ipv4: Add FLOWI_FLAG_KNOWN_NH

Add flag to request that output route should be
returned with known rt_gateway, in case we want to use
it as nexthop for neighbour resolving.

The returned route can be cached as follows:

- in NH exception: because the cached routes are not shared
with other destinations
- in FIB NH: when using gateway because all destinations for
NH share same gateway

As last option, to return rt_gateway!=0 we have to
set DST_NOCACHE.

Signed-off-by: Julian Anastasov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/flow.h

···
 	__u8	flowic_flags;
 #define FLOWI_FLAG_ANYSRC		0x01
 #define FLOWI_FLAG_CAN_SLEEP		0x02
+#define FLOWI_FLAG_KNOWN_NH		0x04
 	__u32	flowic_secid;
 };
 

net/ipv4/route.c

···
 	struct in_device *in_dev;
 	u16 type = res->type;
 	struct rtable *rth;
+	bool do_cache;
 
 	in_dev = __in_dev_get_rcu(dev_out);
 	if (!in_dev)
···
 	}
 
 	fnhe = NULL;
+	do_cache = fi != NULL;
 	if (fi) {
 		struct rtable __rcu **prth;
+		struct fib_nh *nh = &FIB_RES_NH(*res);
 
-		fnhe = find_exception(&FIB_RES_NH(*res), fl4->daddr);
+		fnhe = find_exception(nh, fl4->daddr);
 		if (fnhe)
 			prth = &fnhe->fnhe_rth;
-		else
-			prth = __this_cpu_ptr(FIB_RES_NH(*res).nh_pcpu_rth_output);
+		else {
+			if (unlikely(fl4->flowi4_flags &
+				     FLOWI_FLAG_KNOWN_NH &&
+				     !(nh->nh_gw &&
+				       nh->nh_scope == RT_SCOPE_LINK))) {
+				do_cache = false;
+				goto add;
+			}
+			prth = __this_cpu_ptr(nh->nh_pcpu_rth_output);
+		}
 		rth = rcu_dereference(*prth);
 		if (rt_cache_valid(rth)) {
 			dst_hold(&rth->dst);
 			return rth;
 		}
 	}
+
+add:
 	rth = rt_dst_alloc(dev_out,
 			   IN_DEV_CONF_GET(in_dev, NOPOLICY),
 			   IN_DEV_CONF_GET(in_dev, NOXFRM),
-			   fi);
+			   do_cache);
 	if (!rth)
 		return ERR_PTR(-ENOBUFS);