netdev - Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

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Message-ID: <3e3ef9d0-f1df-4568-a207-2a121ca76def@redhat.com>
Date: Tue, 13 Jan 2026 15:14:26 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Mariusz Klimek <maklimek97@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH
 header in seglen calculation

On 1/6/26 10:52 AM, Mariusz Klimek wrote:
> @@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
>   */
>  static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>  {
> -	unsigned int hdr_len = skb_transport_header(skb) -
> -			       skb_network_header(skb);
> +	unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
> +	unsigned int hdr_len = skb_network_header_len(skb);
> +
> +	/* Jumbogram HBH header is removed upon segmentation. */
> +	if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
> +	    skb->len - off > IPV6_MAXPLEN)
> +		hdr_len -= sizeof(struct hop_jumbo_hdr);

I'm sorry for splitting the feedback in multiple replies.

I think the concern I expressed on v1:

https://lore.kernel.org/netdev/a7b90a3a-79ed-42a4-a782-17cde1b9a2d6@redhat.com/

is still not addressed here. What I fear is:

- TCP cooks a plain GSO packet just below the 64K limit.
- Such packet goes trough UDP (or gre) encapsulation, the skb->len size
(including outer network header) grows above the 64K limit.
- the above check is satisfied, but no jumbo hop option is present.

I think you could use the `ipv6_has_hopopt_jumbo()` helper to be on the
safe side.

/P