netdev - Re: [PATCH net-next 05/15] ipv6/gso: remove temporary HBH/jumbo header

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Date:   Thu, 3 Feb 2022 17:48:09 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 05/15] ipv6/gso: remove temporary HBH/jumbo header

On Thu, Feb 3, 2022 at 5:14 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Feb 3, 2022 at 4:27 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Thu, Feb 3, 2022 at 4:05 PM Alexander Duyck
> > <alexander.duyck@...il.com> wrote:
> > >
> >
> > > I get that. What I was getting at was that we might be able to process
> > > it in ipv6_gso_segment before we hand it off to either TCP or UDP gso
> > > handlers to segment.
> > >
> > > The general idea being we keep the IPv6 specific bits in the IPv6
> > > specific code instead of having the skb_segment function now have to
> > > understand IPv6 packets. So what we would end up doing is having to do
> > > an skb_cow to replace the skb->head if any clones might be holding on
> > > it, and then just chop off the HBH jumbo header before we start the
> > > segmenting.
> > >
> > > The risk would be that we waste cycles removing the HBH header for a
> > > frame that is going to fail, but I am not sure how likely a scenario
> > > that is or if we need to optimize for that.
> >
> > I guess I can try this for the next version, thanks.
>
> I came up with:
>
> ommit 147f17169ccc6c2c38ea802e5728528ed54f492d
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Sat Nov 20 16:49:35 2021 -0800
>
>     ipv6/gso: remove temporary HBH/jumbo header
>
>     ipv6 tcp and gro stacks will soon be able to build big TCP packets,
>     with an added temporary Hop By Hop header.
>
>     If GSO is involved for these large packets, we need to remove
>     the temporary HBH header before segmentation happens.
>
>     v2: perform HBH removal from ipv6_gso_segment() instead of
>         skb_segment() (Alexander feedback)
>
>     Signed-off-by: Eric Dumazet <edumazet@...gle.com>


Well, this does not work at all.




>  static inline bool ipv6_accept_ra(struct inet6_dev *idev)
>  {
>         /* If forwarding is enabled, RA are not accepted unless the special
> diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
> index d37a79a8554e92a1dcaa6fd023cafe2114841ece..7f65097c8f30fa19a8c9c265eb4f027e91848021
> 100644
> --- a/net/ipv6/ip6_offload.c
> +++ b/net/ipv6/ip6_offload.c
> @@ -87,6 +87,27 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
>         bool gso_partial;
>
>         skb_reset_network_header(skb);
> +       if (ipv6_has_hopopt_jumbo(skb)) {
> +               const int hophdr_len = sizeof(struct hop_jumbo_hdr);
> +               int err;
> +
> +               err = skb_cow_head(skb, 0);
> +               if (err < 0)
> +                       return ERR_PTR(err);
> +
> +               /* remove the HBH header.
> +                * Layout: [Ethernet header][IPv6 header][HBH][TCP header]
> +                */
> +               memmove(skb->data + hophdr_len,
> +                       skb->data,
> +                       ETH_HLEN + sizeof(struct ipv6hdr));
> +               skb->data += hophdr_len;
> +               skb->len -= hophdr_len;
> +               skb->network_header += hophdr_len;
> +               skb->mac_header += hophdr_len;
> +               ipv6h = (struct ipv6hdr *)skb->data;
> +               ipv6h->nexthdr = IPPROTO_TCP;
> +       }
>         nhoff = skb_network_header(skb) - skb_mac_header(skb);
>         if (unlikely(!pskb_may_pull(skb, sizeof(*ipv6h))))
>                 goto out;