The tun_vnet_udptnl fixture creates a fresh tap device, assigns it a fixed MAC address and installs the outer neighbor entry as NUD_PERMANENT. 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 lands after the fixture has configured the device, both directions break: - recv_gso_packet transmits through the tap, and the address change flushes the neighbor entry the fixture installed: do_setlink -> netif_set_mac_address -> call_netdevice_notifiers(NETDEV_CHANGEADDR) -> ndisc_netdev_event (arp_netdev_event for an IPv4 outer) -> neigh_changeaddr -> neigh_flush_dev(tbl, dev, /* skip_perm = */ false) so the packet hits __neigh_create() and waits on neighbor resolution that never completes. - send_gso_packet writes frames addressed to the MAC the fixture assigned. Once the tap has a different address, eth_type_trans() marks them PACKET_OTHERHOST and the IP receive path drops them. Either way nothing arrives before the receive timeout, and the test fails with, for example: tun.c:947:send_gso_packet:Expected ret (0) == variant->data_size (1) tun.c:948:send_gso_packet:Expected r_num_mss (0) == variant->r_num_mss (1) tun.c:962:recv_gso_packet:Expected ret (0) == variant->data_size (1) The failure is non-deterministic and reproduces on a plain systemd-based VM with no containers. Fix by calling unshare(CLONE_NEWNET) from both fixture setups. The harness runs each test in its own forked process, so every test gets a private network namespace that is torn down with it, and all tap and geneve devices are created in a namespace that systemd-udevd (running in the init netns) does not watch, so its RTM_SETLINK never fires against them. Creating a network namespace needs CAP_SYS_ADMIN in the current user namespace and CONFIG_NET_NS=y, neither of which the tests required before. Where they are unavailable the unshare() is reported with SKIP() rather than aborting, so the binary still emits a full TAP stream and a runner can tell "network namespaces unavailable" apart from a real tun/tap regression. Verified on a plain systemd-based VM running the affected kernel. Without the fix, 1000 sequential invocations of tun -r tun_vnet_udptnl.4in6_nogsosz_1byte.recv_gso_packet fail 10 times, and 2 out of 20 full runs of the test binary fail. With the fix there are no failures in either case. 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 --- v3: - Drop the claim that a failure leaves the tap and geneve devices behind. The failing checks are EXPECT_EQ()s in the test body, so FIXTURE_TEARDOWN still runs (Sashiko). - Explain the send_gso_packet failure correctly: its frames are dropped as PACKET_OTHERHOST once the tap's MAC changes. The neighbor flush only affects recv_gso_packet. - Redo the measurements on a freshly booted host. The v2 numbers were taken in an unclean environment. The v1 numbers were fine, and the v2 entry saying otherwise was wrong. - Go back to "Assisted-by: LLM", the current format in Documentation/process/coding-assistants.rst. v2 followed an older copy of that document (Sashiko). v2: https://lore.kernel.org/netdev/20260914211921.3786609-1-edoardo.canepa@canonical.com/ - Add the unshare(CLONE_NEWNET) to FIXTURE_SETUP(tun) and FIXTURE_SETUP(tun_vnet_udptnl) instead of replacing TEST_HARNESS_MAIN with a hand-written main() (Jakub). - Report an unshare() failure with SKIP() instead of aborting the binary before the harness starts (Sashiko). - Mention the new CAP_SYS_ADMIN / CONFIG_NET_NS prerequisite in the commit message (Sashiko). - Change the Assisted-by: format (reverted in v3). - Redo the measurements (redone again in v3). v1: https://lore.kernel.org/netdev/20260905085318.3416670-1-edoardo.canepa@canonical.com/ tools/testing/selftests/net/tun.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/tools/testing/selftests/net/tun.c b/tools/testing/selftests/net/tun.c index abe488bac50b..6db21dad0efe 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 @@ -488,6 +489,10 @@ FIXTURE(tun) FIXTURE_SETUP(tun) { + if (unshare(CLONE_NEWNET)) + SKIP(return, "Cannot create network namespace: %s", + strerror(errno)); + memset(self->ifname, 0, sizeof(self->ifname)); self->fd = tun_alloc(self->ifname); @@ -732,6 +737,10 @@ FIXTURE_SETUP(tun_vnet_udptnl) struct sockaddr_storage ssa, dsa; void *sip, *dip, *smac, *dmac; + if (unshare(CLONE_NEWNET)) + SKIP(return, "Cannot create network namespace: %s", + strerror(errno)); + flags = (variant->is_tap ? IFF_TAP : IFF_TUN) | IFF_VNET_HDR | IFF_MULTI_QUEUE | IFF_NO_PI; features = TUN_F_CSUM | TUN_F_UDP_TUNNEL_GSO | -- 2.53.0