[PATCH net] selftests/net: run tun tests in a dedicated network namespace

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 <redacted>
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 <redacted>
---
 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 <errno.h>
 #include <fcntl.h>
+#include <sched.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

@@ -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