Re: [PATCH net-next v2 2/3] net: gro: parse ipv6 ext headers without frag0 invalidation

[Richard Gobert <richardbgobert@gmail.com>]

Eric Dumazet wrote:
> On Tue, Jan 2, 2024 at 2:25 PM Richard Gobert <richardbgobert@gmail.com> wrote:
>>
>> The existing code always pulls the IPv6 header and sets the transport
>> offset initially. Then optionally again pulls any extension headers in
>> ipv6_gso_pull_exthdrs and sets the transport offset again on return from
>> that call. skb->data is set at the start of the first extension header
>> before calling ipv6_gso_pull_exthdrs, and must disable the frag0
>> optimization because that function uses pskb_may_pull/pskb_pull instead of
>> skb_gro_ helpers. It sets the GRO offset to the TCP header with
>> skb_gro_pull and sets the transport header. Then returns skb->data to its
>> position before this block.
>>
>> This commit introduces a new helper function - ipv6_gro_pull_exthdrs -
>> which is used in ipv6_gro_receive to pull ipv6 ext headers instead of
>> ipv6_gso_pull_exthdrs. Thus, there is no modification of skb->data, all
>> operations use skb_gro_* helpers, and the frag0 fast path can be taken for
>> IPv6 packets with ext headers.
>>
>> Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
>> Reviewed-by: Willem de Bruijn <willemb@google.com>
>> ---
>>  include/net/ipv6.h     |  1 +
>>  net/ipv6/ip6_offload.c | 51 +++++++++++++++++++++++++++++++++---------
>>  2 files changed, 42 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/net/ipv6.h b/include/net/ipv6.h
>> index 78d38dd88aba..217240efa182 100644
>> --- a/include/net/ipv6.h
>> +++ b/include/net/ipv6.h
>> @@ -26,6 +26,7 @@ struct ip_tunnel_info;
>>  #define SIN6_LEN_RFC2133       24
>>
>>  #define IPV6_MAXPLEN           65535
>> +#define IPV6_MIN_EXTHDR_LEN    8
> 
> // Hmm see my following comment.
> 
>>
>>  /*
>>   *     NextHeader field of IPv6 header
>> diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
>> index 0e0b5fed0995..c07111d8f56a 100644
>> --- a/net/ipv6/ip6_offload.c
>> +++ b/net/ipv6/ip6_offload.c
>> @@ -37,6 +37,40 @@
>>                 INDIRECT_CALL_L4(cb, f2, f1, head, skb);        \
>>  })
>>
>> +static int ipv6_gro_pull_exthdrs(struct sk_buff *skb, int off, int proto)
>> +{
>> +       const struct net_offload *ops = NULL;
>> +       struct ipv6_opt_hdr *opth;
>> +
>> +       for (;;) {
>> +               int len;
>> +
>> +               ops = rcu_dereference(inet6_offloads[proto]);
>> +
>> +               if (unlikely(!ops))
>> +                       break;
>> +
>> +               if (!(ops->flags & INET6_PROTO_GSO_EXTHDR))
>> +                       break;
>> +
>> +               opth = skb_gro_header(skb, off + IPV6_MIN_EXTHDR_LEN, off);
> 
> I do not see a compelling reason for adding yet another constant here.
> 
> I would stick to
> 
>    opth = skb_gro_header(skb, off + sizeof(*opth), off);
> 
> Consistency with similar helpers is desirable.
> 

In terms of consistency - similar helper functions (ipv6_gso_pull_exthdrs,
ipv6_parse_hopopts) also pull 8 bytes at the beginning of every IPv6
extension header, because the minimum extension header length is 8 bytes.

sizeof(*opth) = 2, so for an IPv6 packet with one extension header with a
common length of 8 bytes, pskb_may_pull will be called twice: first with
length = 2 and again with length = 8, which might not be ideal when parsing
non-linear packets.

Willem suggested adding a constant to make the code more self-documenting.

>> +               if (unlikely(!opth))
>> +                       break;
>> +
>> +               len = ipv6_optlen(opth);
>> +
>> +               opth = skb_gro_header(skb, off + len, off);
> 
> Note this call will take care of precise pull.
> 
>> +               if (unlikely(!opth))
>> +                       break;
>> +               proto = opth->nexthdr;
>> +
>> +               off += len;
>> +       }
>> +
>> +       skb_gro_pull(skb, off - skb_network_offset(skb));
>> +       return proto;
>> +}
>> +
>>  static int ipv6_gso_pull_exthdrs(struct sk_buff *skb, int proto)
>>  {
>>         const struct net_offload *ops = NULL;
>> @@ -203,28 +237,25 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
>>                 goto out;
>>
>>         skb_set_network_header(skb, off);
>> -       skb_gro_pull(skb, sizeof(*iph));
>> -       skb_set_transport_header(skb, skb_gro_offset(skb));
>>
>> -       flush += ntohs(iph->payload_len) != skb_gro_len(skb);
>> +       flush += ntohs(iph->payload_len) != skb->len - hlen;
>>
>>         proto = iph->nexthdr;
>>         ops = rcu_dereference(inet6_offloads[proto]);
>>         if (!ops || !ops->callbacks.gro_receive) {
>> -               pskb_pull(skb, skb_gro_offset(skb));
>> -               skb_gro_frag0_invalidate(skb);
>> -               proto = ipv6_gso_pull_exthdrs(skb, proto);
>> -               skb_gro_pull(skb, -skb_transport_offset(skb));
>> -               skb_reset_transport_header(skb);
>> -               __skb_push(skb, skb_gro_offset(skb));
>> +               proto = ipv6_gro_pull_exthdrs(skb, hlen, proto);
>>
>>                 ops = rcu_dereference(inet6_offloads[proto]);
>>                 if (!ops || !ops->callbacks.gro_receive)
>>                         goto out;
>>
>> -               iph = ipv6_hdr(skb);
>> +               iph = skb_gro_network_header(skb);
>> +       } else {
>> +               skb_gro_pull(skb, sizeof(*iph));
>>         }
>>
>> +       skb_set_transport_header(skb, skb_gro_offset(skb));
>> +
>>         NAPI_GRO_CB(skb)->proto = proto;
>>
>>         flush--;
>> --
>> 2.36.1
>>