The tun_vnet_udptnl fixture creates a fresh tap device and installs an IPv6 outer neighbor entry as NUD_PERMANENT before sending packets. On systems where systemd-udevd is running and a systemd .link file sets MACAddressPolicy=persistent (the default shipped by systemd in 99-default.link, so this is what most systemd-based hosts inherit), systemd-udevd's net_setup_link builtin asynchronously sends an RTM_SETLINK to reassign the freshly created tap device's MAC to a machine-persistent value. When that netlink message races the test's ip_neigh_add() call, the address change kicks the following path: do_setlink -> netif_set_mac_address -> call_netdevice_notifiers_info -> ndisc_netdev_event -> neigh_changeaddr -> neigh_flush_dev(tbl, dev, /* skip_perm = */ false) which flushes every neighbor entry on the interface, including the one the test just installed as NUD_PERMANENT. The subsequent packet therefore hits __neigh_create(), triggers NDISC, and times out with: tun.c:947:send_gso_packet:Expected ret (0) == variant->data_size (1423) tun.c:948:send_gso_packet:Expected r_num_mss (0) == variant->r_num_mss (2) The failure is non-deterministic and can affect both directions. Both recv_gso_packet and send_gso_packet variants can hit it; the failure reproduces on a plain systemd-based VM with no containers, and is triggered whenever the udev worker's RTM_SETLINK lands after the test has installed its neighbor entry. Fix by calling unshare(CLONE_NEWNET) once, from main(), before invoking the kselftest_harness. All tap and geneve devices are then created in a namespace that systemd-udevd (running in the init netns) does not watch, so its RTM_SETLINK never fires against them. This mirrors the approach used by selftests/net/ipsec.c, which also unshares from main() rather than per-fixture; it keeps the harness output ordering intact and avoids paying the netns-creation cost on every variant. Verified on a plain systemd-based VM running the affected kernel: 1000 repeated invocations of tun_vnet_udptnl.4in6_nogsosz_1byte.recv_gso_packet produce 7 failures without the fix and zero failures with it. A 20-iteration run of the full test binary produces 4 failed runs (across different variants, all send_gso_packet) without the fix and zero failed runs with it. Reported-by: Po-Hsu Lin Closes: https://bugs.launchpad.net/bugs/2158217 Fixes: 24e59f26eef2 ("selftest: tun: Add helpers for GSO over UDP tunnel") Assisted-by: LLM Signed-off-by: Edoardo Canepa --- tools/testing/selftests/net/tun.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/tools/testing/selftests/net/tun.c b/tools/testing/selftests/net/tun.c index abe488bac50b..7f118ba3ae0b 100644 --- a/tools/testing/selftests/net/tun.c +++ b/tools/testing/selftests/net/tun.c @@ -4,6 +4,7 @@ #include #include +#include #include #include #include @@ -985,4 +986,12 @@ XFAIL_ADD(tun_vnet_udptnl, 6in4_over_maxbytes, recv_gso_packet); XFAIL_ADD(tun_vnet_udptnl, 4in6_over_maxbytes, recv_gso_packet); XFAIL_ADD(tun_vnet_udptnl, 6in6_over_maxbytes, recv_gso_packet); -TEST_HARNESS_MAIN +int main(int argc, char **argv) +{ + if (unshare(CLONE_NEWNET) < 0) { + perror("unshare(CLONE_NEWNET)"); + return 1; + } + + return test_harness_run(argc, argv); +} base-commit: 80dd7e754b3aa9637a0758ad93fa209f9650ec48 -- 2.53.0