netfilter: bridge: move DNAT helper to br_netfilter

Only one caller, there is no need to keep this in a header.
Move it to br_netfilter.c where this belongs to.

Based on patch from Florian Westphal.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter_bridge.h

@@ -44,18 +44,6 @@ static inline unsigned int nf_bridge_mtu_reduction(const struct sk_buff *skb)
 }
 
 int br_handle_frame_finish(struct sk_buff *skb);
-/* Only used in br_device.c */
-static inline int br_nf_pre_routing_finish_bridge_slow(struct sk_buff *skb)
-{
-	struct nf_bridge_info *nf_bridge = skb->nf_bridge;
-
-	skb_pull(skb, ETH_HLEN);
-	nf_bridge->mask ^= BRNF_BRIDGED_DNAT;
-	skb_copy_to_linear_data_offset(skb, -(ETH_HLEN-ETH_ALEN),
-				       skb->nf_bridge->data, ETH_HLEN-ETH_ALEN);
-	skb->dev = nf_bridge->physindev;
-	return br_handle_frame_finish(skb);
-}
 
 /* This is called by the IP fragmenting code and it ensures there is
  * enough room for the encapsulating header (if there is one). */

net/bridge/br_device.c

@@ -36,13 +36,10 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 	u16 vid = 0;
 
 	rcu_read_lock();
-#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
-	if (skb->nf_bridge && (skb->nf_bridge->mask & BRNF_BRIDGED_DNAT)) {
-		br_nf_pre_routing_finish_bridge_slow(skb);
+	if (br_nf_prerouting_finish_bridge(skb)) {
 		rcu_read_unlock();
 		return NETDEV_TX_OK;
 	}
-#endif
 
 	u64_stats_update_begin(&brstats->syncp);
 	brstats->tx_packets++;

net/bridge/br_netfilter.c

@@ -892,6 +892,38 @@ static unsigned int ip_sabotage_in(const struct nf_hook_ops *ops,
 	return NF_ACCEPT;
 }
 
+/* This is called when br_netfilter has called into iptables/netfilter,
+ * and DNAT has taken place on a bridge-forwarded packet.
+ *
+ * neigh->output has created a new MAC header, with local br0 MAC
+ * as saddr.
+ *
+ * This restores the original MAC saddr of the bridged packet
+ * before invoking bridge forward logic to transmit the packet.
+ */
+static void br_nf_pre_routing_finish_bridge_slow(struct sk_buff *skb)
+{
+	struct nf_bridge_info *nf_bridge = skb->nf_bridge;
+
+	skb_pull(skb, ETH_HLEN);
+	nf_bridge->mask &= ~BRNF_BRIDGED_DNAT;
+
+	skb_copy_to_linear_data_offset(skb, -(ETH_HLEN-ETH_ALEN),
+				       skb->nf_bridge->data, ETH_HLEN-ETH_ALEN);
+	skb->dev = nf_bridge->physindev;
+	br_handle_frame_finish(skb);
+}
+
+int br_nf_prerouting_finish_bridge(struct sk_buff *skb)
+{
+	if (skb->nf_bridge && (skb->nf_bridge->mask & BRNF_BRIDGED_DNAT)) {
+		br_nf_pre_routing_finish_bridge_slow(skb);
+		return 1;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(br_nf_prerouting_finish_bridge);
+
 void br_netfilter_enable(void)
 {
 }

net/bridge/br_private.h

@@ -764,10 +764,15 @@ static inline int br_vlan_enabled(struct net_bridge *br)
 
 /* br_netfilter.c */
 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
+int br_nf_prerouting_finish_bridge(struct sk_buff *skb);
 int br_nf_core_init(void);
 void br_nf_core_fini(void);
 void br_netfilter_rtable_init(struct net_bridge *);
 #else
+static inline int br_nf_prerouting_finish_bridge(struct sk_buff *skb)
+{
+        return 0;
+}
 static inline int br_nf_core_init(void) { return 0; }
 static inline void br_nf_core_fini(void) {}
 #define br_netfilter_rtable_init(x)