netdev - Re: [PATCH net-next] ipv6: gro: do not use slow memcmp() in ipv6_gro

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Date:   Tue, 6 Nov 2018 14:51:15 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] ipv6: gro: do not use slow memcmp() in ipv6_gro_receive()

On Tue, Nov 6, 2018 at 2:41 PM David Miller <davem@...emloft.net> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Tue,  6 Nov 2018 14:25:52 -0800
>
> > +             if (unlikely(nlen > sizeof(struct ipv6hdr))) {
> > +                     if (memcmp(iph + 1, iph2 + 1,
> > +                                nlen - sizeof(struct ipv6hdr)))
> > +                             goto not_same_flow;
> > +             }
>
> Is this even possible?

I believe that nlen can be indeed > sizeof(struct ipv6hdr) in presence
of exthdrs,
eg if ipv6_gso_pull_exthdrs() had to be called (line 201)

 I admit I have not checked if this was actually possible.


>
>         off = skb_gro_offset(skb);
>         hlen = off + sizeof(*iph);
>         iph = skb_gro_header_fast(skb, off);
>
> off is some offset to the ipv6hdr in skb.  This is GRO's CB data_offset.
>
>         skb_set_network_header(skb, off);
>         skb_gro_pull(skb, sizeof(*iph));
>         skb_set_transport_header(skb, skb_gro_offset(skb));
>
> Set network header to location of iph in SKB.
>
> GRO pull causes an incremebt of data_offset by sizeof(*iph) bytes.
>
> Set transport header to new data_offset value.
>
>         nlen = skb_network_header_len(skb);
>
> This is transport_header - network_header.
>
> From what I can see, it is impossible for this to take on any value
> other than sizeof(*ipv6hdr).
>
> If you agree, please let's get rid of nlen and this useless code, and
> replace with sizeof(*ipv6hdr) as needed.
>
> Thanks.