Set rule handle during flush

This change makes it possible to delete rules after inserting them,
without needing to query the rules first. Additionally, this allows
positioning a new rule next to an existing rule.

There are two ways to refer to a rule: Either by ID or by handle. The ID
is assigned by userspace, and is only valid within a transaction, so it
can only be used before the flush. The handle is assigned by the kernel
when the transaction is committed, and can thus only be used after the
flush. We thus need to set an ID on each newly created rule, and
retrieve the handle of the rule during the flush.

I extended the message struct with a pointer to the Rule which the
message creates. This allows calling the reply handler callback which
sets the handle.

I updated tests to add a handle to generated replies for the
NFT_MSG_NEWRULE messages.

chain.go

@@ -140,7 +140,7 @@ func (cc *Conn) AddChain(c *Chain) *Chain {
 			{Type: unix.NFTA_CHAIN_TYPE, Data: []byte(c.Type + "\x00")},
 		})...)
 	}
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_NEWCHAIN),
 			Flags: netlink.Request | netlink.Acknowledge | netlink.Create,
@@ -161,7 +161,7 @@ func (cc *Conn) DelChain(c *Chain) {
 		{Type: unix.NFTA_CHAIN_NAME, Data: []byte(c.Name + "\x00")},
 	})
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELCHAIN),
 			Flags: netlink.Request | netlink.Acknowledge,
@@ -179,7 +179,7 @@ func (cc *Conn) FlushChain(c *Chain) {
 		{Type: unix.NFTA_RULE_TABLE, Data: []byte(c.Table.Name + "\x00")},
 		{Type: unix.NFTA_RULE_CHAIN, Data: []byte(c.Name + "\x00")},
 	})
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELRULE),
 			Flags: netlink.Request | netlink.Acknowledge,

conn.go

@@ -41,7 +41,7 @@ type Conn struct {
 
 	lasting      bool       // establish a lasting connection to be used across multiple netlink operations.
 	mu           sync.Mutex // protects the following state
-	messages     []netlink.Message
+	messages     []netlinkMessage
 	err          error
 	nlconn       *netlink.Conn // netlink socket using NETLINK_NETFILTER protocol.
 	sockOptions  []SockOption
@@ -49,6 +49,12 @@ type Conn struct {
 	allocatedIDs uint32
 }
 
+type netlinkMessage struct {
+	Header netlink.Header
+	Data   []byte
+	rule   *Rule
+}
+
 // ConnOption is an option to change the behavior of the nftables Conn returned by Open.
 type ConnOption func(*Conn)
 
@@ -171,24 +177,6 @@ func receiveAckAware(nlconn *netlink.Conn, sentMsgFlags netlink.HeaderFlags) ([]
 		return reply, nil
 	}
 
-	if len(reply) != 0 {
-		last := reply[len(reply)-1]
-		for re := last.Header.Type; (re&netlink.Overrun) == netlink.Overrun && (re&netlink.Done) != netlink.Done; re = last.Header.Type {
-			// we are not finished, the message is overrun
-			r, err := nlconn.Receive()
-			if err != nil {
-				return nil, err
-			}
-			reply = append(reply, r...)
-			last = reply[len(reply)-1]
-		}
-
-		if last.Header.Type == netlink.Error && binaryutil.BigEndian.Uint32(last.Data[:4]) == 0 {
-			// we have already collected an ack
-			return reply, nil
-		}
-	}
-
 	// Now we expect an ack
 	ack, err := nlconn.Receive()
 	if err != nil {
@@ -196,8 +184,7 @@ func receiveAckAware(nlconn *netlink.Conn, sentMsgFlags netlink.HeaderFlags) ([]
 	}
 
 	if len(ack) == 0 {
-		// received an empty ack?
-		return reply, nil
+		return nil, errors.New("received an empty ack")
 	}
 
 	msg := ack[0]
@@ -263,15 +250,54 @@ func (cc *Conn) Flush() error {
 	}
 	defer func() { _ = closer() }()
 
-	if _, err := conn.SendMessages(batch(cc.messages)); err != nil {
+	messages, err := conn.SendMessages(batch(cc.messages))
+	if err != nil {
 		return fmt.Errorf("SendMessages: %w", err)
 	}
 
 	var errs error
+
+	// Fetch replies. Each message with the Echo flag triggers a reply of the same
+	// type. Additionally, if the first message of the batch has the Echo flag, we
+	// get a reply of type NFT_MSG_NEWGEN, which we ignore.
+	replyIndex := 0
+	for replyIndex < len(cc.messages) && cc.messages[replyIndex].Header.Flags&netlink.Echo == 0 {
+		replyIndex++
+	}
+	replies, err := conn.Receive()
+	for err == nil && len(replies) != 0 {
+		reply := replies[0]
+		if reply.Header.Type == netlink.Error && reply.Header.Sequence == messages[1].Header.Sequence {
+			// The next message is the acknowledgement for the first message in the
+			// batch; stop looking for replies.
+			break
+		} else if replyIndex < len(cc.messages) {
+			msg := messages[replyIndex+1]
+			if msg.Header.Sequence == reply.Header.Sequence && msg.Header.Type == reply.Header.Type {
+				// The only messages which set the echo flag are rule create messages.
+				err := cc.messages[replyIndex].rule.handleCreateReply(reply)
+				if err != nil {
+					errs = errors.Join(errs, err)
+				}
+				replyIndex++
+				for replyIndex < len(cc.messages) && cc.messages[replyIndex].Header.Flags&netlink.Echo == 0 {
+					replyIndex++
+				}
+			}
+		}
+		replies = replies[1:]
+		if len(replies) == 0 {
+			replies, err = conn.Receive()
+		}
+	}
+
 	// Fetch the requested acknowledgement for each message we sent.
-	for _, msg := range cc.messages {
-		if _, err := receiveAckAware(conn, msg.Header.Flags); err != nil {
-			if errors.Is(err, os.ErrPermission) || errors.Is(err, syscall.ENOBUFS) {
+	for i := range cc.messages {
+		if i != 0 {
+			_, err = conn.Receive()
+		}
+		if err != nil {
+			if errors.Is(err, os.ErrPermission) || errors.Is(err, syscall.ENOBUFS) || errors.Is(err, syscall.ENOMEM) {
 				// Kernel will only send one error to user space.
 				return err
 			}
@@ -282,6 +308,9 @@ func (cc *Conn) Flush() error {
 	if errs != nil {
 		return fmt.Errorf("conn.Receive: %w", errs)
 	}
+	if replyIndex < len(cc.messages) {
+		return fmt.Errorf("missing reply for message %d in batch", replyIndex)
+	}
 
 	return nil
 }
@@ -291,7 +320,7 @@ func (cc *Conn) Flush() error {
 func (cc *Conn) FlushRuleset() {
 	cc.mu.Lock()
 	defer cc.mu.Unlock()
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELTABLE),
 			Flags: netlink.Request | netlink.Acknowledge | netlink.Create,
@@ -350,26 +379,30 @@ func (cc *Conn) marshalExpr(fam byte, e expr.Any) []byte {
 	return b
 }
 
-func batch(messages []netlink.Message) []netlink.Message {
-	batch := []netlink.Message{
-		{
-			Header: netlink.Header{
-				Type:  netlink.HeaderType(unix.NFNL_MSG_BATCH_BEGIN),
-				Flags: netlink.Request,
-			},
-			Data: extraHeader(0, unix.NFNL_SUBSYS_NFTABLES),
+func batch(messages []netlinkMessage) []netlink.Message {
+	batch := make([]netlink.Message, len(messages)+2)
+	batch[0] = netlink.Message{
+		Header: netlink.Header{
+			Type:  netlink.HeaderType(unix.NFNL_MSG_BATCH_BEGIN),
+			Flags: netlink.Request,
 		},
+		Data: extraHeader(0, unix.NFNL_SUBSYS_NFTABLES),
 	}
 
-	batch = append(batch, messages...)
+	for i, msg := range messages {
+		batch[i+1] = netlink.Message{
+			Header: msg.Header,
+			Data:   msg.Data,
+		}
+	}
 
-	batch = append(batch, netlink.Message{
+	batch[len(messages)+1] = netlink.Message{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType(unix.NFNL_MSG_BATCH_END),
 			Flags: netlink.Request,
 		},
 		Data: extraHeader(0, unix.NFNL_SUBSYS_NFTABLES),
-	})
+	}
 
 	return batch
 }

flowtable.go

@@ -142,7 +142,7 @@ func (cc *Conn) AddFlowtable(f *Flowtable) *Flowtable {
 		{Type: unix.NLA_F_NESTED | NFTA_FLOWTABLE_HOOK, Data: cc.marshalAttr(hookAttr)},
 	})...)
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | NFT_MSG_NEWFLOWTABLE),
 			Flags: netlink.Request | netlink.Acknowledge | netlink.Create,
@@ -162,7 +162,7 @@ func (cc *Conn) DelFlowtable(f *Flowtable) {
 		{Type: NFTA_FLOWTABLE_NAME, Data: []byte(f.Name)},
 	})
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | NFT_MSG_DELFLOWTABLE),
 			Flags: netlink.Request | netlink.Acknowledge,

internal/nftest/nftest.go

@@ -8,7 +8,9 @@ import (
 	"testing"
 
 	"github.com/google/nftables"
+	"github.com/google/nftables/binaryutil"
 	"github.com/mdlayher/netlink"
+	"golang.org/x/sys/unix"
 )
 
 // Recorder provides an nftables connection that does not send to the Linux
@@ -21,14 +23,34 @@ type Recorder struct {
 // Conn opens an nftables connection that records netlink messages into the
 // Recorder.
 func (r *Recorder) Conn() (*nftables.Conn, error) {
+	nextHandle := uint64(1)
 	return nftables.New(nftables.WithTestDial(
 		func(req []netlink.Message) ([]netlink.Message, error) {
 			r.requests = append(r.requests, req...)
 
-			acks := make([]netlink.Message, 0, len(req))
+			replies := make([]netlink.Message, 0, len(req))
+			// Generate replies.
+			for _, msg := range req {
+				if msg.Header.Flags&netlink.Echo != 0 {
+					data := append([]byte{}, msg.Data...)
+					switch msg.Header.Type {
+					case netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_NEWRULE):
+						attrs, _ := netlink.MarshalAttributes([]netlink.Attribute{
+							{Type: unix.NFTA_RULE_HANDLE, Data: binaryutil.BigEndian.PutUint64(nextHandle)},
+						})
+						nextHandle++
+						data = append(data, attrs...)
+					}
+					replies = append(replies, netlink.Message{
+						Header: msg.Header,
+						Data:   data,
+					})
+				}
+			}
+			// Generate acknowledgements.
 			for _, msg := range req {
 				if msg.Header.Flags&netlink.Acknowledge != 0 {
-					acks = append(acks, netlink.Message{
+					replies = append(replies, netlink.Message{
 						Header: netlink.Header{
 							Length:   4,
 							Type:     netlink.Error,
@@ -39,7 +61,7 @@ func (r *Recorder) Conn() (*nftables.Conn, error) {
 					})
 				}
 			}
-			return acks, nil
+			return replies, nil
 		}))
 }
 

obj.go

@@ -124,7 +124,7 @@ func (cc *Conn) AddObj(o Obj) Obj {
 		attrs = append(attrs, netlink.Attribute{Type: unix.NLA_F_NESTED | unix.NFTA_OBJ_DATA, Data: data})
 	}
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_NEWOBJ),
 			Flags: netlink.Request | netlink.Acknowledge | netlink.Create,
@@ -146,7 +146,7 @@ func (cc *Conn) DeleteObject(o Obj) {
 	data := cc.marshalAttr(attrs)
 	data = append(data, cc.marshalAttr([]netlink.Attribute{{Type: unix.NLA_F_NESTED | unix.NFTA_OBJ_DATA}})...)
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELOBJ),
 			Flags: netlink.Request | netlink.Acknowledge,

rule.go

@@ -48,10 +48,13 @@ const (
 type Rule struct {
 	Table *Table
 	Chain *Chain
-	// Handle identifies an existing Rule.
+	// Handle identifies an existing Rule. For a new Rule, this field is set
+	// during the Flush() in which the rule is committed. Make sure to not access
+	// this field concurrently with this Flush() to avoid data races.
 	Handle uint64
 	// ID is an identifier for a new Rule, which is assigned by
 	// AddRule/InsertRule, and only valid before the rule is committed by Flush().
+	// The field is set to 0 during Flush().
 	ID uint32
 	// Position can be set to the Handle of another Rule to insert the new Rule
 	// before (InsertRule) or after (AddRule) the existing rule.
@@ -94,7 +97,7 @@ func (cc *Conn) GetRules(t *Table, c *Chain) ([]*Rule, error) {
 	message := netlink.Message{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_GETRULE),
-			Flags: netlink.Request | netlink.Acknowledge | netlink.Dump | unix.NLM_F_ECHO,
+			Flags: netlink.Request | netlink.Acknowledge | netlink.Dump,
 		},
 		Data: append(extraHeader(uint8(t.Family), 0), data...),
 	}
@@ -164,20 +167,23 @@ func (cc *Conn) newRule(r *Rule, op ruleOperation) *Rule {
 	msgData := []byte{}
 
 	msgData = append(msgData, data...)
-	var flags netlink.HeaderFlags
 	if r.UserData != nil {
 		msgData = append(msgData, cc.marshalAttr([]netlink.Attribute{
 			{Type: unix.NFTA_RULE_USERDATA, Data: r.UserData},
 		})...)
 	}
 
+	var flags netlink.HeaderFlags
+	var ruleRef *Rule
 	switch op {
 	case operationAdd:
-		flags = netlink.Request | netlink.Acknowledge | netlink.Create | unix.NLM_F_ECHO | unix.NLM_F_APPEND
+		flags = netlink.Request | netlink.Acknowledge | netlink.Create | netlink.Echo | netlink.Append
+		ruleRef = r
 	case operationInsert:
-		flags = netlink.Request | netlink.Acknowledge | netlink.Create | unix.NLM_F_ECHO
+		flags = netlink.Request | netlink.Acknowledge | netlink.Create | netlink.Echo
+		ruleRef = r
 	case operationReplace:
-		flags = netlink.Request | netlink.Acknowledge | netlink.Replace | unix.NLM_F_ECHO | unix.NLM_F_REPLACE
+		flags = netlink.Request | netlink.Acknowledge | netlink.Replace
 	}
 
 	if r.Position != 0 || (r.Flags&(1<<unix.NFTA_RULE_POSITION)) != 0 {
@@ -190,17 +196,42 @@ func (cc *Conn) newRule(r *Rule, op ruleOperation) *Rule {
 		})...)
 	}
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  newRuleHeaderType,
 			Flags: flags,
 		},
 		Data: append(extraHeader(uint8(r.Table.Family), 0), msgData...),
+		rule: ruleRef,
 	})
 
 	return r
 }
 
+func (r *Rule) handleCreateReply(reply netlink.Message) error {
+	ad, err := netlink.NewAttributeDecoder(reply.Data[4:])
+	if err != nil {
+		return err
+	}
+	ad.ByteOrder = binary.BigEndian
+	var handle uint64
+	for ad.Next() {
+		switch ad.Type() {
+		case unix.NFTA_RULE_HANDLE:
+			handle = ad.Uint64()
+		}
+	}
+	if ad.Err() != nil {
+		return ad.Err()
+	}
+	if handle == 0 {
+		return fmt.Errorf("missing rule handle in create reply")
+	}
+	r.Handle = handle
+	r.ID = 0
+	return nil
+}
+
 func (cc *Conn) ReplaceRule(r *Rule) *Rule {
 	return cc.newRule(r, operationReplace)
 }
@@ -247,7 +278,7 @@ func (cc *Conn) DelRule(r *Rule) error {
 	}
 	flags := netlink.Request | netlink.Acknowledge
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  delRuleHeaderType,
 			Flags: flags,

set.go

@@ -506,7 +506,7 @@ func (cc *Conn) appendElemList(s *Set, vals []SetElement, hdrType uint16) error
 			{Type: unix.NFTA_SET_ELEM_LIST_ELEMENTS | unix.NLA_F_NESTED, Data: encodedElem},
 		}
 
-		cc.messages = append(cc.messages, netlink.Message{
+		cc.messages = append(cc.messages, netlinkMessage{
 			Header: netlink.Header{
 				Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | hdrType),
 				Flags: netlink.Request | netlink.Acknowledge | netlink.Create,
@@ -680,7 +680,7 @@ func (cc *Conn) AddSet(s *Set, vals []SetElement) error {
 		tableInfo = append(tableInfo, netlink.Attribute{Type: unix.NLA_F_NESTED | NFTA_SET_ELEM_EXPRESSIONS, Data: data})
 	}
 
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_NEWSET),
 			Flags: netlink.Request | netlink.Acknowledge | netlink.Create,
@@ -700,7 +700,7 @@ func (cc *Conn) DelSet(s *Set) {
 		{Type: unix.NFTA_SET_TABLE, Data: []byte(s.Table.Name + "\x00")},
 		{Type: unix.NFTA_SET_NAME, Data: []byte(s.Name + "\x00")},
 	})
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELSET),
 			Flags: netlink.Request | netlink.Acknowledge,
@@ -717,7 +717,7 @@ func (cc *Conn) FlushSet(s *Set) {
 		{Type: unix.NFTA_SET_TABLE, Data: []byte(s.Table.Name + "\x00")},
 		{Type: unix.NFTA_SET_NAME, Data: []byte(s.Name + "\x00")},
 	})
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELSETELEM),
 			Flags: netlink.Request | netlink.Acknowledge,

set_test.go

@@ -254,7 +254,10 @@ func TestMarshalSet(t *testing.T) {
 			}
 			msg := c.messages[connMsgSetIdx]
 
-			nset, err := setsFromMsg(msg)
+			nset, err := setsFromMsg(netlink.Message{
+				Header: msg.Header,
+				Data:   msg.Data,
+			})
 			if err != nil {
 				t.Fatalf("setsFromMsg() error: %+v", err)
 			}

table.go

@@ -57,7 +57,7 @@ func (cc *Conn) DelTable(t *Table) {
 		{Type: unix.NFTA_TABLE_NAME, Data: []byte(t.Name + "\x00")},
 		{Type: unix.NFTA_TABLE_FLAGS, Data: []byte{0, 0, 0, 0}},
 	})
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELTABLE),
 			Flags: netlink.Request | netlink.Acknowledge,
@@ -73,7 +73,7 @@ func (cc *Conn) addTable(t *Table, flag netlink.HeaderFlags) *Table {
 		{Type: unix.NFTA_TABLE_NAME, Data: []byte(t.Name + "\x00")},
 		{Type: unix.NFTA_TABLE_FLAGS, Data: []byte{0, 0, 0, 0}},
 	})
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_NEWTABLE),
 			Flags: netlink.Request | netlink.Acknowledge | flag,
@@ -103,7 +103,7 @@ func (cc *Conn) FlushTable(t *Table) {
 	data := cc.marshalAttr([]netlink.Attribute{
 		{Type: unix.NFTA_RULE_TABLE, Data: []byte(t.Name + "\x00")},
 	})
-	cc.messages = append(cc.messages, netlink.Message{
+	cc.messages = append(cc.messages, netlinkMessage{
 		Header: netlink.Header{
 			Type:  netlink.HeaderType((unix.NFNL_SUBSYS_NFTABLES << 8) | unix.NFT_MSG_DELRULE),
 			Flags: netlink.Request | netlink.Acknowledge,