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

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Message-ID: <20250617144017.82931-16-maxim@isovalent.com>
Date: Tue, 17 Jun 2025 16:40:14 +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 15/17] udp: Set length in UDP header to 0 for big GSO packets

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 2326548997d3..5c76a4aaab7d 100644
--- a/net/ipv4/udp_tunnel_core.c
+++ b/net/ipv4/udp_tunnel_core.c
@@ -179,7 +179,7 @@ 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 c99053189ea8..e318c89663cb 100644
--- a/net/ipv6/ip6_udp_tunnel.c
+++ b/net/ipv6/ip6_udp_tunnel.c
@@ -92,7 +92,7 @@ int 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);
 
-- 
2.49.0