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

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Message-ID: <a7b90a3a-79ed-42a4-a782-17cde1b9a2d6@redhat.com>
Date: Tue, 2 Dec 2025 12:36:59 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Mariusz Klimek <maklimek97@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: gso: do not include jumbogram HBH
 header in seglen calculation

On 11/27/25 10:13 AM, Mariusz Klimek wrote:
> 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);

Isn't the above condition a bit too course-grain? Specifically, can
UDP-encapsulated GSO packets wrongly hit it?

/P