IsDestRegSet unmarshaling fix (#178)

Fixes https://github.com/google/nftables/issues/176 | Added test case

expr/lookup.go

@@ -76,6 +76,7 @@ func (e *Lookup) unmarshal(fam byte, data []byte) error {
 			e.SourceRegister = ad.Uint32()
 		case unix.NFTA_LOOKUP_DREG:
 			e.DestRegister = ad.Uint32()
+			e.IsDestRegSet = true
 		case unix.NFTA_LOOKUP_FLAGS:
 			e.Invert = (ad.Uint32() & unix.NFT_LOOKUP_F_INV) != 0
 		}

nftables_test.go

@@ -3095,6 +3095,102 @@ func TestFlushTable(t *testing.T) {
 	}
 }
 
+func TestGetLookupExprDestSet(t *testing.T) {
+	c, newNS := openSystemNFTConn(t)
+	defer cleanupSystemNFTConn(t, newNS)
+	c.FlushRuleset()
+	defer c.FlushRuleset()
+
+	filter := c.AddTable(&nftables.Table{
+		Family: nftables.TableFamilyIPv4,
+		Name:   "filter",
+	})
+	forward := c.AddChain(&nftables.Chain{
+		Name:     "forward",
+		Table:    filter,
+		Type:     nftables.ChainTypeFilter,
+		Hooknum:  nftables.ChainHookForward,
+		Priority: nftables.ChainPriorityFilter,
+	})
+
+	set := &nftables.Set{
+		Table:    filter,
+		Name:     "kek",
+		IsMap:    true,
+		KeyType:  nftables.TypeInetService,
+		DataType: nftables.TypeVerdict,
+	}
+	if err := c.AddSet(set, nil); err != nil {
+		t.Errorf("c.AddSet(set) failed: %v", err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Errorf("c.Flush() failed: %v", err)
+	}
+
+	c.AddRule(&nftables.Rule{
+		Table: filter,
+		Chain: forward,
+		Exprs: []expr.Any{
+			&expr.Meta{Key: expr.MetaKeyL4PROTO, Register: 1},
+			&expr.Cmp{
+				Op:       expr.CmpOpEq,
+				Register: 1,
+				Data:     []byte{unix.IPPROTO_TCP},
+			},
+			&expr.Payload{
+				DestRegister: 1,
+				Base:         expr.PayloadBaseTransportHeader,
+				Offset:       2,
+				Len:          2,
+			},
+			&expr.Lookup{
+				SourceRegister: 1,
+				SetName:        set.Name,
+				SetID:          set.ID,
+				DestRegister:   0,
+				IsDestRegSet:   true,
+			},
+		},
+	})
+
+	if err := c.Flush(); err != nil {
+		t.Errorf("c.Flush() failed: %v", err)
+	}
+
+	rules, err := c.GetRules(
+		&nftables.Table{
+			Family: nftables.TableFamilyIPv4,
+			Name:   "filter",
+		},
+		&nftables.Chain{
+			Name: "forward",
+		},
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got, want := len(rules), 1; got != want {
+		t.Fatalf("unexpected number of rules: got %d, want %d", got, want)
+	}
+	if got, want := len(rules[0].Exprs), 4; got != want {
+		t.Fatalf("unexpected number of exprs: got %d, want %d", got, want)
+	}
+
+	lookup, lookupOk := rules[0].Exprs[3].(*expr.Lookup)
+	if !lookupOk {
+		t.Fatalf("Exprs[3] is type %T, want *expr.Lookup", rules[0].Exprs[3])
+	}
+	if want := (&expr.Lookup{
+		SourceRegister: 1,
+		SetName:        set.Name,
+		DestRegister:   0,
+		IsDestRegSet:   true,
+	}); !reflect.DeepEqual(lookup, want) {
+		t.Errorf("lookup expr = %+v, wanted %+v", lookup, want)
+	}
+}
+
 func TestGetRuleLookupVerdictImmediate(t *testing.T) {
 	// Create a new network namespace to test these operations,
 	// and tear down the namespace at test completion.