rtnetlink: Add RTNH_F_TRAP flag

The flag indicates to user space that the nexthop is not programmed to
forward packets in hardware, but rather to trap them to the CPU. This is
needed, for example, when the MAC of the nexthop neighbour is not
resolved and packets should reach the CPU to trigger neighbour
resolution.

The flag will be used in subsequent patches by netdevsim to test nexthop
objects programming to device drivers and in the future by mlxsw as
well.

Changes since RFC:
* Reword commit message
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/rtnetlink.h

@@ -396,11 +396,13 @@ struct rtnexthop {
 #define RTNH_F_DEAD		1	/* Nexthop is dead (used by multipath)	*/
 #define RTNH_F_PERVASIVE	2	/* Do recursive gateway lookup	*/
 #define RTNH_F_ONLINK		4	/* Gateway is forced on link	*/
-#define RTNH_F_OFFLOAD		8	/* offloaded route */
+#define RTNH_F_OFFLOAD		8	/* Nexthop is offloaded */
 #define RTNH_F_LINKDOWN		16	/* carrier-down on nexthop */
 #define RTNH_F_UNRESOLVED	32	/* The entry is unresolved (ipmr) */
+#define RTNH_F_TRAP		64	/* Nexthop is trapping packets */
 
-#define RTNH_COMPARE_MASK	(RTNH_F_DEAD | RTNH_F_LINKDOWN | RTNH_F_OFFLOAD)
+#define RTNH_COMPARE_MASK	(RTNH_F_DEAD | RTNH_F_LINKDOWN | \
+				 RTNH_F_OFFLOAD | RTNH_F_TRAP)
 
 /* Macros to handle hexthops */
 

net/ipv4/fib_semantics.c

@@ -1644,6 +1644,8 @@ int fib_nexthop_info(struct sk_buff *skb, const struct fib_nh_common *nhc,
 	*flags |= (nhc->nhc_flags & RTNH_F_ONLINK);
 	if (nhc->nhc_flags & RTNH_F_OFFLOAD)
 		*flags |= RTNH_F_OFFLOAD;
+	if (nhc->nhc_flags & RTNH_F_TRAP)
+		*flags |= RTNH_F_TRAP;
 
 	if (!skip_oif && nhc->nhc_dev &&
 	    nla_put_u32(skb, RTA_OIF, nhc->nhc_dev->ifindex))