netdev - [PATCH net-next 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

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Message-ID: <20251127091325.7248-2-maklimek97@gmail.com>
Date: Thu, 27 Nov 2025 10:13:23 +0100
From: Mariusz Klimek <maklimek97@...il.com>
To: netdev@...r.kernel.org
Cc: Mariusz Klimek <maklimek97@...il.com>
Subject: [PATCH net-next 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

This patch fixes an issue in skb_gso_network_seglen where the calculated
segment length includes the HBH headers of BIG TCP jumbograms despite these
headers being removed before segmentation. These headers are added by GRO
or by ip6_xmit for BIG TCP packets and are later removed by GSO. This bug
causes MTU validation of BIG TCP jumbograms to fail.

Signed-off-by: Mariusz Klimek <maklimek97@...il.com>
---
 net/core/gso.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/gso.c b/net/core/gso.c
index bcd156372f4d..251a49181031 100644
--- a/net/core/gso.c
+++ b/net/core/gso.c
@@ -180,6 +180,10 @@ static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
 	unsigned int hdr_len = skb_transport_header(skb) -
 			       skb_network_header(skb);
 
+	/* Jumbogram HBH header is removed upon segmentation. */
+	if (skb->protocol == htons(ETH_P_IPV6) && skb->len > IPV6_MAXPLEN)
+		hdr_len -= sizeof(struct hop_jumbo_hdr);
+
 	return hdr_len + skb_gso_transport_seglen(skb);
 }
 
-- 
2.47.3