Pass IPv6 TCP packets to tcp6 gro_receive in homa_hijack_gro_receive

* homa_hijack_gro_receive is installed for both inet_offloads and
  inet6_offloads, but always passed non-Homa packets to the IPv4 TCP
  gro_receive, so IPv6 TCP packets went through tcp4_gro_receive
  (which treats the IPv6 header as an iphdr). homa_hijack_init runs
  at module load, so this affects all IPv6 TCP traffic.
* Use the saved tcp6_net_offload for IPv6 packets.
* Pin the existing pass-through test to IPv4 (it relied on the
  default) and add an IPv6 one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: randomizedcoder

homa_hijack.c

@@ -76,8 +76,12 @@ struct sk_buff *homa_hijack_gro_receive(struct list_head *held_list,
 	// tt_record4("homa_hijack_gro_receive got type 0x%x, flags 0x%x, "
 	//		"urgent 0x%x, id %d", h->type, h->flags,
 	//		ntohs(h->urgent), homa_local_id(h->sender_id));
-	if (!homa_skb_hijacked(skb))
-		return tcp_net_offload->callbacks.gro_receive(held_list, skb);
+	if (!homa_skb_hijacked(skb)) {
+		const struct net_offload *tcp_offload = skb_is_ipv6(skb) ?
+				tcp6_net_offload : tcp_net_offload;
+
+		return tcp_offload->callbacks.gro_receive(held_list, skb);
+	}
 
 	/* Change the packet's IP protocol to Homa so that it will get
 	 * dispatched directly to Homa in the future.

test/unit_homa_hijack.c

@@ -97,11 +97,12 @@ TEST_F(homa_hijack, homa_hijack_init)
 		  inet6_offloads[IPPROTO_TCP]->callbacks.gro_receive);
 }
 
-TEST_F(homa_hijack, homa_hijack_gro_receive__pass_to_tcp)
+TEST_F(homa_hijack, homa_hijack_gro_receive__pass_to_tcp_ipv4)
 {
 	struct homa_common_hdr *h;
 	struct sk_buff *skb;
 
+	mock_ipv6 = false;
 	homa_hijack_init();
 	self->header.seg.offset = htonl(6000);
 	skb = mock_skb_alloc(&self->src_ip, &self->dst_ip,
@@ -122,6 +123,22 @@ TEST_F(homa_hijack, homa_hijack_gro_receive__pass_to_tcp)
 	kfree_skb(skb);
 	homa_hijack_end();
 }
+TEST_F(homa_hijack, homa_hijack_gro_receive__pass_to_tcp_ipv6)
+{
+	struct homa_common_hdr *h;
+	struct sk_buff *skb;
+
+	mock_ipv6 = true;
+	homa_hijack_init();
+	skb = mock_skb_alloc(&self->src_ip, &self->dst_ip,
+			     &self->header.common, 1400, 0);
+	h = (struct homa_common_hdr *)skb_transport_header(skb);
+	h->flags = 0;
+	EXPECT_EQ(NULL, homa_hijack_gro_receive(&self->empty_list, skb));
+	EXPECT_STREQ("unit_tcp6_gro_receive", unit_log_get());
+	kfree_skb(skb);
+	homa_hijack_end();
+}
 TEST_F(homa_hijack, homa_hijack_gro_receive__pass_to_homa_ipv6)
 {
 	struct homa_common_hdr *h;