On Thu, Sep 17, 2026 at 01:15:20PM +0200, Edoardo Canepa wrote: > 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 <[email protected]> > 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 <[email protected]> > --- > 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/[email protected]/ > - 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/[email protected]/
Reviewed-by: Simon Horman <[email protected]>
