/* SPDX-License-Identifier: MIT
 *
 * Copyright (C) 2017-2022 WireGuard LLC. All Rights Reserved.
 */

package tun

import (
	"errors"
	"fmt"
	"net"
	"os"
	"sync"
	"syscall"
	"unsafe"

	"golang.org/x/net/ipv6"
	"golang.org/x/sys/unix"
)

// Structure for iface mtu get/set ioctls
type ifreq_mtu struct {
	Name [unix.IFNAMSIZ]byte
	MTU  uint32
	Pad0 [12]byte
}

const _TUNSIFMODE = 0x8004745d

type NativeTun struct {
	name        string
	tunFile     *os.File
	events      chan Event
	errors      chan error
	routeSocket int
	closeOnce   sync.Once
}

func (tun *NativeTun) routineRouteListener(tunIfindex int) {
	var (
		statusUp  bool
		statusMTU int
	)

	defer close(tun.events)

	check := func() bool {
		iface, err := net.InterfaceByIndex(tunIfindex)
		if err != nil {
			tun.errors <- err
			return true
		}

		// Up / Down event
		up := (iface.Flags & net.FlagUp) != 0
		if up != statusUp && up {
			tun.events <- EventUp
		}
		if up != statusUp && !up {
			tun.events <- EventDown
		}
		statusUp = up

		// MTU changes
		if iface.MTU != statusMTU {
			tun.events <- EventMTUUpdate
		}
		statusMTU = iface.MTU
		return false
	}

	if check() {
		return
	}

	data := make([]byte, os.Getpagesize())
	for {
		n, err := unix.Read(tun.routeSocket, data)
		if err != nil {
			if errno, ok := err.(syscall.Errno); ok && errno == syscall.EINTR {
				continue
			}
			tun.errors <- err
			return
		}

		if n < 8 {
			continue
		}

		if data[3 /* type */] != unix.RTM_IFINFO {
			continue
		}
		ifindex := int(*(*uint16)(unsafe.Pointer(&data[6 /* ifindex */])))
		if ifindex != tunIfindex {
			continue
		}
		if check() {
			return
		}
	}
}

func CreateTUN(name string, mtu int) (Device, error) {
	ifIndex := -1
	if name != "tun" {
		_, err := fmt.Sscanf(name, "tun%d", &ifIndex)
		if err != nil || ifIndex < 0 {
			return nil, fmt.Errorf("Interface name must be tun[0-9]*")
		}
	}

	var tunfile *os.File
	var err error

	if ifIndex != -1 {
		tunfile, err = os.OpenFile(fmt.Sprintf("/dev/tun%d", ifIndex), unix.O_RDWR|unix.O_CLOEXEC, 0)
	} else {
		for ifIndex = 0; ifIndex < 256; ifIndex++ {
			tunfile, err = os.OpenFile(fmt.Sprintf("/dev/tun%d", ifIndex), unix.O_RDWR|unix.O_CLOEXEC, 0)
			if err == nil || !errors.Is(err, syscall.EBUSY) {
				break
			}
		}
	}

	if err != nil {
		return nil, err
	}

	tun, err := CreateTUNFromFile(tunfile, mtu)

	if err == nil && name == "tun" {
		fname := os.Getenv("WG_TUN_NAME_FILE")
		if fname != "" {
			os.WriteFile(fname, []byte(tun.(*NativeTun).name+"\n"), 0o400)
		}
	}

	return tun, err
}

func CreateTUNFromFile(file *os.File, mtu int) (Device, error) {
	tun := &NativeTun{
		tunFile: file,
		events:  make(chan Event, 10),
		errors:  make(chan error, 1),
	}

	name, err := tun.Name()
	if err != nil {
		tun.tunFile.Close()
		return nil, err
	}

	tunIfindex, err := func() (int, error) {
		iface, err := net.InterfaceByName(name)
		if err != nil {
			return -1, err
		}
		return iface.Index, nil
	}()
	if err != nil {
		tun.tunFile.Close()
		return nil, err
	}

	tun.routeSocket, err = unix.Socket(unix.AF_ROUTE, unix.SOCK_RAW|unix.SOCK_CLOEXEC, unix.AF_UNSPEC)
	if err != nil {
		tun.tunFile.Close()
		return nil, err
	}

	go tun.routineRouteListener(tunIfindex)

	currentMTU, err := tun.MTU()
	if err != nil || currentMTU != mtu {
		err = tun.setMTU(mtu)
		if err != nil {
			tun.Close()
			return nil, err
		}
	}

	return tun, nil
}

func (tun *NativeTun) Name() (string, error) {
	gostat, err := tun.tunFile.Stat()
	if err != nil {
		tun.name = ""
		return "", err
	}
	stat := gostat.Sys().(*syscall.Stat_t)
	tun.name = fmt.Sprintf("tun%d", stat.Rdev%256)
	return tun.name, nil
}

func (tun *NativeTun) File() *os.File {
	return tun.tunFile
}

func (tun *NativeTun) Events() chan Event {
	return tun.events
}

func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
	select {
	case err := <-tun.errors:
		return 0, err
	default:
		buff := buff[offset-4:]
		n, err := tun.tunFile.Read(buff[:])
		if n < 4 {
			return 0, err
		}
		return n - 4, err
	}
}

func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
	// reserve space for header

	buff = buff[offset-4:]

	// add packet information header

	buff[0] = 0x00
	buff[1] = 0x00
	buff[2] = 0x00

	if buff[4]>>4 == ipv6.Version {
		buff[3] = unix.AF_INET6
	} else {
		buff[3] = unix.AF_INET
	}

	// write

	return tun.tunFile.Write(buff)
}

func (tun *NativeTun) Flush() error {
	// TODO: can flushing be implemented by buffering and using sendmmsg?
	return nil
}

func (tun *NativeTun) Close() error {
	var err1, err2 error
	tun.closeOnce.Do(func() {
		err1 = tun.tunFile.Close()
		if tun.routeSocket != -1 {
			unix.Shutdown(tun.routeSocket, unix.SHUT_RDWR)
			err2 = unix.Close(tun.routeSocket)
			tun.routeSocket = -1
		} else if tun.events != nil {
			close(tun.events)
		}
	})
	if err1 != nil {
		return err1
	}
	return err2
}

func (tun *NativeTun) setMTU(n int) error {
	// open datagram socket

	var fd int

	fd, err := unix.Socket(
		unix.AF_INET,
		unix.SOCK_DGRAM|unix.SOCK_CLOEXEC,
		0,
	)
	if err != nil {
		return err
	}

	defer unix.Close(fd)

	// do ioctl call

	var ifr ifreq_mtu
	copy(ifr.Name[:], tun.name)
	ifr.MTU = uint32(n)

	_, _, errno := unix.Syscall(
		unix.SYS_IOCTL,
		uintptr(fd),
		uintptr(unix.SIOCSIFMTU),
		uintptr(unsafe.Pointer(&ifr)),
	)

	if errno != 0 {
		return fmt.Errorf("failed to set MTU on %s", tun.name)
	}

	return nil
}

func (tun *NativeTun) MTU() (int, error) {
	// open datagram socket

	fd, err := unix.Socket(
		unix.AF_INET,
		unix.SOCK_DGRAM|unix.SOCK_CLOEXEC,
		0,
	)
	if err != nil {
		return 0, err
	}

	defer unix.Close(fd)

	// do ioctl call
	var ifr ifreq_mtu
	copy(ifr.Name[:], tun.name)

	_, _, errno := unix.Syscall(
		unix.SYS_IOCTL,
		uintptr(fd),
		uintptr(unix.SIOCGIFMTU),
		uintptr(unsafe.Pointer(&ifr)),
	)
	if errno != 0 {
		return 0, fmt.Errorf("failed to get MTU on %s", tun.name)
	}

	return int(*(*int32)(unsafe.Pointer(&ifr.MTU))), nil
}
