netdev - Re: [RFC net-next v4 1/2] IPv6/GRO: generic helper to remove temporary HBH/jumbo header in driver

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Date:   Sun, 4 Dec 2022 18:23:35 -0800
From:   Michael Chan <michael.chan@...adcom.com>
To:     Coco Li <lixiaoyan@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC net-next v4 1/2] IPv6/GRO: generic helper to remove
 temporary HBH/jumbo header in driver

On Fri, Dec 2, 2022 at 2:12 PM Coco Li <lixiaoyan@...gle.com> wrote:
>
> IPv6/TCP and GRO stacks can build big TCP packets with an added
> temporary Hop By Hop header.
>
> Is GSO is not involved, then the temporary header needs to be removed in
> the driver. This patch provides a generic helper for drivers that need
> to modify their headers in place.
>
> Signed-off-by: Coco Li <lixiaoyan@...gle.com>
> ---
>  include/net/ipv6.h     | 35 +++++++++++++++++++++++++++++++++++
>  net/ipv6/ip6_offload.c | 27 ++++-----------------------
>  2 files changed, 39 insertions(+), 23 deletions(-)
>
> diff --git a/include/net/ipv6.h b/include/net/ipv6.h
> index d383c895592a..08adec74f067 100644
> --- a/include/net/ipv6.h
> +++ b/include/net/ipv6.h
> @@ -500,6 +500,41 @@ static inline int ipv6_has_hopopt_jumbo(const struct sk_buff *skb)
>         return jhdr->nexthdr;
>  }
>
> +/* Return 0 if HBH header is successfully removed
> + * Or if HBH removal is unnecessary (packet is not big TCP)
> + * Return error to indicate dropping the packet
> + */
> +static inline int ipv6_hopopt_jumbo_remove(struct sk_buff *skb)
> +{
> +       const int hophdr_len = sizeof(struct hop_jumbo_hdr);
> +       int nexthdr = ipv6_has_hopopt_jumbo(skb);
> +       struct ipv6hdr *h6;
> +
> +       if (!nexthdr)
> +               return 0;
> +
> +       if (skb_cow_head(skb, 0))
> +               return -1;
> +
> +       /* Remove the HBH header.
> +        * Layout: [Ethernet header][IPv6 header][HBH][L4 Header]
> +        */
> +       memmove(skb_mac_header(skb) + hophdr_len, skb_mac_header(skb),
> +               skb_network_header(skb) - skb_mac_header(skb) +
> +               sizeof(struct ipv6hdr));
> +
> +       if (unlikely(!pskb_may_pull(skb, hophdr_len)))
> +               return -1;
> +

This version doesn't seem to work.  I think you need to add hophdr_len
to skb->data and subtract it from skb->len.

> +       skb->network_header += hophdr_len;
> +       skb->mac_header += hophdr_len;
> +
> +       h6 = ipv6_hdr(skb);
> +       h6->nexthdr = nexthdr;
> +
> +       return 0;
> +}
> +
>  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 3ee345672849..00dc2e3b0184 100644
> --- a/net/ipv6/ip6_offload.c
> +++ b/net/ipv6/ip6_offload.c
> @@ -77,7 +77,7 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
>         struct sk_buff *segs = ERR_PTR(-EINVAL);
>         struct ipv6hdr *ipv6h;
>         const struct net_offload *ops;
> -       int proto, nexthdr;
> +       int proto, err;
>         struct frag_hdr *fptr;
>         unsigned int payload_len;
>         u8 *prevhdr;
> @@ -87,28 +87,9 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
>         bool gso_partial;
>
>         skb_reset_network_header(skb);
> -       nexthdr = ipv6_has_hopopt_jumbo(skb);
> -       if (nexthdr) {
> -               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_mac_header(skb) + hophdr_len,
> -                       skb_mac_header(skb),
> -                       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 = nexthdr;
> -       }
> +       err = ipv6_hopopt_jumbo_remove(skb);
> +       if (err)
> +               return ERR_PTR(err);
>         nhoff = skb_network_header(skb) - skb_mac_header(skb);
>         if (unlikely(!pskb_may_pull(skb, sizeof(*ipv6h))))
>                 goto out;
> --
> 2.39.0.rc0.267.gcb52ba06e7-goog
>

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