netdev - [PATCH RFC net-next 13/17] udp: Support gro_ipv4_max

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Message-ID: <20250617144017.82931-14-maxim@isovalent.com>
Date: Tue, 17 Jun 2025 16:40:12 +0200
From: Maxim Mikityanskiy <maxtram95@...il.com>
To: Daniel Borkmann <daniel@...earbox.net>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	David Ahern <dsahern@...nel.org>,
	Nikolay Aleksandrov <razor@...ckwall.org>
Cc: netdev@...r.kernel.org,
	Maxim Mikityanskiy <maxim@...valent.com>
Subject: [PATCH RFC net-next 13/17] udp: Support gro_ipv4_max_size > 65536

From: Maxim Mikityanskiy <maxim@...valent.com>

Currently, gro_max_size and gro_ipv4_max_size can be set to values
bigger than 65536, and GRO will happily aggregate UDP to the configured
size (for example, with TCP traffic in VXLAN tunnels). However,
udp_gro_complete uses the 16-bit length field in the UDP header to store
the length of the aggregated packet. It leads to the packet truncation
later in __udp4_lib_rcv.

Fix this by storing 0 to the UDP length field and by restoring the real
length from skb->len in __udp4_lib_rcv.

Signed-off-by: Maxim Mikityanskiy <maxim@...valent.com>
---
 net/ipv4/udp.c         | 5 ++++-
 net/ipv4/udp_offload.c | 7 +++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index dde52b8050b8..6200e09d9a37 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2642,7 +2642,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 {
 	struct sock *sk = NULL;
 	struct udphdr *uh;
-	unsigned short ulen;
+	unsigned int ulen;
 	struct rtable *rt = skb_rtable(skb);
 	__be32 saddr, daddr;
 	struct net *net = dev_net(skb->dev);
@@ -2666,6 +2666,9 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		goto short_packet;
 
 	if (proto == IPPROTO_UDP) {
+		if (!ulen)
+			ulen = skb->len;
+
 		/* UDP validates ulen. */
 		if (ulen < sizeof(*uh) || pskb_trim_rcsum(skb, ulen))
 			goto short_packet;
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 85b5aa82d7d7..ee12847a0347 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -930,12 +930,15 @@ static int udp_gro_complete_segment(struct sk_buff *skb)
 int udp_gro_complete(struct sk_buff *skb, int nhoff,
 		     udp_lookup_t lookup)
 {
-	__be16 newlen = htons(skb->len - nhoff);
+	unsigned int newlen = skb->len - nhoff;
 	struct udphdr *uh = (struct udphdr *)(skb->data + nhoff);
 	struct sock *sk;
 	int err;
 
-	uh->len = newlen;
+	if (newlen <= GRO_LEGACY_MAX_SIZE)
+		uh->len = htons(newlen);
+	else
+		uh->len = 0;
 
 	sk = INDIRECT_CALL_INET(lookup, udp6_lib_lookup_skb,
 				udp4_lib_lookup_skb, skb, uh->source, uh->dest);
-- 
2.49.0