdout, check=False) + return res.returncode != 0 and dmesg.returncode == 0 + + +@contextmanager
Interesting approach. In ksft the built-in defer() mechanism may be preferred. Not sure.
+def dummy_netdev(ns: NetNS, mtu: int, ipv6: bool) -> None:
+ try:
+ ip('link add dummy type dummy', ns=ns)
+ ip(f'link set dummy mtu {mtu}', ns=ns)
+ ip('link set dummy up', ns=ns)
+ flag = '-6' if ipv6 else ''
+ nodad = 'nodad' if ipv6 else ''
+ local = 'fd00::1/64' if ipv6 else '10.0.0.1/24'
+ remote = 'fd00::2' if ipv6 else '10.0.0.2'
+ ip(f'{flag} addr add {local} dev dummy {nodad}', ns=ns)
+ ip(f'{flag} neigh add {remote} lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns)
+ yield
+ finally:
+ ip('link del dummy', ns=ns)
+
+