set: Add set support for size specifier

Handle attribute NFTNL_SET_DESC_SIZE, as done in libnftnl:
https://git.netfilter.org/libnftnl/tree/src/set.c#n424

Example:
nft add set ip filter myset { type ipv4_addr\; size 65535\; flags dynamic\; }

nftables_test.go

@@ -4103,6 +4103,46 @@ func TestSetElementsInterval(t *testing.T) {
 	}
 }
 
+func TestSetSizeConcat(t *testing.T) {
+	// Create a new network namespace to test these operations,
+	// and tear down the namespace at test completion.
+	c, newNS := nftest.OpenSystemConn(t, *enableSysTests)
+	defer nftest.CleanupSystemConn(t, newNS)
+	// Clear all rules at the beginning + end of the test.
+	c.FlushRuleset()
+	defer c.FlushRuleset()
+
+	filter := c.AddTable(&nftables.Table{
+		Family: nftables.TableFamilyIPv6,
+		Name:   "filter",
+	})
+
+	set := &nftables.Set{
+		Name:          "test-set",
+		Table:         filter,
+		KeyType:       nftables.MustConcatSetType(nftables.TypeIP6Addr, nftables.TypeInetService, nftables.TypeIP6Addr),
+		Dynamic:       true,
+		Concatenation: true,
+		Size:          200,
+	}
+
+	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)
+	}
+
+	sets, err := c.GetSets(filter)
+	if err != nil {
+		t.Errorf("c.GetSets() failed: %v", err)
+	}
+	if len(sets) != 1 {
+		t.Fatalf("len(sets) = %d, want 1", len(sets))
+	}
+}
+
 func TestCreateListFlowtable(t *testing.T) {
 	c, newNS := nftest.OpenSystemConn(t, *enableSysTests)
 	defer nftest.CleanupSystemConn(t, newNS)

set.go

@@ -267,6 +267,8 @@ type Set struct {
 	// https://git.netfilter.org/nftables/tree/include/datatype.h?id=d486c9e626405e829221b82d7355558005b26d8a#n109
 	KeyByteOrder binaryutil.ByteOrder
 	Comment      string
+	// Indicates that the set has "size" specifier
+	Size uint32
 }
 
 // SetElement represents a data point within a set.
@@ -553,6 +555,21 @@ func (cc *Conn) AddSet(s *Set, vals []SetElement) error {
 		}
 		tableInfo = append(tableInfo, netlink.Attribute{Type: unix.NLA_F_NESTED | unix.NFTA_SET_DESC, Data: numberOfElements})
 	}
+
+	var descBytes []byte
+
+	if s.Size > 0 {
+		// Marshal set size description
+		descSizeBytes, err := netlink.MarshalAttributes([]netlink.Attribute{
+			{Type: unix.NFTA_SET_DESC_SIZE, Data: binaryutil.BigEndian.PutUint32(s.Size)},
+		})
+		if err != nil {
+			return fmt.Errorf("fail to marshal set size description: %w", err)
+		}
+
+		descBytes = append(descBytes, descSizeBytes...)
+	}
+
 	if s.Concatenation {
 		// Length of concatenated types is a must, otherwise segfaults when executing nft list ruleset
 		var concatDefinition []byte
@@ -579,8 +596,13 @@ func (cc *Conn) AddSet(s *Set, vals []SetElement) error {
 		if err != nil {
 			return fmt.Errorf("fail to marshal concat definition %v", err)
 		}
-		// Marshal concat size description as set description
-		tableInfo = append(tableInfo, netlink.Attribute{Type: unix.NLA_F_NESTED | unix.NFTA_SET_DESC, Data: concatBytes})
+
+		descBytes = append(descBytes, concatBytes...)
+	}
+
+	if len(descBytes) > 0 {
+		// Marshal set description
+		tableInfo = append(tableInfo, netlink.Attribute{Type: unix.NLA_F_NESTED | unix.NFTA_SET_DESC, Data: descBytes})
 	}
 
 	// https://git.netfilter.org/libnftnl/tree/include/udata.h#n17
@@ -763,6 +785,20 @@ func setsFromMsg(msg netlink.Message) (*Set, error) {
 			data := ad.Bytes()
 			value, ok := userdata.GetUint32(data, userdata.NFTNL_UDATA_SET_MERGE_ELEMENTS)
 			set.AutoMerge = ok && value == 1
+		case unix.NFTA_SET_DESC:
+			nestedAD, err := netlink.NewAttributeDecoder(ad.Bytes())
+			if err != nil {
+				return nil, fmt.Errorf("nested NewAttributeDecoder() failed: %w", err)
+			}
+			for nestedAD.Next() {
+				switch nestedAD.Type() {
+				case unix.NFTA_SET_DESC_SIZE:
+					set.Size = binary.BigEndian.Uint32(nestedAD.Bytes())
+				}
+			}
+			if nestedAD.Err() != nil {
+				return nil, fmt.Errorf("decoding set description: %w", nestedAD.Err())
+			}
 		}
 	}
 	return &set, nil

set_test.go

@@ -1,7 +1,11 @@
 package nftables
 
 import (
+	"reflect"
 	"testing"
+	"time"
+
+	"github.com/mdlayher/netlink"
 )
 
 // unknownNFTMagic is an nftMagic value that's unhandled by this
@@ -185,3 +189,82 @@ func TestConcatSetTypeElements(t *testing.T) {
 		})
 	}
 }
+
+func TestMarshalSet(t *testing.T) {
+	t.Parallel()
+
+	tbl := &Table{
+		Name:   "ipv4table",
+		Family: TableFamilyIPv4,
+	}
+
+	c, err := New(WithTestDial(
+		func(req []netlink.Message) ([]netlink.Message, error) {
+			return req, nil
+		}))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	c.AddTable(tbl)
+
+	// Ensure the table is added.
+	const connMsgStart = 1
+	if len(c.messages) != connMsgStart {
+		t.Fatalf("AddSet() wrong start message count: %d, expected: %d", len(c.messages), connMsgStart)
+	}
+
+	tests := []struct {
+		name string
+		set  Set
+	}{
+		{
+			name: "Set without flags",
+			set: Set{
+				Name:    "test-set",
+				ID:      uint32(1),
+				Table:   tbl,
+				KeyType: TypeIPAddr,
+			},
+		},
+		{
+			name: "Set with size, timeout, dynamic flag specified",
+			set: Set{
+				Name:       "test-set",
+				ID:         uint32(2),
+				HasTimeout: true,
+				Dynamic:    true,
+				Size:       10,
+				Table:      tbl,
+				KeyType:    TypeIPAddr,
+				Timeout:    30 * time.Second,
+			},
+		},
+	}
+
+	for i, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := c.AddSet(&tt.set, nil); err != nil {
+				t.Fatal(err)
+			}
+
+			connMsgSetIdx := connMsgStart + i
+			if len(c.messages) != connMsgSetIdx+1 {
+				t.Fatalf("AddSet() wrong message count: %d, expected: %d", len(c.messages), connMsgSetIdx+1)
+			}
+			msg := c.messages[connMsgSetIdx]
+
+			nset, err := setsFromMsg(msg)
+			if err != nil {
+				t.Fatalf("setsFromMsg() error: %+v", err)
+			}
+
+			// Table pointer is set after flush, which is not implemented in the test.
+			tt.set.Table = nil
+
+			if !reflect.DeepEqual(&tt.set, nset) {
+				t.Fatalf("original %+v and recovered %+v Set structs are different", tt.set, nset)
+			}
+		})
+	}
+}