[PATCH net v2] 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) 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 <redacted>
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 <redacted>
---
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/ (local)
 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 <errno.h>
 #include <fcntl.h>
+#include <sched.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

@@ -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