selftests: fib_test: Add test case for ipv4 multi nexthops

The test checks that with multi nexthops route the preferred route is the
one which matches source ip. In case when source ip is on dummy
interface, it checks that the routes are balanced.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20251221192639.3911901-2-vadim.fedorenko@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Vadim Fedorenko

tools/testing/selftests/net/fib_tests.sh

@@ -12,7 +12,7 @@ TESTS="unregister down carrier nexthop suppress ipv6_notify ipv4_notify \
        ipv4_route_metrics ipv4_route_v6_gw rp_filter ipv4_del_addr \
        ipv6_del_addr ipv4_mangle ipv6_mangle ipv4_bcast_neigh fib6_gc_test \
        ipv4_mpath_list ipv6_mpath_list ipv4_mpath_balance ipv6_mpath_balance \
-       fib6_ra_to_static"
+       ipv4_mpath_balance_preferred fib6_ra_to_static"
 
 VERBOSE=0
 PAUSE_ON_FAIL=no
@@ -2751,6 +2751,73 @@ ipv4_mpath_balance_test()
 	forwarding_cleanup
 }
 
+get_route_dev_src()
+{
+	local pfx="$1"
+	local src="$2"
+	local out
+
+	if out=$($IP -j route get "$pfx" from "$src" | jq -re ".[0].dev"); then
+		echo "$out"
+	fi
+}
+
+ipv4_mpath_preferred()
+{
+	local src_ip=$1
+	local pref_dev=$2
+	local dev routes
+	local route0=0
+	local route1=0
+	local pref_route=0
+	num_routes=254
+
+	for i in $(seq 1 $num_routes) ; do
+		dev=$(get_route_dev_src 172.16.105.$i $src_ip)
+		if [ "$dev" = "$pref_dev" ]; then
+			pref_route=$((pref_route+1))
+		elif [ "$dev" = "veth1" ]; then
+			route0=$((route0+1))
+		elif [ "$dev" = "veth3" ]; then
+			route1=$((route1+1))
+		fi
+	done
+
+	routes=$((route0+route1))
+
+	[ "$VERBOSE" = "1" ] && echo "multipath: routes seen: ($route0,$route1,$pref_route)"
+
+	if [ x"$pref_dev" = x"" ]; then
+		[[ $routes -ge $num_routes ]] && [[ $route0 -gt 0 ]] && [[ $route1 -gt 0 ]]
+	else
+		[[ $pref_route -ge $num_routes ]]
+	fi
+
+}
+
+ipv4_mpath_balance_preferred_test()
+{
+	echo
+	echo "IPv4 multipath load balance preferred route"
+
+	forwarding_setup
+
+	$IP route add 172.16.105.0/24 \
+		nexthop via 172.16.101.2 \
+		nexthop via 172.16.103.2
+
+	ipv4_mpath_preferred 172.16.101.1 veth1
+	log_test $? 0 "IPv4 multipath loadbalance from veth1"
+
+	ipv4_mpath_preferred 172.16.103.1 veth3
+	log_test $? 0 "IPv4 multipath loadbalance from veth3"
+
+	ipv4_mpath_preferred 198.51.100.1
+	log_test $? 0 "IPv4 multipath loadbalance from dummy"
+
+	forwarding_cleanup
+}
+
 ipv6_mpath_balance_test()
 {
 	echo
@@ -2861,6 +2928,7 @@ do
 	ipv6_mpath_list)		ipv6_mpath_list_test;;
 	ipv4_mpath_balance)		ipv4_mpath_balance_test;;
 	ipv6_mpath_balance)		ipv6_mpath_balance_test;;
+	ipv4_mpath_balance_preferred)	ipv4_mpath_balance_preferred_test;;
 	fib6_ra_to_static)		fib6_ra_to_static;;
 
 	help) echo "Test names: $TESTS"; exit 0;;