can: gw: make routing to the incoming CAN interface configurable

Introduce new configuration flag CGW_FLAGS_CAN_IIF_TX_OK to configure if a
CAN sk_buff that has been routed with can-gw is allowed to be send back to
the originating CAN interface.
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

include/uapi/linux/can/gw.h

@@ -51,6 +51,7 @@ enum {
 
 #define CGW_FLAGS_CAN_ECHO 0x01
 #define CGW_FLAGS_CAN_SRC_TSTAMP 0x02
+#define CGW_FLAGS_CAN_IIF_TX_OK 0x04
 
 #define CGW_MOD_FUNCS 4 /* AND OR XOR SET */
 

net/can/gw.c

@@ -52,6 +52,7 @@
 #include <linux/skbuff.h>
 #include <linux/can.h>
 #include <linux/can/core.h>
+#include <linux/can/skb.h>
 #include <linux/can/gw.h>
 #include <net/rtnetlink.h>
 #include <net/net_namespace.h>
@@ -347,6 +348,13 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
 		return;
 	}
 
+	/* is sending the skb back to the incoming interface not allowed? */
+	if (!(gwj->flags & CGW_FLAGS_CAN_IIF_TX_OK) &&
+	    skb_headroom(skb) == sizeof(struct can_skb_priv) &&
+	    (((struct can_skb_priv *)(skb->head))->ifindex ==
+	     gwj->dst.dev->ifindex))
+		return;
+
 	/*
 	 * clone the given skb, which has not been done in can_rcv()
 	 *