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

Richard Gobert
2023-12-28T16:46:21Z

Willem de Bruijn wrote:
> Richard Gobert wrote:
>> This commit utilizes 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. To use ipv6_gso_pull_exthdr, pskb_pull and
>> __skb_push must be used, and frag0 must be invalidated. This commit
>> removes unnecessary code around the call to ipv6_gso_pull_exthdrs and
>> enables the frag0 fast path in IPv6 packets with ext headers.
>>
>> Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
>> ---
>> net/ipv6/ip6_offload.c | 51 +++++++++++++++++++++++++++++++++---------
>> 1 file changed, 41 insertions(+), 10 deletions(-)
>>
>> diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
>> index 0e0b5fed0995..a3b8d9127dbb 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 + 8, off);
>
> When changing this code, it would be great to make it more self
> documenting. It's not entirely clear what that 8 is based on.
> sizeof(*opth) is only 2. Probably an optimization to handle the most
> common extension headers in a single pskb_may_pull? If so, this new
> code does not have that concern, so can just use sizeof(*opth). Or
> else add a const int likely_max_opt_hdr_len = 8 or so.
>
>
>> + if (unlikely(!opth))
>> + break;
>> +
>> + len = ipv6_optlen(opth);
>> +
>> + opth = skb_gro_header(skb, off + len, off);
>> + 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));
>> }
>
> This code is non-obvious and has proven fragile (57ea52a8651). Changes
> are best as simple as they can be, with ample documentation. My
> attempt, as arrived at during review:
>
> The existing 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.
>
> The new code adds a small optimization to only pull and set transport
> offset once.
>
> The existing code needs to set skb->data 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 inner TCP header with skb_gro_pull and sets the
> transport header. Then returns skb->data to its position before
> this block.
>
> The new code is much simpler: it does not have to modify skb->data,
> as all operations are with skb_gro_ helpers.
>
> Aside from the small comment above, and suggestion to include
> something like this summary in the code and/or avoid the extra
> optimization,
>
> Reviewed-by: Willem de Bruijn <willemb@google.com>
>

Thanks for the review, I'll submit v2 with an extended commit message
summary as suggested.

>>
>> + skb_set_transport_header(skb, skb_gro_offset(skb));
>> +
>> NAPI_GRO_CB(skb)->proto = proto;
>>
>> flush--;
>> --
>> 2.36.1
>>
>
>