Implement socket expr

Signed-off-by: black-desk <me@black-desk.cn>

expr/socket.go

@@ -0,0 +1,75 @@
+package expr
+
+import (
+	"encoding/binary"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/google/nftables/binaryutil"
+	"github.com/mdlayher/netlink"
+)
+
+type Socket struct {
+	Key      SocketKey
+	Level    uint32
+	Register uint32
+}
+
+type SocketKey uint32
+
+const (
+	// TODO, Once the constants below are available in golang.org/x/sys/unix, switch to use those.
+	NFTA_SOCKET_KEY   = 1
+	NFTA_SOCKET_DREG  = 2
+	NFTA_SOCKET_LEVEL = 3
+
+	NFT_SOCKET_TRANSPARENT = 0
+	NFT_SOCKET_MARK        = 1
+	NFT_SOCKET_WILDCARD    = 2
+	NFT_SOCKET_CGROUPV2    = 3
+
+	SocketKeyTransparent SocketKey = NFT_SOCKET_TRANSPARENT
+	SocketKeyMark        SocketKey = NFT_SOCKET_MARK
+	SocketKeyWildcard    SocketKey = NFT_SOCKET_WILDCARD
+	SocketKeyCgroupv2    SocketKey = NFT_SOCKET_CGROUPV2
+)
+
+func (e *Socket) marshal(fam byte) ([]byte, error) {
+	// NOTE: Socket.Level is only used when Socket.Key == SocketKeyCgroupv2. But `nft` always encoding it. Check link below:
+	// http://git.netfilter.org/nftables/tree/src/netlink_linearize.c?id=0583bac241ea18c9d7f61cb20ca04faa1e043b78#n319
+	exprData, err := netlink.MarshalAttributes(
+		[]netlink.Attribute{
+			{Type: NFTA_SOCKET_DREG, Data: binaryutil.BigEndian.PutUint32(e.Register)},
+			{Type: NFTA_SOCKET_KEY, Data: binaryutil.BigEndian.PutUint32(uint32(e.Key))},
+			{Type: NFTA_SOCKET_LEVEL, Data: binaryutil.BigEndian.PutUint32(uint32(e.Level))},
+		},
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return netlink.MarshalAttributes([]netlink.Attribute{
+		{Type: unix.NFTA_EXPR_NAME, Data: []byte("socket\x00")},
+		{Type: unix.NLA_F_NESTED | unix.NFTA_EXPR_DATA, Data: exprData},
+	})
+}
+
+func (e *Socket) unmarshal(fam byte, data []byte) error {
+	ad, err := netlink.NewAttributeDecoder(data)
+	if err != nil {
+		return err
+	}
+
+	ad.ByteOrder = binary.BigEndian
+	for ad.Next() {
+		switch ad.Type() {
+		case NFTA_SOCKET_DREG:
+			e.Register = ad.Uint32()
+		case NFTA_SOCKET_KEY:
+			e.Key = SocketKey(ad.Uint32())
+		case NFTA_SOCKET_LEVEL:
+			e.Level = ad.Uint32()
+		}
+	}
+	return ad.Err()
+}