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) 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, with the tap and geneve devices removed between iterations so that each one starts from a clean state. 1000 repeated invocations of tun -r tun_vnet_udptnl.4in6_nogsosz_1byte.recv_gso_packet produce 266 failures without the fix and zero failures with it, and a full run of the test binary fails in 20 out of 20 attempts without the fix and in zero out of 20 with it. Note that without the fix a failure is not self-contained: the fixture setup aborts before FIXTURE_TEARDOWN runs, so the tap and geneve devices are left behind in the init netns and every later run fails right away in geneve_create(). Running in a private namespace also removes that, since the namespace is torn down with the test process. 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: Claude:claude-opus-5 Signed-off-by: Edoardo Canepa --- v2: - 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(), as suggested by Jakub. - Report an unshare() failure with SKIP() instead of aborting the binary before the harness starts, so the TAP stream stays complete and a runner can tell "no network namespaces" apart from a real tun/tap regression (raised by Sashiko). - Mention the new CAP_SYS_ADMIN / CONFIG_NET_NS prerequisite in the commit message (raised by Sashiko). - Use the Assisted-by: format documented in Documentation/process/coding-assistants.rst. - Redo the measurements in the commit message. The v1 numbers were taken without cleaning up the tap and geneve devices that a failed run leaves behind, which made runs after the first failure fail in geneve_create() rather than on the race being fixed here. 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