Re: [PATCH net-next 06/15] ipv6/gro: insert temporary HBH/jumbo header

On Thu, Feb 3, 2022 at 1:20 AM Paolo Abeni [off-list ref] wrote:

On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

Following patch will add GRO_IPV6_MAX_SIZE, allowing gro to build
BIG TCP ipv6 packets (bigger than 64K).

This patch changes ipv6_gro_complete() to insert a HBH/jumbo header
so that resulting packet can go through IPv6/TCP stacks.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv6/ip6_offload.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index d37a79a8554e92a1dcaa6fd023cafe2114841ece..dac6f60436e167a3d979fef02f25fc039c6ed37d 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c

@@ -318,15 +318,43 @@ static struct sk_buff *ip4ip6_gro_receive(struct list_head *head,
 INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 {
      const struct net_offload *ops;
-     struct ipv6hdr *iph = (struct ipv6hdr *)(skb->data + nhoff);
+     struct ipv6hdr *iph;
      int err = -ENOSYS;
+     u32 payload_len;

      if (skb->encapsulation) {
              skb_set_inner_protocol(skb, cpu_to_be16(ETH_P_IPV6));
              skb_set_inner_network_header(skb, nhoff);
      }

-     iph->payload_len = htons(skb->len - nhoff - sizeof(*iph));
+     payload_len = skb->len - nhoff - sizeof(*iph);
+     if (unlikely(payload_len > IPV6_MAXPLEN)) {
+             struct hop_jumbo_hdr *hop_jumbo;
+             int hoplen = sizeof(*hop_jumbo);
+
+             /* Move network header left */
+             memmove(skb_mac_header(skb) - hoplen, skb_mac_header(skb),
+                     skb->transport_header - skb->mac_header);

I was wondering if we should check for enough headroom and what about
TCP over UDP tunnel, then I read the next patch ;)

The check about headroom is provided in the following patch (ipv6: add
GRO_IPV6_MAX_SIZE),
which allows GRO stack to build packets bigger than 64KB,
if drivers provided enough headroom (8 bytes).
They usually provide NET_SKB_PAD (64 bytes or more)

Before the next patch, this code is dead.

Also current patch set does not cook BIG TCP packets for tunneled traffic
(look at skb_gro_receive() changes in following patch)

I think a comment here referring to the constraint enforced by
skb_gro_receive() could help, or perhaps squashing the 2 patches?!?

Well no, we spent time making small patches to ease review, and these patches
have different authors anyway.

Thanks!

Paolo