[RFC PATCH net-next v3 1/2] udp: fix encapsulation packet resubmit in multicast deliver

[Anton Danilov <littlesmilingcloud@gmail.com>]

When a UDP encapsulation socket (e.g., FOU) receives a multicast
packet, __udp4_lib_mcast_deliver() and __udp6_lib_mcast_deliver()
call consume_skb() when udp_queue_rcv_skb() returns a positive value.
A positive return value from udp_queue_rcv_skb() indicates that the
encap_rcv handler (e.g., fou_udp_recv) has consumed the UDP header
and wants the packet to be resubmitted to the IP protocol handler
for further processing (e.g., as a GRE packet).

The unicast path in udp_unicast_rcv_skb() handles this correctly by
returning -ret, which propagates up to ip_protocol_deliver_rcu() for
resubmission. However, the multicast path destroys the packet via
consume_skb() instead of resubmitting it, causing silent packet loss.

This affects any UDP encapsulation (FOU, GUE) combined with multicast
destination addresses.

Fix this by returning -ret instead of calling consume_skb() when the
return value is positive, matching the behavior of the unicast path.
This avoids growing the call stack compared to calling
ip_protocol_deliver_rcu() directly.

Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Assisted-by: Claude:claude-opus-4-6
---
 net/ipv4/udp.c | 6 ++++--
 net/ipv6/udp.c | 6 ++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index e9e2ce9522ef..84a98631b556 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2467,6 +2467,7 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	struct udp_hslot *hslot;
 	struct sk_buff *nskb;
 	bool use_hash2;
+	int ret;
 
 	hash2_any = 0;
 	hash2 = 0;
@@ -2511,8 +2512,9 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	}
 
 	if (first) {
-		if (udp_queue_rcv_skb(first, skb) > 0)
-			consume_skb(skb);
+		ret = udp_queue_rcv_skb(first, skb);
+		if (ret > 0)
+			return -ret;
 	} else {
 		kfree_skb(skb);
 		__UDP_INC_STATS(net, UDP_MIB_IGNOREDMULTI);
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 15e032194ecc..ff2e389e286b 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -949,6 +949,7 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	struct udp_hslot *hslot;
 	struct sk_buff *nskb;
 	bool use_hash2;
+	int ret;
 
 	hash2_any = 0;
 	hash2 = 0;
@@ -998,8 +999,9 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	}
 
 	if (first) {
-		if (udpv6_queue_rcv_skb(first, skb) > 0)
-			consume_skb(skb);
+		ret = udpv6_queue_rcv_skb(first, skb);
+		if (ret > 0)
+			return -ret;
 	} else {
 		kfree_skb(skb);
 		__UDP6_INC_STATS(net, UDP_MIB_IGNOREDMULTI);
-- 
2.47.3