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() which calls
consume_skb() on packets that need resubmission to the inner protocol
handler, silently dropping them instead.

The unicast delivery path handles this correctly by returning -ret,
but the multicast path was never updated to support UDP encapsulation
resubmit.

This causes silent packet loss for FOU/GRETAP tunnels configured with
multicast remote addresses. The loss ratio depends on the early demux
cache hit rate - packets that hit early demux bypass the multicast path
and work correctly, masking the issue.

Reproducing the issue:

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

  # Multicast routes
  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

AI assistance (Claude, claude-opus-4-6) was used during root cause
analysis of the kernel source code (tracing the call chain from
udp_queue_rcv_skb through encap_rcv to ip_protocol_deliver_rcu,
comparing unicast/GSO/multicast paths) and during patch and selftest
authoring. The fix approach was identified by observing that the
unicast path (udp_unicast_rcv_skb) already handles encap resubmit
correctly via return -ret, while the multicast path did not.

v4:
  - Promoted from RFC to PATCH; no functional changes since v3.
    v3 was posted as RFC and consequently dropped from patchwork,
    which explains the lack of review feedback.
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 and avoiding call stack
    growth with nested encapsulations (Kuniyuki Iwashima)
  - Only change the first-socket path; the clone loop is not
    reachable for tunnel sockets (no SO_REUSEADDR/SO_REUSEPORT)
  - Replace Python packet generator with ping through a properly
    configured FOU/GRETAP tunnel in the selftest
  - Add static neighbor entry (ARP replies cannot traverse the
    unidirectional multicast tunnel)
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 +
 .../testing/selftests/net/fou_mcast_encap.sh  | 112 ++++++++++++++++++
 4 files changed, 121 insertions(+), 4 deletions(-)
 create mode 100755 tools/testing/selftests/net/fou_mcast_encap.sh

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2.47.3