[PATCH nf-next v2 5/6] net: netfilter: nf_flow_table: refactor MTU check for tunnel offload

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.
Please note IPv4 over IPv6 and SIT tunnel flowtable offloading is not
enabled yet.

Signed-off-by: Lorenzo Bianconi <[email protected]>
---
 net/netfilter/nf_flow_table_ip.c | 41 ++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/net/netfilter/nf_flow_table_ip.c b/net/netfilter/nf_flow_table_ip.c
index 1b9360d54dfc..96dbdadba4e7 100644
--- a/net/netfilter/nf_flow_table_ip.c
+++ b/net/netfilter/nf_flow_table_ip.c
@@ -472,6 +472,31 @@ 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 *flow,
+				     enum flow_offload_tuple_dir dir,
+				     struct sk_buff *skb)
+{
+	unsigned int mtu;
+
+	mtu = flow->tuplehash[dir].tuple.mtu + ctx->offset;
+	switch (flow->tuplehash[!dir].tuple.tun.encap_proto) {
+	case AF_INET:
+		mtu -= sizeof(struct iphdr);
+		break;
+	case AF_INET6:
+		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,
@@ -479,17 +504,13 @@ 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, flow, dir, skb))
 		return 0;
 
 	iph = (struct iphdr *)(skb_network_header(skb) + ctx->offset);
@@ -1075,17 +1096,13 @@ 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, flow, dir, skb))
 		return 0;
 
 	ip6h = (struct ipv6hdr *)(skb_network_header(skb) + ctx->offset);

-- 
2.55.0