// Copyright 2018 The gVisor Authors.
//
// 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 ipv4 contains the implementation of the ipv4 network protocol. To use
// it in the networking stack, this package must be added to the project, and
// activated on the stack by passing ipv4.NewProtocol() as one of the network
// protocols when calling stack.New(). Then endpoints can be created by passing
// ipv4.ProtocolNumber as the network protocol number when calling
// Stack.NewEndpoint().
package ipv4

import (
	"sync/atomic"

	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/header"
	"github.com/google/netstack/tcpip/network/fragmentation"
	"github.com/google/netstack/tcpip/network/hash"
	"github.com/google/netstack/tcpip/stack"
)

const (
	// ProtocolNumber is the ipv4 protocol number.
	ProtocolNumber = header.IPv4ProtocolNumber

	// MaxTotalSize is maximum size that can be encoded in the 16-bit
	// TotalLength field of the ipv4 header.
	MaxTotalSize = 0xffff

	// DefaultTTL is the default time-to-live value for this endpoint.
	DefaultTTL = 64

	// buckets is the number of identifier buckets.
	buckets = 2048
)

type endpoint struct {
	nicid         tcpip.NICID
	id            stack.NetworkEndpointID
	prefixLen     int
	linkEP        stack.LinkEndpoint
	dispatcher    stack.TransportDispatcher
	fragmentation *fragmentation.Fragmentation
	protocol      *protocol
}

// NewEndpoint creates a new ipv4 endpoint.
func (p *protocol) NewEndpoint(nicid tcpip.NICID, addrWithPrefix tcpip.AddressWithPrefix, linkAddrCache stack.LinkAddressCache, dispatcher stack.TransportDispatcher, linkEP stack.LinkEndpoint) (stack.NetworkEndpoint, *tcpip.Error) {
	e := &endpoint{
		nicid:         nicid,
		id:            stack.NetworkEndpointID{LocalAddress: addrWithPrefix.Address},
		prefixLen:     addrWithPrefix.PrefixLen,
		linkEP:        linkEP,
		dispatcher:    dispatcher,
		fragmentation: fragmentation.NewFragmentation(fragmentation.HighFragThreshold, fragmentation.LowFragThreshold, fragmentation.DefaultReassembleTimeout),
		protocol:      p,
	}

	return e, nil
}

// DefaultTTL is the default time-to-live value for this endpoint.
func (e *endpoint) DefaultTTL() uint8 {
	return e.protocol.DefaultTTL()
}

// MTU implements stack.NetworkEndpoint.MTU. It returns the link-layer MTU minus
// the network layer max header length.
func (e *endpoint) MTU() uint32 {
	return calculateMTU(e.linkEP.MTU())
}

// Capabilities implements stack.NetworkEndpoint.Capabilities.
func (e *endpoint) Capabilities() stack.LinkEndpointCapabilities {
	return e.linkEP.Capabilities()
}

// NICID returns the ID of the NIC this endpoint belongs to.
func (e *endpoint) NICID() tcpip.NICID {
	return e.nicid
}

// ID returns the ipv4 endpoint ID.
func (e *endpoint) ID() *stack.NetworkEndpointID {
	return &e.id
}

// PrefixLen returns the ipv4 endpoint subnet prefix length in bits.
func (e *endpoint) PrefixLen() int {
	return e.prefixLen
}

// MaxHeaderLength returns the maximum length needed by ipv4 headers (and
// underlying protocols).
func (e *endpoint) MaxHeaderLength() uint16 {
	return e.linkEP.MaxHeaderLength() + header.IPv4MinimumSize
}

// GSOMaxSize returns the maximum GSO packet size.
func (e *endpoint) GSOMaxSize() uint32 {
	if gso, ok := e.linkEP.(stack.GSOEndpoint); ok {
		return gso.GSOMaxSize()
	}
	return 0
}

// writePacketFragments calls e.linkEP.WritePacket with each packet fragment to
// write. It assumes that the IP header is entirely in hdr but does not assume
// that only the IP header is in hdr. It assumes that the input packet's stated
// length matches the length of the hdr+payload. mtu includes the IP header and
// options. This does not support the DontFragment IP flag.
func (e *endpoint) writePacketFragments(r *stack.Route, gso *stack.GSO, hdr buffer.Prependable, payload buffer.VectorisedView, mtu int) *tcpip.Error {
	// This packet is too big, it needs to be fragmented.
	ip := header.IPv4(hdr.View())
	flags := ip.Flags()

	// Update mtu to take into account the header, which will exist in all
	// fragments anyway.
	innerMTU := mtu - int(ip.HeaderLength())

	// Round the MTU down to align to 8 bytes. Then calculate the number of
	// fragments. Calculate fragment sizes as in RFC791.
	innerMTU &^= 7
	n := (int(ip.PayloadLength()) + innerMTU - 1) / innerMTU

	outerMTU := innerMTU + int(ip.HeaderLength())
	offset := ip.FragmentOffset()
	originalAvailableLength := hdr.AvailableLength()
	for i := 0; i < n; i++ {
		// Where possible, the first fragment that is sent has the same
		// hdr.UsedLength() as the input packet. The link-layer endpoint may depends
		// on this for looking at, eg, L4 headers.
		h := ip
		if i > 0 {
			hdr = buffer.NewPrependable(int(ip.HeaderLength()) + originalAvailableLength)
			h = header.IPv4(hdr.Prepend(int(ip.HeaderLength())))
			copy(h, ip[:ip.HeaderLength()])
		}
		if i != n-1 {
			h.SetTotalLength(uint16(outerMTU))
			h.SetFlagsFragmentOffset(flags|header.IPv4FlagMoreFragments, offset)
		} else {
			h.SetTotalLength(uint16(h.HeaderLength()) + uint16(payload.Size()))
			h.SetFlagsFragmentOffset(flags, offset)
		}
		h.SetChecksum(0)
		h.SetChecksum(^h.CalculateChecksum())
		offset += uint16(innerMTU)
		if i > 0 {
			newPayload := payload.Clone([]buffer.View{})
			newPayload.CapLength(innerMTU)
			if err := e.linkEP.WritePacket(r, gso, hdr, newPayload, ProtocolNumber); err != nil {
				return err
			}
			r.Stats().IP.PacketsSent.Increment()
			payload.TrimFront(newPayload.Size())
			continue
		}
		// Special handling for the first fragment because it comes from the hdr.
		if outerMTU >= hdr.UsedLength() {
			// This fragment can fit all of hdr and possibly some of payload, too.
			newPayload := payload.Clone([]buffer.View{})
			newPayloadLength := outerMTU - hdr.UsedLength()
			newPayload.CapLength(newPayloadLength)
			if err := e.linkEP.WritePacket(r, gso, hdr, newPayload, ProtocolNumber); err != nil {
				return err
			}
			r.Stats().IP.PacketsSent.Increment()
			payload.TrimFront(newPayloadLength)
		} else {
			// The fragment is too small to fit all of hdr.
			startOfHdr := hdr
			startOfHdr.TrimBack(hdr.UsedLength() - outerMTU)
			emptyVV := buffer.NewVectorisedView(0, []buffer.View{})
			if err := e.linkEP.WritePacket(r, gso, startOfHdr, emptyVV, ProtocolNumber); err != nil {
				return err
			}
			r.Stats().IP.PacketsSent.Increment()
			// Add the unused bytes of hdr into the payload that remains to be sent.
			restOfHdr := hdr.View()[outerMTU:]
			tmp := buffer.NewVectorisedView(len(restOfHdr), []buffer.View{buffer.NewViewFromBytes(restOfHdr)})
			tmp.Append(payload)
			payload = tmp
		}
	}
	return nil
}

// WritePacket writes a packet to the given destination address and protocol.
func (e *endpoint) WritePacket(r *stack.Route, gso *stack.GSO, hdr buffer.Prependable, payload buffer.VectorisedView, params stack.NetworkHeaderParams, loop stack.PacketLooping) *tcpip.Error {
	ip := header.IPv4(hdr.Prepend(header.IPv4MinimumSize))
	length := uint16(hdr.UsedLength() + payload.Size())
	id := uint32(0)
	if length > header.IPv4MaximumHeaderSize+8 {
		// Packets of 68 bytes or less are required by RFC 791 to not be
		// fragmented, so we only assign ids to larger packets.
		id = atomic.AddUint32(&e.protocol.ids[hashRoute(r, params.Protocol, e.protocol.hashIV)%buckets], 1)
	}
	ip.Encode(&header.IPv4Fields{
		IHL:         header.IPv4MinimumSize,
		TotalLength: length,
		ID:          uint16(id),
		TTL:         params.TTL,
		TOS:         params.TOS,
		Protocol:    uint8(params.Protocol),
		SrcAddr:     r.LocalAddress,
		DstAddr:     r.RemoteAddress,
	})
	ip.SetChecksum(^ip.CalculateChecksum())

	if loop&stack.PacketLoop != 0 {
		views := make([]buffer.View, 1, 1+len(payload.Views()))
		views[0] = hdr.View()
		views = append(views, payload.Views()...)
		vv := buffer.NewVectorisedView(len(views[0])+payload.Size(), views)
		loopedR := r.MakeLoopedRoute()
		e.HandlePacket(&loopedR, vv)
		loopedR.Release()
	}
	if loop&stack.PacketOut == 0 {
		return nil
	}
	if hdr.UsedLength()+payload.Size() > int(e.linkEP.MTU()) && (gso == nil || gso.Type == stack.GSONone) {
		return e.writePacketFragments(r, gso, hdr, payload, int(e.linkEP.MTU()))
	}
	if err := e.linkEP.WritePacket(r, gso, hdr, payload, ProtocolNumber); err != nil {
		return err
	}
	r.Stats().IP.PacketsSent.Increment()
	return nil
}

// WriteHeaderIncludedPacket writes a packet already containing a network
// header through the given route.
func (e *endpoint) WriteHeaderIncludedPacket(r *stack.Route, payload buffer.VectorisedView, loop stack.PacketLooping) *tcpip.Error {
	// The packet already has an IP header, but there are a few required
	// checks.
	ip := header.IPv4(payload.First())
	if !ip.IsValid(payload.Size()) {
		return tcpip.ErrInvalidOptionValue
	}

	// Always set the total length.
	ip.SetTotalLength(uint16(payload.Size()))

	// Set the source address when zero.
	if ip.SourceAddress() == tcpip.Address(([]byte{0, 0, 0, 0})) {
		ip.SetSourceAddress(r.LocalAddress)
	}

	// Set the destination. If the packet already included a destination,
	// it will be part of the route.
	ip.SetDestinationAddress(r.RemoteAddress)

	// Set the packet ID when zero.
	if ip.ID() == 0 {
		id := uint32(0)
		if payload.Size() > header.IPv4MaximumHeaderSize+8 {
			// Packets of 68 bytes or less are required by RFC 791 to not be
			// fragmented, so we only assign ids to larger packets.
			id = atomic.AddUint32(&e.protocol.ids[hashRoute(r, 0 /* protocol */, e.protocol.hashIV)%buckets], 1)
		}
		ip.SetID(uint16(id))
	}

	// Always set the checksum.
	ip.SetChecksum(0)
	ip.SetChecksum(^ip.CalculateChecksum())

	if loop&stack.PacketLoop != 0 {
		e.HandlePacket(r, payload)
	}
	if loop&stack.PacketOut == 0 {
		return nil
	}

	hdr := buffer.NewPrependableFromView(payload.ToView())
	r.Stats().IP.PacketsSent.Increment()
	return e.linkEP.WritePacket(r, nil /* gso */, hdr, buffer.VectorisedView{}, ProtocolNumber)
}

// HandlePacket is called by the link layer when new ipv4 packets arrive for
// this endpoint.
func (e *endpoint) HandlePacket(r *stack.Route, vv buffer.VectorisedView) {
	headerView := vv.First()
	h := header.IPv4(headerView)
	if !h.IsValid(vv.Size()) {
		r.Stats().IP.MalformedPacketsReceived.Increment()
		return
	}

	hlen := int(h.HeaderLength())
	tlen := int(h.TotalLength())
	vv.TrimFront(hlen)
	vv.CapLength(tlen - hlen)

	more := (h.Flags() & header.IPv4FlagMoreFragments) != 0
	if more || h.FragmentOffset() != 0 {
		if vv.Size() == 0 {
			// Drop the packet as it's marked as a fragment but has
			// no payload.
			r.Stats().IP.MalformedPacketsReceived.Increment()
			r.Stats().IP.MalformedFragmentsReceived.Increment()
			return
		}
		// The packet is a fragment, let's try to reassemble it.
		last := h.FragmentOffset() + uint16(vv.Size()) - 1
		// Drop the packet if the fragmentOffset is incorrect. i.e the
		// combination of fragmentOffset and vv.size() causes a wrap
		// around resulting in last being less than the offset.
		if last < h.FragmentOffset() {
			r.Stats().IP.MalformedPacketsReceived.Increment()
			r.Stats().IP.MalformedFragmentsReceived.Increment()
			return
		}
		var ready bool
		var err error
		vv, ready, err = e.fragmentation.Process(hash.IPv4FragmentHash(h), h.FragmentOffset(), last, more, vv)
		if err != nil {
			r.Stats().IP.MalformedPacketsReceived.Increment()
			r.Stats().IP.MalformedFragmentsReceived.Increment()
			return
		}
		if !ready {
			return
		}
	}
	p := h.TransportProtocol()
	if p == header.ICMPv4ProtocolNumber {
		headerView.CapLength(hlen)
		e.handleICMP(r, headerView, vv)
		return
	}
	r.Stats().IP.PacketsDelivered.Increment()
	e.dispatcher.DeliverTransportPacket(r, p, headerView, vv)
}

// Close cleans up resources associated with the endpoint.
func (e *endpoint) Close() {}

type protocol struct {
	ids    []uint32
	hashIV uint32

	// defaultTTL is the current default TTL for the protocol. Only the
	// uint8 portion of it is meaningful and it must be accessed
	// atomically.
	defaultTTL uint32
}

// Number returns the ipv4 protocol number.
func (p *protocol) Number() tcpip.NetworkProtocolNumber {
	return ProtocolNumber
}

// MinimumPacketSize returns the minimum valid ipv4 packet size.
func (p *protocol) MinimumPacketSize() int {
	return header.IPv4MinimumSize
}

// DefaultPrefixLen returns the IPv4 default prefix length.
func (p *protocol) DefaultPrefixLen() int {
	return header.IPv4AddressSize * 8
}

// ParseAddresses implements NetworkProtocol.ParseAddresses.
func (*protocol) ParseAddresses(v buffer.View) (src, dst tcpip.Address) {
	h := header.IPv4(v)
	return h.SourceAddress(), h.DestinationAddress()
}

// SetOption implements NetworkProtocol.SetOption.
func (p *protocol) SetOption(option interface{}) *tcpip.Error {
	switch v := option.(type) {
	case tcpip.DefaultTTLOption:
		p.SetDefaultTTL(uint8(v))
		return nil
	default:
		return tcpip.ErrUnknownProtocolOption
	}
}

// Option implements NetworkProtocol.Option.
func (p *protocol) Option(option interface{}) *tcpip.Error {
	switch v := option.(type) {
	case *tcpip.DefaultTTLOption:
		*v = tcpip.DefaultTTLOption(p.DefaultTTL())
		return nil
	default:
		return tcpip.ErrUnknownProtocolOption
	}
}

// SetDefaultTTL sets the default TTL for endpoints created with this protocol.
func (p *protocol) SetDefaultTTL(ttl uint8) {
	atomic.StoreUint32(&p.defaultTTL, uint32(ttl))
}

// DefaultTTL returns the default TTL for endpoints created with this protocol.
func (p *protocol) DefaultTTL() uint8 {
	return uint8(atomic.LoadUint32(&p.defaultTTL))
}

// calculateMTU calculates the network-layer payload MTU based on the link-layer
// payload mtu.
func calculateMTU(mtu uint32) uint32 {
	if mtu > MaxTotalSize {
		mtu = MaxTotalSize
	}
	return mtu - header.IPv4MinimumSize
}

// hashRoute calculates a hash value for the given route. It uses the source &
// destination address, the transport protocol number, and a random initial
// value (generated once on initialization) to generate the hash.
func hashRoute(r *stack.Route, protocol tcpip.TransportProtocolNumber, hashIV uint32) uint32 {
	t := r.LocalAddress
	a := uint32(t[0]) | uint32(t[1])<<8 | uint32(t[2])<<16 | uint32(t[3])<<24
	t = r.RemoteAddress
	b := uint32(t[0]) | uint32(t[1])<<8 | uint32(t[2])<<16 | uint32(t[3])<<24
	return hash.Hash3Words(a, b, uint32(protocol), hashIV)
}

// NewProtocol returns an IPv4 network protocol.
func NewProtocol() stack.NetworkProtocol {
	ids := make([]uint32, buckets)

	// Randomly initialize hashIV and the ids.
	r := hash.RandN32(1 + buckets)
	for i := range ids {
		ids[i] = r[i]
	}
	hashIV := r[buckets]

	return &protocol{ids: ids, hashIV: hashIV, defaultTTL: DefaultTTL}
}