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

import (
	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/header"
	"github.com/google/netstack/tcpip/stack"
	"github.com/google/netstack/tcpip/transport/raw"
	"github.com/google/netstack/waiter"
)

const (
	// ProtocolNumber is the udp protocol number.
	ProtocolNumber = header.UDPProtocolNumber
)

type protocol struct{}

// Number returns the udp protocol number.
func (*protocol) Number() tcpip.TransportProtocolNumber {
	return ProtocolNumber
}

// NewEndpoint creates a new udp endpoint.
func (*protocol) NewEndpoint(stack *stack.Stack, netProto tcpip.NetworkProtocolNumber, waiterQueue *waiter.Queue) (tcpip.Endpoint, *tcpip.Error) {
	return newEndpoint(stack, netProto, waiterQueue), nil
}

// NewRawEndpoint creates a new raw UDP endpoint. It implements
// stack.TransportProtocol.NewRawEndpoint.
func (p *protocol) NewRawEndpoint(stack *stack.Stack, netProto tcpip.NetworkProtocolNumber, waiterQueue *waiter.Queue) (tcpip.Endpoint, *tcpip.Error) {
	return raw.NewEndpoint(stack, netProto, header.UDPProtocolNumber, waiterQueue)
}

// MinimumPacketSize returns the minimum valid udp packet size.
func (*protocol) MinimumPacketSize() int {
	return header.UDPMinimumSize
}

// ParsePorts returns the source and destination ports stored in the given udp
// packet.
func (*protocol) ParsePorts(v buffer.View) (src, dst uint16, err *tcpip.Error) {
	h := header.UDP(v)
	return h.SourcePort(), h.DestinationPort(), nil
}

// HandleUnknownDestinationPacket handles packets targeted at this protocol but
// that don't match any existing endpoint.
func (p *protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.TransportEndpointID, netHeader buffer.View, vv buffer.VectorisedView) bool {
	// Get the header then trim it from the view.
	hdr := header.UDP(vv.First())
	if int(hdr.Length()) > vv.Size() {
		// Malformed packet.
		r.Stack().Stats().UDP.MalformedPacketsReceived.Increment()
		return true
	}
	// TODO(b/129426613): only send an ICMP message if UDP checksum is valid.

	// Only send ICMP error if the address is not a multicast/broadcast
	// v4/v6 address or the source is not the unspecified address.
	//
	// See: point e) in https://tools.ietf.org/html/rfc4443#section-2.4
	if id.LocalAddress == header.IPv4Broadcast || header.IsV4MulticastAddress(id.LocalAddress) || header.IsV6MulticastAddress(id.LocalAddress) || id.RemoteAddress == header.IPv6Any || id.RemoteAddress == header.IPv4Any {
		return true
	}

	// As per RFC: 1122 Section 3.2.2.1 A host SHOULD generate Destination
	//   Unreachable messages with code:
	//
	//     2 (Protocol Unreachable), when the designated transport protocol
	//     is not supported; or
	//
	//     3 (Port Unreachable), when the designated transport protocol
	//     (e.g., UDP) is unable to demultiplex the datagram but has no
	//     protocol mechanism to inform the sender.
	switch len(id.LocalAddress) {
	case header.IPv4AddressSize:
		if !r.Stack().AllowICMPMessage() {
			r.Stack().Stats().ICMP.V4PacketsSent.RateLimited.Increment()
			return true
		}
		// As per RFC 1812 Section 4.3.2.3
		//
		//   ICMP datagram SHOULD contain as much of the original
		//   datagram as possible without the length of the ICMP
		//   datagram exceeding 576 bytes
		//
		// NOTE: The above RFC referenced is different from the original
		// recommendation in RFC 1122 where it mentioned that at least 8
		// bytes of the payload must be included. Today linux and other
		// systems implement the] RFC1812 definition and not the original
		// RFC 1122 requirement.
		mtu := int(r.MTU())
		if mtu > header.IPv4MinimumProcessableDatagramSize {
			mtu = header.IPv4MinimumProcessableDatagramSize
		}
		headerLen := int(r.MaxHeaderLength()) + header.ICMPv4MinimumSize
		available := int(mtu) - headerLen
		payloadLen := len(netHeader) + vv.Size()
		if payloadLen > available {
			payloadLen = available
		}

		// The buffers used by vv and netHeader may be used elsewhere
		// in the system.  For example, a raw or packet socket may use
		// what UDP considers an unreachable destination. Thus we deep
		// copy vv and netHeader to prevent multiple ownership and SR
		// errors.
		newNetHeader := make(buffer.View, len(netHeader))
		copy(newNetHeader, netHeader)
		payload := buffer.NewVectorisedView(len(newNetHeader), []buffer.View{newNetHeader})
		payload.Append(vv.ToView().ToVectorisedView())
		payload.CapLength(payloadLen)

		hdr := buffer.NewPrependable(headerLen)
		pkt := header.ICMPv4(hdr.Prepend(header.ICMPv4MinimumSize))
		pkt.SetType(header.ICMPv4DstUnreachable)
		pkt.SetCode(header.ICMPv4PortUnreachable)
		pkt.SetChecksum(header.ICMPv4Checksum(pkt, payload))
		r.WritePacket(nil /* gso */, hdr, payload, stack.NetworkHeaderParams{Protocol: header.ICMPv4ProtocolNumber, TTL: r.DefaultTTL(), TOS: stack.DefaultTOS})

	case header.IPv6AddressSize:
		if !r.Stack().AllowICMPMessage() {
			r.Stack().Stats().ICMP.V6PacketsSent.RateLimited.Increment()
			return true
		}

		// As per RFC 4443 section 2.4
		//
		//    (c) Every ICMPv6 error message (type < 128) MUST include
		//    as much of the IPv6 offending (invoking) packet (the
		//    packet that caused the error) as possible without making
		//    the error message packet exceed the minimum IPv6 MTU
		//    [IPv6].
		mtu := int(r.MTU())
		if mtu > header.IPv6MinimumMTU {
			mtu = header.IPv6MinimumMTU
		}
		headerLen := int(r.MaxHeaderLength()) + header.ICMPv6DstUnreachableMinimumSize
		available := int(mtu) - headerLen
		payloadLen := len(netHeader) + vv.Size()
		if payloadLen > available {
			payloadLen = available
		}
		payload := buffer.NewVectorisedView(len(netHeader), []buffer.View{netHeader})
		payload.Append(vv)
		payload.CapLength(payloadLen)

		hdr := buffer.NewPrependable(headerLen)
		pkt := header.ICMPv6(hdr.Prepend(header.ICMPv6DstUnreachableMinimumSize))
		pkt.SetType(header.ICMPv6DstUnreachable)
		pkt.SetCode(header.ICMPv6PortUnreachable)
		pkt.SetChecksum(header.ICMPv6Checksum(pkt, r.LocalAddress, r.RemoteAddress, payload))
		r.WritePacket(nil /* gso */, hdr, payload, stack.NetworkHeaderParams{Protocol: header.ICMPv6ProtocolNumber, TTL: r.DefaultTTL(), TOS: stack.DefaultTOS})
	}
	return true
}

// SetOption implements TransportProtocol.SetOption.
func (p *protocol) SetOption(option interface{}) *tcpip.Error {
	return tcpip.ErrUnknownProtocolOption
}

// Option implements TransportProtocol.Option.
func (p *protocol) Option(option interface{}) *tcpip.Error {
	return tcpip.ErrUnknownProtocolOption
}

// NewProtocol returns a UDP transport protocol.
func NewProtocol() stack.TransportProtocol {
	return &protocol{}
}