UDP encapsulation (FOU, GUE) has never worked correctly with multicast
destination addresses. When a FOU-encapsulated packet arrives at a
multicast address, it enters __udp4_lib_mcast_deliver() /
__udp6_lib_mcast_deliver() which call consume_skb() on packets that
need resubmission to the inner protocol handler, silently dropping
them instead.

The unicast delivery paths handle this correctly by propagating the
return value up to ip[6]_protocol_deliver_rcu() for resubmission, but
the multicast paths were never updated to support UDP encapsulation
resubmit.

This causes silent packet loss for FOU/GRETAP tunnels configured with
multicast remote addresses (both IPv4 and IPv6).

Reproducing the issue (IPv4):

  ip netns add ns_a && ip netns add ns_b
  ip link add veth0 netns ns_a type veth peer name veth1 netns ns_b

  ip -n ns_a addr add 10.0.0.1/24 dev veth0 && ip -n ns_a link set veth0 up
  ip -n ns_b addr add 10.0.0.2/24 dev veth1 && ip -n ns_b link set veth1 up

  ip -n ns_a route add 239.0.0.0/8 dev veth0
  ip -n ns_b route add 239.0.0.0/8 dev veth1

  # Disable early demux to expose the issue (otherwise it's partially masked)
  ip netns exec ns_b sysctl -w net.ipv4.ip_early_demux=0

  # Join multicast group on receiver
  ip -n ns_b addr add 239.0.0.1/32 dev veth1 autojoin

  # Sender: GRETAP with FOU encap
  ip -n ns_a link add eoudp0 type gretap \
      remote 239.0.0.1 local 10.0.0.1 \
      encap fou encap-sport 4797 encap-dport 4797 key 239.0.0.1
  ip -n ns_a link set eoudp0 up
  ip -n ns_a addr add 192.168.99.1/24 dev eoudp0

  # Receiver: FOU listener + GRETAP
  ip netns exec ns_b ip fou add port 4797 ipproto 47
  ip -n ns_b link add eoudp0 type gretap \
      remote 239.0.0.1 local 10.0.0.2 \
      encap fou encap-sport 4797 encap-dport 4797 key 239.0.0.1
  ip -n ns_b link set eoudp0 up
  ip -n ns_b addr add 192.168.99.2/24 dev eoudp0

  # Static neigh: ARP replies can't traverse unidirectional mcast tunnel
  recv_mac=$(ip -n ns_b link show eoudp0 | awk '/ether/{print $2}')
  ip -n ns_a neigh add 192.168.99.2 lladdr $recv_mac dev eoudp0

  # Test: ping through the FOU/GRETAP tunnel
  ip netns exec ns_a ping -c 100 192.168.99.2
  # -> without this patch: 0 packets received on eoudp0
  # -> with this patch: all packets received on eoudp0

IPv6 (using fou6 + ip6gretap) exhibits the same silent drop with a
different fix (see 1/2 for the sign-of-ret difference between
ip_protocol_deliver_rcu() and ip6_protocol_deliver_rcu()).

AI assistance (Claude, claude-opus-4-6) was used during root cause
analysis of the kernel source code (tracing the call chain from
udp[6]_queue_rcv_skb through encap_rcv to ip[6]_protocol_deliver_rcu,
comparing unicast/GSO/multicast paths) and during patch and selftest
authoring.

v6:
  - selftest: use the lib.sh ktap helpers (log_test / log_test_skip)
    so the IPv6-unavailable case reports KSFT_SKIP properly (Willem
    de Bruijn)
  - selftest: add CONFIG_IPV6_FOU / CONFIG_IPV6_FOU_TUNNEL to
    tools/testing/selftests/net/config (Willem de Bruijn)
  - selftest: speed up the ping loop with -i 0.01
v5: https://lore.kernel.org/netdev/cover.1783218197.git.littlesmilingcloud@gmail.com/ (local)
  - Fix IPv6 patch: return ret, not -ret (Willem de Bruijn)
  - selftest: add IPv6 test case
  - selftest: create the veth pair inside the namespaces (Willem de Bruijn)
v4: https://lore.kernel.org/netdev/cover.1782945956.git.littlesmilingcloud@gmail.com/ (local)
  - Promoted from RFC to PATCH; no functional changes since v3.
v3: https://lore.kernel.org/netdev/cover.1777934869.git.littlesmilingcloud@gmail.com/ (local)
  - Use return -ret instead of calling ip_protocol_deliver_rcu()
    directly, matching the unicast path (Kuniyuki Iwashima)
  - Replace Python packet generator with ping through a real
    FOU/GRETAP tunnel in the selftest
v2: https://lore.kernel.org/netdev/ad_dal164gVmImWl@dau-home-pc/ (local)
  - Moved inline Python packet generator into a separate helper
  - Fixed author email typo in Signed-off-by
v1 (RFC): https://lore.kernel.org/netdev/ad7MsSJOuUU6EGwS@dau-home-pc/ (local)

Anton Danilov (2):
  udp: fix encapsulation packet resubmit in multicast deliver
  selftests: net: add FOU multicast encapsulation resubmit test

 net/ipv4/udp.c                                |   6 +-
 net/ipv6/udp.c                                |   6 +-
 tools/testing/selftests/net/Makefile          |   1 +
 tools/testing/selftests/net/config            |   2 +
 .../testing/selftests/net/fou_mcast_encap.sh  | 172 ++++++++++++++++++
 5 files changed, 183 insertions(+), 4 deletions(-)
 create mode 100755 tools/testing/selftests/net/fou_mcast_encap.sh

-- 
2.47.3