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]/
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
