Introduce nf_flow_offload_check_mtu() helper and use the encapsulated
protocol (tun.encap_proto) to compute the tunnel overhead instead of
relying on tun_num, so the correct inner header size (IPv4 vs IPv6) is
accounted for. Use it in both the IPv4 and IPv6 forward paths.
This is a preliminary patch to support IPv4 over IPv6 and SIT flowtable
tunnel offload.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
---
 net/netfilter/nf_flow_table_ip.c | 47 ++++++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 12 deletions(-)

diff --git a/net/netfilter/nf_flow_table_ip.c b/net/netfilter/nf_flow_table_ip.c
index 3ecc47b57854..0aaf1e07f320 100644
--- a/net/netfilter/nf_flow_table_ip.c
+++ b/net/netfilter/nf_flow_table_ip.c
@@ -470,6 +470,37 @@ nf_flow_offload_lookup(struct nf_flowtable_ctx *ctx,
 	return flow_offload_lookup(flow_table, &tuple);
 }
 
+static int nf_flow_offload_check_mtu(struct nf_flowtable_ctx *ctx,
+				     struct flow_offload_tuple_rhash *tuplehash,
+				     struct sk_buff *skb)
+{
+	enum flow_offload_tuple_dir dir;
+	struct flow_offload *flow;
+	unsigned int mtu;
+
+	dir = tuplehash->tuple.dir;
+	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
+
+	mtu = flow->tuplehash[dir].tuple.mtu + ctx->offset;
+	if (unlikely(nf_flow_is_tunnel_ip(ctx))) {
+		switch (ctx->ether_type) {
+		case htons(ETH_P_IP):
+			mtu -= sizeof(struct iphdr);
+			break;
+		case htons(ETH_P_IPV6):
+			mtu -= sizeof(struct ipv6hdr);
+			break;
+		default:
+			break;
+		}
+	}
+
+	if (unlikely(nf_flow_exceeds_mtu(skb, mtu)))
+		return -EINVAL;
+
+	return 0;
+}
+
 static int nf_flow_offload_forward(struct nf_flowtable_ctx *ctx,
 				   struct nf_flowtable *flow_table,
 				   struct flow_offload_tuple_rhash *tuplehash,
@@ -477,17 +508,13 @@ static int nf_flow_offload_forward(struct nf_flowtable_ctx *ctx,
 {
 	enum flow_offload_tuple_dir dir;
 	struct flow_offload *flow;
-	unsigned int thoff, mtu;
+	unsigned int thoff;
 	struct iphdr *iph;
 
 	dir = tuplehash->tuple.dir;
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
 
-	mtu = flow->tuplehash[dir].tuple.mtu + ctx->offset;
-	if (flow->tuplehash[!dir].tuple.tun_num)
-		mtu -= sizeof(*iph);
-
-	if (unlikely(nf_flow_exceeds_mtu(skb, mtu)))
+	if (nf_flow_offload_check_mtu(ctx, tuplehash, skb))
 		return 0;
 
 	iph = (struct iphdr *)(skb_network_header(skb) + ctx->offset);
@@ -1073,17 +1100,13 @@ static int nf_flow_offload_ipv6_forward(struct nf_flowtable_ctx *ctx,
 {
 	enum flow_offload_tuple_dir dir;
 	struct flow_offload *flow;
-	unsigned int thoff, mtu;
 	struct ipv6hdr *ip6h;
+	unsigned int thoff;
 
 	dir = tuplehash->tuple.dir;
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
 
-	mtu = flow->tuplehash[dir].tuple.mtu + ctx->offset;
-	if (flow->tuplehash[!dir].tuple.tun_num)
-		mtu -= sizeof(*ip6h);
-
-	if (unlikely(nf_flow_exceeds_mtu(skb, mtu)))
+	if (nf_flow_offload_check_mtu(ctx, tuplehash, skb))
 		return 0;
 
 	ip6h = (struct ipv6hdr *)(skb_network_header(skb) + ctx->offset);

-- 
2.55.0