getDefaultGatewayInterfaceByFamily: custom filter for MultiHop

Due to the way how IPv6 multipath is implemented, it is not possible
to filter by ifindex as the ifindex for multihop routes will always be
0. Instead, implement a custom filter.

Reported-at: https://issues.redhat.com/browse/OCPBUGS-1318
Signed-off-by: Andreas Karis <ak.karis@gmail.com>

go-controller/pkg/node/helper_linux.go

@@ -58,6 +58,7 @@ func getDefaultGatewayInterfaceByFamily(family int, gwIface string) (string, net
 	// filter the default route to obtain the gateway
 	filter := &netlink.Route{Dst: nil}
 	mask := netlink.RT_FILTER_DST
+	gwIfIdx := 0
 	// gw interface provided
 	if len(gwIface) > 0 {
 		link, err := netlink.LinkByName(gwIface)
@@ -67,16 +68,15 @@ func getDefaultGatewayInterfaceByFamily(family int, gwIface string) (string, net
 		if link.Attrs() == nil {
 			return "", nil, fmt.Errorf("no attributes found for link: %#v", link)
 		}
-		gwIfIdx := link.Attrs().Index
+		gwIfIdx = link.Attrs().Index
 		klog.Infof("Provided gateway interface %q, found as index: %d", gwIface, gwIfIdx)
-		filter.LinkIndex = gwIfIdx
-		mask = mask | netlink.RT_FILTER_OIF
 	}
 
-	routes, err := netlink.RouteListFiltered(family, filter, mask)
+	routeList, err := netlink.RouteListFiltered(family, filter, mask)
 	if err != nil {
 		return "", nil, errors.Wrapf(err, "failed to get routing table in node")
 	}
+	routes := filterRoutesByIfIndex(routeList, gwIfIdx)
 	// use the first valid default gateway
 	for _, r := range routes {
 		// no multipath
@@ -148,3 +148,44 @@ func getIntfName(gatewayIntf string) (string, error) {
 	}
 	return intfName, nil
 }
+
+// filterRoutesByIfIndex is a helper function that will sieve the provided routes and check
+// if they match the provided index. This used to be implemented with netlink.RT_FILTER_OIF,
+// however the problem is that this filtered out MultiPath IPv6 routes which have a LinkIndex of 0.
+// filterRoutesByIfIndex will return:
+//   a) if ifidx <= 0: routesUnfiltered
+//   b) else: a filtered list of routes
+// The filter works as follows:
+//   i)  return routes with LinkIndex == ifIdx
+//   ii) return routes with LinkIndex == 0 and where *all* MultiPaths have LinkIndex == ifIdx
+// That also means: If a MultiPath route points out different interfaces, then skip that route and
+// do not return it.
+func filterRoutesByIfIndex(routesUnfiltered []netlink.Route, ifIdx int) []netlink.Route {
+	if ifIdx <= 0 {
+		return routesUnfiltered
+	}
+	var routes []netlink.Route
+	for _, r := range routesUnfiltered {
+		if r.LinkIndex == ifIdx ||
+			multipathRouteMatchesIfIndex(r, ifIdx) {
+			routes = append(routes, r)
+		}
+	}
+	return routes
+}
+
+// multipathRouteMatchesIfIndex is a helper for filterRoutesByIfIndex.
+func multipathRouteMatchesIfIndex(r netlink.Route, ifIdx int) bool {
+	if r.LinkIndex != 0 {
+		return false
+	}
+	if len(r.MultiPath) == 0 {
+		return false
+	}
+	for _, mr := range r.MultiPath {
+		if mr.LinkIndex != ifIdx {
+			return false
+		}
+	}
+	return true
+}

go-controller/pkg/node/helper_linux_test.go

@@ -0,0 +1,176 @@
+package node
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/vishvananda/netlink"
+)
+
+func TestFilterRoutesByIfIndex(t *testing.T) {
+	tcs := []struct {
+		routesUnfiltered []netlink.Route
+		gwIfIdx          int
+		routesFiltered   []netlink.Route
+	}{
+		// tc0 - baseline.
+		{
+			routesUnfiltered: []netlink.Route{},
+			gwIfIdx:          0,
+			routesFiltered:   []netlink.Route{},
+		},
+		// tc1 - do not filter.
+		{
+			routesUnfiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 2,
+					Gw:        []byte{2, 2, 2, 2},
+				},
+				{
+					LinkIndex: 1,
+					Gw:        []byte{3, 3, 3, 3},
+				},
+			},
+			gwIfIdx: 0,
+			routesFiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 2,
+					Gw:        []byte{2, 2, 2, 2},
+				},
+				{
+					LinkIndex: 1,
+					Gw:        []byte{3, 3, 3, 3},
+				},
+			},
+		},
+		// tc2 - filter by link index.
+		{
+			routesUnfiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 2,
+					Gw:        []byte{2, 2, 2, 2},
+				},
+				{
+					LinkIndex: 1,
+					Gw:        []byte{3, 3, 3, 3},
+				},
+			},
+			gwIfIdx: 1,
+			routesFiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 1,
+					Gw:        []byte{3, 3, 3, 3},
+				},
+			},
+		},
+		// tc3 - filter by MultiPath index.
+		{
+			routesUnfiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 2,
+					Gw:        []byte{2, 2, 2, 2},
+				},
+				{
+					LinkIndex: 0,
+					MultiPath: []*netlink.NexthopInfo{
+						{
+							LinkIndex: 1,
+							Hops:      0,
+							Gw:        []byte{3, 3, 3, 3},
+						},
+						{
+							LinkIndex: 1,
+							Hops:      0,
+							Gw:        []byte{4, 4, 4, 4},
+						},
+					},
+				},
+			},
+			gwIfIdx: 1,
+			routesFiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 0,
+					MultiPath: []*netlink.NexthopInfo{
+						{
+							LinkIndex: 1,
+							Hops:      0,
+							Gw:        []byte{3, 3, 3, 3},
+						},
+						{
+							LinkIndex: 1,
+							Hops:      0,
+							Gw:        []byte{4, 4, 4, 4},
+						},
+					},
+				},
+			},
+		},
+		// tc4 - filter by MultiPath index - MultiPath with multiple interfaces.
+		{
+			routesUnfiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+				{
+					LinkIndex: 2,
+					Gw:        []byte{2, 2, 2, 2},
+				},
+				{
+					LinkIndex: 0,
+					MultiPath: []*netlink.NexthopInfo{
+						{
+							LinkIndex: 1,
+							Hops:      0,
+							Gw:        []byte{3, 3, 3, 3},
+						},
+						{
+							LinkIndex: 2,
+							Hops:      0,
+							Gw:        []byte{4, 4, 4, 4},
+						},
+					},
+				},
+			},
+			gwIfIdx: 1,
+			routesFiltered: []netlink.Route{
+				{
+					LinkIndex: 1,
+					Gw:        []byte{1, 1, 1, 1},
+				},
+			},
+		},
+	}
+
+	for i, tc := range tcs {
+		routesFiltered := filterRoutesByIfIndex(tc.routesUnfiltered, tc.gwIfIdx)
+		if !reflect.DeepEqual(tc.routesFiltered, routesFiltered) {
+			t.Fatalf("TestFilterRoutesByIfIndex(%d): Filtering '%v' by index '%d' should have yielded '%v' but got '%v'",
+				i, tc.routesUnfiltered, tc.gwIfIdx, tc.routesFiltered, routesFiltered)
+		}
+	}
+}