selftests: forwarding: Add blackhole nexthops tests

Test that IPv4 and IPv6 ping fail when the route is using a blackhole
nexthop or a group with a blackhole nexthop. Test that ping passes when
the route starts using a valid nexthop.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/router_mpath_nh.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
-ALL_TESTS="ping_ipv4 ping_ipv6 multipath_test"
+ALL_TESTS="
+	ping_ipv4
+	ping_ipv6
+	multipath_test
+	ping_ipv4_blackhole
+	ping_ipv6_blackhole
+"
 NUM_NETIFS=8
 source lib.sh
 
@@ -302,6 +308,56 @@ multipath_test()
 	multipath6_l4_test "Weighted MP 11:45" 11 45
 }
 
+ping_ipv4_blackhole()
+{
+	RET=0
+
+	ip nexthop add id 1001 blackhole
+	ip nexthop add id 1002 group 1001
+
+	ip route replace 198.51.100.0/24 vrf vrf-r1 nhid 1001
+	ping_do $h1 198.51.100.2
+	check_fail $? "ping did not fail when using a blackhole nexthop"
+
+	ip route replace 198.51.100.0/24 vrf vrf-r1 nhid 1002
+	ping_do $h1 198.51.100.2
+	check_fail $? "ping did not fail when using a blackhole nexthop group"
+
+	ip route replace 198.51.100.0/24 vrf vrf-r1 nhid 103
+	ping_do $h1 198.51.100.2
+	check_err $? "ping failed with a valid nexthop"
+
+	log_test "IPv4 blackhole ping"
+
+	ip nexthop del id 1002
+	ip nexthop del id 1001
+}
+
+ping_ipv6_blackhole()
+{
+	RET=0
+
+	ip -6 nexthop add id 1001 blackhole
+	ip nexthop add id 1002 group 1001
+
+	ip route replace 2001:db8:2::/64 vrf vrf-r1 nhid 1001
+	ping6_do $h1 2001:db8:2::2
+	check_fail $? "ping did not fail when using a blackhole nexthop"
+
+	ip route replace 2001:db8:2::/64 vrf vrf-r1 nhid 1002
+	ping6_do $h1 2001:db8:2::2
+	check_fail $? "ping did not fail when using a blackhole nexthop group"
+
+	ip route replace 2001:db8:2::/64 vrf vrf-r1 nhid 106
+	ping6_do $h1 2001:db8:2::2
+	check_err $? "ping failed with a valid nexthop"
+
+	log_test "IPv6 blackhole ping"
+
+	ip nexthop del id 1002
+	ip -6 nexthop del id 1001
+}
+
 setup_prepare()
 {
 	h1=${NETIFS[p1]}