net: vrf: Implement get_saddr for IPv6

IPv6 source address selection needs to consider the real egress route.
Similar to IPv4 implement a get_saddr6 method which is called if
source address has not been set.  The get_saddr6 method does a full
lookup which means pulling a route from the VRF FIB table and properly
considering linklocal/multicast destination addresses. Lookup failures
(eg., unreachable) then cause the source address selection to fail
which gets propagated back to the caller.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vrf.c

@@ -999,6 +999,46 @@ static struct dst_entry *vrf_get_rt6_dst(const struct net_device *dev,
 
 	return dst;
 }
+
+/* called under rcu_read_lock */
+static int vrf_get_saddr6(struct net_device *dev, const struct sock *sk,
+			  struct flowi6 *fl6)
+{
+	struct net *net = dev_net(dev);
+	struct dst_entry *dst;
+	struct rt6_info *rt;
+	int err;
+
+	if (rt6_need_strict(&fl6->daddr)) {
+		rt = vrf_ip6_route_lookup(net, dev, fl6, fl6->flowi6_oif,
+					  RT6_LOOKUP_F_IFACE);
+		if (unlikely(!rt))
+			return 0;
+
+		dst = &rt->dst;
+	} else {
+		__u8 flags = fl6->flowi6_flags;
+
+		fl6->flowi6_flags |= FLOWI_FLAG_L3MDEV_SRC;
+		fl6->flowi6_flags |= FLOWI_FLAG_SKIP_NH_OIF;
+
+		dst = ip6_route_output(net, sk, fl6);
+		rt = (struct rt6_info *)dst;
+
+		fl6->flowi6_flags = flags;
+	}
+
+	err = dst->error;
+	if (!err) {
+		err = ip6_route_get_saddr(net, rt, &fl6->daddr,
+					  sk ? inet6_sk(sk)->srcprefs : 0,
+					  &fl6->saddr);
+	}
+
+	dst_release(dst);
+
+	return err;
+}
 #endif
 
 static const struct l3mdev_ops vrf_l3mdev_ops = {
@@ -1008,6 +1048,7 @@ static const struct l3mdev_ops vrf_l3mdev_ops = {
 	.l3mdev_l3_rcv		= vrf_l3_rcv,
 #if IS_ENABLED(CONFIG_IPV6)
 	.l3mdev_get_rt6_dst	= vrf_get_rt6_dst,
+	.l3mdev_get_saddr6	= vrf_get_saddr6,
 #endif
 };