// Copyright 2018 Google LLC. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package expr

import (
	"encoding/binary"

	"github.com/google/nftables/binaryutil"
	"github.com/mdlayher/netlink"
	"golang.org/x/sys/unix"
)

type ExthdrOp uint32

const (
	ExthdrOpIpv6   ExthdrOp = unix.NFT_EXTHDR_OP_IPV6
	ExthdrOpTcpopt ExthdrOp = unix.NFT_EXTHDR_OP_TCPOPT
)

type Exthdr struct {
	DestRegister   uint32
	Type           uint8
	Offset         uint32
	Len            uint32
	Flags          uint32
	Op             ExthdrOp
	SourceRegister uint32
}

func (e *Exthdr) marshal() ([]byte, error) {
	var attr []netlink.Attribute

	// Operations are differentiated by the Op and whether the SourceRegister
	// or DestRegister is set. Mixing them results in EOPNOTSUPP.
	if e.SourceRegister != 0 {
		attr = []netlink.Attribute{
			{Type: unix.NFTA_EXTHDR_SREG, Data: binaryutil.BigEndian.PutUint32(e.SourceRegister)}}
	} else {
		attr = []netlink.Attribute{
			netlink.Attribute{Type: unix.NFTA_EXTHDR_DREG, Data: binaryutil.BigEndian.PutUint32(e.DestRegister)}}
	}

	attr = append(attr,
		netlink.Attribute{Type: unix.NFTA_EXTHDR_TYPE, Data: []byte{e.Type}},
		netlink.Attribute{Type: unix.NFTA_EXTHDR_OFFSET, Data: binaryutil.BigEndian.PutUint32(e.Offset)},
		netlink.Attribute{Type: unix.NFTA_EXTHDR_LEN, Data: binaryutil.BigEndian.PutUint32(e.Len)},
		netlink.Attribute{Type: unix.NFTA_EXTHDR_OP, Data: binaryutil.BigEndian.PutUint32(uint32(e.Op))})

	// Flags is only set if DREG is set
	if e.DestRegister != 0 {
		attr = append(attr,
			netlink.Attribute{Type: unix.NFTA_EXTHDR_FLAGS, Data: binaryutil.BigEndian.PutUint32(e.Flags)})
	}

	data, err := netlink.MarshalAttributes(attr)
	if err != nil {
		return nil, err
	}
	return netlink.MarshalAttributes([]netlink.Attribute{
		{Type: unix.NFTA_EXPR_NAME, Data: []byte("exthdr\x00")},
		{Type: unix.NLA_F_NESTED | unix.NFTA_EXPR_DATA, Data: data},
	})
}

func (e *Exthdr) unmarshal(data []byte) error {
	ad, err := netlink.NewAttributeDecoder(data)
	if err != nil {
		return err
	}
	ad.ByteOrder = binary.BigEndian
	for ad.Next() {
		switch ad.Type() {
		case unix.NFTA_EXTHDR_DREG:
			e.DestRegister = ad.Uint32()
		case unix.NFTA_EXTHDR_TYPE:
			e.Type = ad.Uint8()
		case unix.NFTA_EXTHDR_OFFSET:
			e.Offset = ad.Uint32()
		case unix.NFTA_EXTHDR_LEN:
			e.Len = ad.Uint32()
		case unix.NFTA_EXTHDR_FLAGS:
			e.Flags = ad.Uint32()
		case unix.NFTA_EXTHDR_OP:
			e.Op = ExthdrOp(ad.Uint32())
		case unix.NFTA_EXTHDR_SREG:
			e.SourceRegister = ad.Uint32()
		}
	}
	return ad.Err()
}