netdev - Re: [PATCH net-next 08/15] ipv6: Add hop-by-hop header to jumbograms in ip6

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Date:   Thu, 03 Feb 2022 10:07:32 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 08/15] ipv6: Add hop-by-hop header to
 jumbograms in ip6_output

On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote:
> From: Coco Li <lixiaoyan@...gle.com>
> 
> Instead of simply forcing a 0 payload_len in IPv6 header,
> implement RFC 2675 and insert a custom extension header.
> 
> Note that only TCP stack is currently potentially generating
> jumbograms, and that this extension header is purely local,
> it wont be sent on a physical link.
> 
> This is needed so that packet capture (tcpdump and friends)
> can properly dissect these large packets.
> 
> Signed-off-by: Coco Li <lixiaoyan@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/ipv6.h  |  1 +
>  net/ipv6/ip6_output.c | 22 ++++++++++++++++++++--
>  2 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
> index 1e0f8a31f3de175659dca9ecee9f97d8b01e2b68..d3fb87e1589997570cde9cb5d92b2222008a229d 100644
> --- a/include/linux/ipv6.h
> +++ b/include/linux/ipv6.h
> @@ -144,6 +144,7 @@ struct inet6_skb_parm {
>  #define IP6SKB_L3SLAVE         64
>  #define IP6SKB_JUMBOGRAM      128
>  #define IP6SKB_SEG6	      256
> +#define IP6SKB_FAKEJUMBO      512
>  };
>  
>  #if defined(CONFIG_NET_L3_MASTER_DEV)
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 0c6c971ce0a58b50f8a9349b8507dffac9c7818c..f78ba145620560e5d7cb25aaf16fec61ddd9ed40 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -180,7 +180,9 @@ static int __ip6_finish_output(struct net *net, struct sock *sk, struct sk_buff
>  #endif
>  
>  	mtu = ip6_skb_dst_mtu(skb);
> -	if (skb_is_gso(skb) && !skb_gso_validate_network_len(skb, mtu))
> +	if (skb_is_gso(skb) &&
> +	    !(IP6CB(skb)->flags & IP6SKB_FAKEJUMBO) &&
> +	    !skb_gso_validate_network_len(skb, mtu))
>  		return ip6_finish_output_gso_slowpath_drop(net, sk, skb, mtu);

If I read correctly jumbogram with gso len not fitting the egress
device MTU will not be fragmented, as opposed to plain old GSO packets.
Am I correct? why fragmentation is not needed for jumbogram?

Thanks!

Paolo