netdev - [PATCH net-next 15/17] udp: Set length in UDP header to 0 for big GSO packets

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Message-ID: <20250923134742.1399800-16-maxtram95@gmail.com>
Date: Tue, 23 Sep 2025 16:47:40 +0300
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,
	tcpdump-workers@...ts.tcpdump.org,
	Guy Harris <gharris@...ic.net>,
	Michael Richardson <mcr@...delman.ca>,
	Denis Ovsienko <denis@...ienko.info>,
	Xin Long <lucien.xin@...il.com>,
	Maxim Mikityanskiy <maxim@...valent.com>
Subject: [PATCH net-next 15/17] udp: Set length in UDP header to 0 for big GSO packets

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

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

skb->len may be bigger than 65535 in UDP-based tunnels that have BIG TCP
enabled. If GSO aggregates packets that large, set the length in the UDP
header to 0, so that tcpdump can print such packets properly (treating
them as RFC 2675 jumbograms). Later in the pipeline, __udp_gso_segment
will set uh->len to the size of individual packets.

Signed-off-by: Maxim Mikityanskiy <maxim@...valent.com>
---
 net/ipv4/udp_tunnel_core.c | 2 +-
 net/ipv6/ip6_udp_tunnel.c  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/udp_tunnel_core.c b/net/ipv4/udp_tunnel_core.c
index 54386e06a813..98faddb7b4bf 100644
--- a/net/ipv4/udp_tunnel_core.c
+++ b/net/ipv4/udp_tunnel_core.c
@@ -184,7 +184,7 @@ void udp_tunnel_xmit_skb(struct rtable *rt, struct sock *sk, struct sk_buff *skb
 
 	uh->dest = dst_port;
 	uh->source = src_port;
-	uh->len = htons(skb->len);
+	uh->len = skb->len <= GRO_LEGACY_MAX_SIZE ? htons(skb->len) : 0;
 
 	memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
 
diff --git a/net/ipv6/ip6_udp_tunnel.c b/net/ipv6/ip6_udp_tunnel.c
index 0ff547a4bff7..0fb85f490f8c 100644
--- a/net/ipv6/ip6_udp_tunnel.c
+++ b/net/ipv6/ip6_udp_tunnel.c
@@ -93,7 +93,7 @@ void udp_tunnel6_xmit_skb(struct dst_entry *dst, struct sock *sk,
 	uh->dest = dst_port;
 	uh->source = src_port;
 
-	uh->len = htons(skb->len);
+	uh->len = skb->len <= GRO_LEGACY_MAX_SIZE ? htons(skb->len) : 0;
 
 	skb_dst_set(skb, dst);
 
-- 
2.50.1