github.com/metacubex/gvisor@v0.0.0-20240320004321-933faba989ec/pkg/tcpip/stack/packet_buffer.go

// Copyright 2019 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 stack

import (
	"fmt"
	"io"

	"github.com/metacubex/gvisor/pkg/buffer"
	"github.com/metacubex/gvisor/pkg/sync"
	"github.com/metacubex/gvisor/pkg/tcpip"
	"github.com/metacubex/gvisor/pkg/tcpip/header"
)

type headerType int

const (
	virtioNetHeader headerType = iota
	linkHeader
	networkHeader
	transportHeader
	numHeaderType
)

var pkPool = sync.Pool{
	New: func() any {
		return &PacketBuffer{}
	},
}

// PacketBufferOptions specifies options for PacketBuffer creation.
type PacketBufferOptions struct {
	// ReserveHeaderBytes is the number of bytes to reserve for headers. Total
	// number of bytes pushed onto the headers must not exceed this value.
	ReserveHeaderBytes int

	// Payload is the initial unparsed data for the new packet. If set, it will
	// be owned by the new packet.
	Payload buffer.Buffer

	// IsForwardedPacket identifies that the PacketBuffer being created is for a
	// forwarded packet.
	IsForwardedPacket bool

	// OnRelease is a function to be run when the packet buffer is no longer
	// referenced (released back to the pool).
	OnRelease func()
}

// A PacketBuffer contains all the data of a network packet.
//
// As a PacketBuffer traverses up the stack, it may be necessary to pass it to
// multiple endpoints.
//
// The whole packet is expected to be a series of bytes in the following order:
// LinkHeader, NetworkHeader, TransportHeader, and Data. Any of them can be
// empty. Use of PacketBuffer in any other order is unsupported.
//
// PacketBuffer must be created with NewPacketBuffer, which sets the initial
// reference count to 1. Owners should call `DecRef()` when they are finished
// with the buffer to return it to the pool.
//
// Internal structure: A PacketBuffer holds a pointer to buffer.Buffer, which
// exposes a logically-contiguous byte storage. The underlying storage structure
// is abstracted out, and should not be a concern here for most of the time.
//
//	|- reserved ->|
//								|--->| consumed (incoming)
//	0             V    V
//	+--------+----+----+--------------------+
//	|        |    |    | current data ...   | (buf)
//	+--------+----+----+--------------------+
//					 ^    |
//					 |<---| pushed (outgoing)
//
// When a PacketBuffer is created, a `reserved` header region can be specified,
// which stack pushes headers in this region for an outgoing packet. There could
// be no such region for an incoming packet, and `reserved` is 0. The value of
// `reserved` never changes in the entire lifetime of the packet.
//
// Outgoing Packet: When a header is pushed, `pushed` gets incremented by the
// pushed length, and the current value is stored for each header. PacketBuffer
// subtracts this value from `reserved` to compute the starting offset of each
// header in `buf`.
//
// Incoming Packet: When a header is consumed (a.k.a. parsed), the current
// `consumed` value is stored for each header, and it gets incremented by the
// consumed length. PacketBuffer adds this value to `reserved` to compute the
// starting offset of each header in `buf`.
//
// +stateify savable
type PacketBuffer struct {
	_ sync.NoCopy

	packetBufferRefs

	// buf is the underlying buffer for the packet. See struct level docs for
	// details.
	buf      buffer.Buffer
	reserved int
	pushed   int
	consumed int

	// headers stores metadata about each header.
	headers [numHeaderType]headerInfo

	// NetworkProtocolNumber is only valid when NetworkHeader().View().IsEmpty()
	// returns false.
	// TODO(gvisor.dev/issue/3574): Remove the separately passed protocol
	// numbers in registration APIs that take a PacketBuffer.
	NetworkProtocolNumber tcpip.NetworkProtocolNumber

	// TransportProtocol is only valid if it is non zero.
	// TODO(gvisor.dev/issue/3810): This and the network protocol number should
	// be moved into the headerinfo. This should resolve the validity issue.
	TransportProtocolNumber tcpip.TransportProtocolNumber

	// Hash is the transport layer hash of this packet. A value of zero
	// indicates no valid hash has been set.
	Hash uint32

	// Owner is implemented by task to get the uid and gid.
	// Only set for locally generated packets.
	Owner tcpip.PacketOwner

	// The following fields are only set by the qdisc layer when the packet
	// is added to a queue.
	EgressRoute RouteInfo
	GSOOptions  GSO

	// snatDone indicates if the packet's source has been manipulated as per
	// iptables NAT table.
	snatDone bool

	// dnatDone indicates if the packet's destination has been manipulated as per
	// iptables NAT table.
	dnatDone bool

	// PktType indicates the SockAddrLink.PacketType of the packet as defined in
	// https://www.man7.org/linux/man-pages/man7/packet.7.html.
	PktType tcpip.PacketType

	// NICID is the ID of the last interface the network packet was handled at.
	NICID tcpip.NICID

	// RXChecksumValidated indicates that checksum verification may be
	// safely skipped.
	RXChecksumValidated bool

	// NetworkPacketInfo holds an incoming packet's network-layer information.
	NetworkPacketInfo NetworkPacketInfo

	tuple *tuple

	// onRelease is a function to be run when the packet buffer is no longer
	// referenced (released back to the pool).
	onRelease func() `state:"nosave"`
}

// NewPacketBuffer creates a new PacketBuffer with opts.
func NewPacketBuffer(opts PacketBufferOptions) *PacketBuffer {
	pk := pkPool.Get().(*PacketBuffer)
	pk.reset()
	if opts.ReserveHeaderBytes != 0 {
		v := buffer.NewViewSize(opts.ReserveHeaderBytes)
		pk.buf.Append(v)
		pk.reserved = opts.ReserveHeaderBytes
	}
	if opts.Payload.Size() > 0 {
		pk.buf.Merge(&opts.Payload)
	}
	pk.NetworkPacketInfo.IsForwardedPacket = opts.IsForwardedPacket
	pk.onRelease = opts.OnRelease
	pk.InitRefs()
	return pk
}

// IncRef increments the PacketBuffer's refcount.
func (pk *PacketBuffer) IncRef() *PacketBuffer {
	pk.packetBufferRefs.IncRef()
	return pk
}

// DecRef decrements the PacketBuffer's refcount. If the refcount is
// decremented to zero, the PacketBuffer is returned to the PacketBuffer
// pool.
func (pk *PacketBuffer) DecRef() {
	pk.packetBufferRefs.DecRef(func() {
		if pk.onRelease != nil {
			pk.onRelease()
		}

		pk.buf.Release()
		pkPool.Put(pk)
	})
}

func (pk *PacketBuffer) reset() {
	*pk = PacketBuffer{}
}

// ReservedHeaderBytes returns the number of bytes initially reserved for
// headers.
func (pk *PacketBuffer) ReservedHeaderBytes() int {
	return pk.reserved
}

// AvailableHeaderBytes returns the number of bytes currently available for
// headers. This is relevant to PacketHeader.Push method only.
func (pk *PacketBuffer) AvailableHeaderBytes() int {
	return pk.reserved - pk.pushed
}

// VirtioNetHeader returns the handle to virtio-layer header.
func (pk *PacketBuffer) VirtioNetHeader() PacketHeader {
	return PacketHeader{
		pk:  pk,
		typ: virtioNetHeader,
	}
}

// LinkHeader returns the handle to link-layer header.
func (pk *PacketBuffer) LinkHeader() PacketHeader {
	return PacketHeader{
		pk:  pk,
		typ: linkHeader,
	}
}

// NetworkHeader returns the handle to network-layer header.
func (pk *PacketBuffer) NetworkHeader() PacketHeader {
	return PacketHeader{
		pk:  pk,
		typ: networkHeader,
	}
}

// TransportHeader returns the handle to transport-layer header.
func (pk *PacketBuffer) TransportHeader() PacketHeader {
	return PacketHeader{
		pk:  pk,
		typ: transportHeader,
	}
}

// HeaderSize returns the total size of all headers in bytes.
func (pk *PacketBuffer) HeaderSize() int {
	return pk.pushed + pk.consumed
}

// Size returns the size of packet in bytes.
func (pk *PacketBuffer) Size() int {
	return int(pk.buf.Size()) - pk.headerOffset()
}

// MemSize returns the estimation size of the pk in memory, including backing
// buffer data.
func (pk *PacketBuffer) MemSize() int {
	return int(pk.buf.Size()) + PacketBufferStructSize
}

// Data returns the handle to data portion of pk.
func (pk *PacketBuffer) Data() PacketData {
	return PacketData{pk: pk}
}

// AsSlices returns the underlying storage of the whole packet.
//
// Note that AsSlices can allocate a lot. In hot paths it may be preferable to
// iterate over a PacketBuffer's data via AsViewList.
func (pk *PacketBuffer) AsSlices() [][]byte {
	vl := pk.buf.AsViewList()
	views := make([][]byte, 0, vl.Len())
	offset := pk.headerOffset()
	pk.buf.SubApply(offset, int(pk.buf.Size())-offset, func(v *buffer.View) {
		views = append(views, v.AsSlice())
	})
	return views
}

// AsViewList returns the list of Views backing the PacketBuffer along with the
// header offset into them. Users may not save or modify the ViewList returned.
func (pk *PacketBuffer) AsViewList() (buffer.ViewList, int) {
	return pk.buf.AsViewList(), pk.headerOffset()
}

// ToBuffer returns a caller-owned copy of the underlying storage of the whole
// packet.
func (pk *PacketBuffer) ToBuffer() buffer.Buffer {
	b := pk.buf.Clone()
	b.TrimFront(int64(pk.headerOffset()))
	return b
}

// ToView returns a caller-owned copy of the underlying storage of the whole
// packet as a view.
func (pk *PacketBuffer) ToView() *buffer.View {
	p := buffer.NewView(int(pk.buf.Size()))
	offset := pk.headerOffset()
	pk.buf.SubApply(offset, int(pk.buf.Size())-offset, func(v *buffer.View) {
		p.Write(v.AsSlice())
	})
	return p
}

func (pk *PacketBuffer) headerOffset() int {
	return pk.reserved - pk.pushed
}

func (pk *PacketBuffer) headerOffsetOf(typ headerType) int {
	return pk.reserved + pk.headers[typ].offset
}

func (pk *PacketBuffer) dataOffset() int {
	return pk.reserved + pk.consumed
}

func (pk *PacketBuffer) push(typ headerType, size int) []byte {
	h := &pk.headers[typ]
	if h.length > 0 {
		panic(fmt.Sprintf("push(%s, %d) called after previous push", typ, size))
	}
	if pk.pushed+size > pk.reserved {
		panic(fmt.Sprintf("push(%s, %d) overflows; pushed=%d reserved=%d", typ, size, pk.pushed, pk.reserved))
	}
	pk.pushed += size
	h.offset = -pk.pushed
	h.length = size
	view := pk.headerView(typ)
	return view.AsSlice()
}

func (pk *PacketBuffer) consume(typ headerType, size int) (v []byte, consumed bool) {
	h := &pk.headers[typ]
	if h.length > 0 {
		panic(fmt.Sprintf("consume must not be called twice: type %s", typ))
	}
	if pk.reserved+pk.consumed+size > int(pk.buf.Size()) {
		return nil, false
	}
	h.offset = pk.consumed
	h.length = size
	pk.consumed += size
	view := pk.headerView(typ)
	return view.AsSlice(), true
}

func (pk *PacketBuffer) headerView(typ headerType) buffer.View {
	h := &pk.headers[typ]
	if h.length == 0 {
		return buffer.View{}
	}
	v, ok := pk.buf.PullUp(pk.headerOffsetOf(typ), h.length)
	if !ok {
		panic("PullUp failed")
	}
	return v
}

// Clone makes a semi-deep copy of pk. The underlying packet payload is
// shared. Hence, no modifications is done to underlying packet payload.
func (pk *PacketBuffer) Clone() *PacketBuffer {
	newPk := pkPool.Get().(*PacketBuffer)
	newPk.reset()
	newPk.buf = pk.buf.Clone()
	newPk.reserved = pk.reserved
	newPk.pushed = pk.pushed
	newPk.consumed = pk.consumed
	newPk.headers = pk.headers
	newPk.Hash = pk.Hash
	newPk.Owner = pk.Owner
	newPk.GSOOptions = pk.GSOOptions
	newPk.NetworkProtocolNumber = pk.NetworkProtocolNumber
	newPk.dnatDone = pk.dnatDone
	newPk.snatDone = pk.snatDone
	newPk.TransportProtocolNumber = pk.TransportProtocolNumber
	newPk.PktType = pk.PktType
	newPk.NICID = pk.NICID
	newPk.RXChecksumValidated = pk.RXChecksumValidated
	newPk.NetworkPacketInfo = pk.NetworkPacketInfo
	newPk.tuple = pk.tuple
	newPk.InitRefs()
	return newPk
}

// ReserveHeaderBytes prepends reserved space for headers at the front
// of the underlying buf. Can only be called once per packet.
func (pk *PacketBuffer) ReserveHeaderBytes(reserved int) {
	if pk.reserved != 0 {
		panic(fmt.Sprintf("ReserveHeaderBytes(...) called on packet with reserved=%d, want reserved=0", pk.reserved))
	}
	pk.reserved = reserved
	pk.buf.Prepend(buffer.NewViewSize(reserved))
}

// Network returns the network header as a header.Network.
//
// Network should only be called when NetworkHeader has been set.
func (pk *PacketBuffer) Network() header.Network {
	switch netProto := pk.NetworkProtocolNumber; netProto {
	case header.IPv4ProtocolNumber:
		return header.IPv4(pk.NetworkHeader().Slice())
	case header.IPv6ProtocolNumber:
		return header.IPv6(pk.NetworkHeader().Slice())
	default:
		panic(fmt.Sprintf("unknown network protocol number %d", netProto))
	}
}

// CloneToInbound makes a semi-deep copy of the packet buffer (similar to
// Clone) to be used as an inbound packet.
//
// See PacketBuffer.Data for details about how a packet buffer holds an inbound
// packet.
func (pk *PacketBuffer) CloneToInbound() *PacketBuffer {
	newPk := pkPool.Get().(*PacketBuffer)
	newPk.reset()
	newPk.buf = pk.buf.Clone()
	newPk.InitRefs()
	// Treat unfilled header portion as reserved.
	newPk.reserved = pk.AvailableHeaderBytes()
	newPk.tuple = pk.tuple
	return newPk
}

// DeepCopyForForwarding creates a deep copy of the packet buffer for
// forwarding.
//
// The returned packet buffer will have the network and transport headers
// set if the original packet buffer did.
func (pk *PacketBuffer) DeepCopyForForwarding(reservedHeaderBytes int) *PacketBuffer {
	payload := BufferSince(pk.NetworkHeader())
	defer payload.Release()
	newPk := NewPacketBuffer(PacketBufferOptions{
		ReserveHeaderBytes: reservedHeaderBytes,
		Payload:            payload.DeepClone(),
		IsForwardedPacket:  true,
	})

	{
		consumeBytes := len(pk.NetworkHeader().Slice())
		if _, consumed := newPk.NetworkHeader().Consume(consumeBytes); !consumed {
			panic(fmt.Sprintf("expected to consume network header %d bytes from new packet", consumeBytes))
		}
		newPk.NetworkProtocolNumber = pk.NetworkProtocolNumber
	}

	{
		consumeBytes := len(pk.TransportHeader().Slice())
		if _, consumed := newPk.TransportHeader().Consume(consumeBytes); !consumed {
			panic(fmt.Sprintf("expected to consume transport header %d bytes from new packet", consumeBytes))
		}
		newPk.TransportProtocolNumber = pk.TransportProtocolNumber
	}

	newPk.tuple = pk.tuple

	return newPk
}

// IsNil returns whether the pointer is logically nil.
func (pk *PacketBuffer) IsNil() bool {
	return pk == nil
}

// headerInfo stores metadata about a header in a packet.
//
// +stateify savable
type headerInfo struct {
	// offset is the offset of the header in pk.buf relative to
	// pk.buf[pk.reserved]. See the PacketBuffer struct for details.
	offset int

	// length is the length of this header.
	length int
}

// PacketHeader is a handle object to a header in the underlying packet.
type PacketHeader struct {
	pk  *PacketBuffer
	typ headerType
}

// View returns an caller-owned copy of the underlying storage of h as a
// *buffer.View.
func (h PacketHeader) View() *buffer.View {
	view := h.pk.headerView(h.typ)
	if view.Size() == 0 {
		return nil
	}
	return view.Clone()
}

// Slice returns the underlying storage of h as a []byte. The returned slice
// should not be modified if the underlying packet could be shared, cloned, or
// borrowed.
func (h PacketHeader) Slice() []byte {
	view := h.pk.headerView(h.typ)
	return view.AsSlice()
}

// Push pushes size bytes in the front of its residing packet, and returns the
// backing storage. Callers may only call one of Push or Consume once on each
// header in the lifetime of the underlying packet.
func (h PacketHeader) Push(size int) []byte {
	return h.pk.push(h.typ, size)
}

// Consume moves the first size bytes of the unparsed data portion in the packet
// to h, and returns the backing storage. In the case of data is shorter than
// size, consumed will be false, and the state of h will not be affected.
// Callers may only call one of Push or Consume once on each header in the
// lifetime of the underlying packet.
func (h PacketHeader) Consume(size int) (v []byte, consumed bool) {
	return h.pk.consume(h.typ, size)
}

// PacketData represents the data portion of a PacketBuffer.
//
// +stateify savable
type PacketData struct {
	pk *PacketBuffer
}

// PullUp returns a contiguous slice of size bytes from the beginning of d.
// Callers should not keep the view for later use. Callers can write to the
// returned slice if they have singular ownership over the underlying
// Buffer.
func (d PacketData) PullUp(size int) (b []byte, ok bool) {
	view, ok := d.pk.buf.PullUp(d.pk.dataOffset(), size)
	return view.AsSlice(), ok
}

// Consume is the same as PullUp except that is additionally consumes the
// returned bytes. Subsequent PullUp or Consume will not return these bytes.
func (d PacketData) Consume(size int) ([]byte, bool) {
	v, ok := d.PullUp(size)
	if ok {
		d.pk.consumed += size
	}
	return v, ok
}

// ReadTo reads bytes from d to dst. It also removes these bytes from d
// unless peek is true.
func (d PacketData) ReadTo(dst io.Writer, peek bool) (int, error) {
	var (
		err  error
		done int
	)
	offset := d.pk.dataOffset()
	d.pk.buf.SubApply(offset, int(d.pk.buf.Size())-offset, func(v *buffer.View) {
		if err != nil {
			return
		}
		var n int
		n, err = dst.Write(v.AsSlice())
		done += n
		if err != nil {
			return
		}
		if n != v.Size() {
			panic(fmt.Sprintf("io.Writer.Write succeeded with incomplete write: %d != %d", n, v.Size()))
		}
	})
	if !peek {
		d.pk.buf.TrimFront(int64(done))
	}
	return done, err
}

// CapLength reduces d to at most length bytes.
func (d PacketData) CapLength(length int) {
	if length < 0 {
		panic("length < 0")
	}
	d.pk.buf.Truncate(int64(length + d.pk.dataOffset()))
}

// ToBuffer returns the underlying storage of d in a buffer.Buffer.
func (d PacketData) ToBuffer() buffer.Buffer {
	buf := d.pk.buf.Clone()
	offset := d.pk.dataOffset()
	buf.TrimFront(int64(offset))
	return buf
}

// AppendView appends v into d, taking the ownership of v.
func (d PacketData) AppendView(v *buffer.View) {
	d.pk.buf.Append(v)
}

// MergeBuffer merges b into d and clears b.
func (d PacketData) MergeBuffer(b *buffer.Buffer) {
	d.pk.buf.Merge(b)
}

// MergeFragment appends the data portion of frag to dst. It modifies
// frag and frag should not be used again.
func MergeFragment(dst, frag *PacketBuffer) {
	frag.buf.TrimFront(int64(frag.dataOffset()))
	dst.buf.Merge(&frag.buf)
}

// ReadFrom moves at most count bytes from the beginning of src to the end
// of d and returns the number of bytes moved.
func (d PacketData) ReadFrom(src *buffer.Buffer, count int) int {
	toRead := int64(count)
	if toRead > src.Size() {
		toRead = src.Size()
	}
	clone := src.Clone()
	clone.Truncate(toRead)
	d.pk.buf.Merge(&clone)
	src.TrimFront(toRead)
	return int(toRead)
}

// ReadFromPacketData moves count bytes from the beginning of oth to the end of
// d.
func (d PacketData) ReadFromPacketData(oth PacketData, count int) {
	buf := oth.ToBuffer()
	buf.Truncate(int64(count))
	d.MergeBuffer(&buf)
	oth.TrimFront(count)
	buf.Release()
}

// Merge clears headers in oth and merges its data with d.
func (d PacketData) Merge(oth PacketData) {
	oth.pk.buf.TrimFront(int64(oth.pk.dataOffset()))
	d.pk.buf.Merge(&oth.pk.buf)
}

// TrimFront removes up to count bytes from the front of d's payload.
func (d PacketData) TrimFront(count int) {
	if count > d.Size() {
		count = d.Size()
	}
	buf := d.pk.Data().ToBuffer()
	buf.TrimFront(int64(count))
	d.pk.buf.Truncate(int64(d.pk.dataOffset()))
	d.pk.buf.Merge(&buf)
}

// Size returns the number of bytes in the data payload of the packet.
func (d PacketData) Size() int {
	return int(d.pk.buf.Size()) - d.pk.dataOffset()
}

// AsRange returns a Range representing the current data payload of the packet.
func (d PacketData) AsRange() Range {
	return Range{
		pk:     d.pk,
		offset: d.pk.dataOffset(),
		length: d.Size(),
	}
}

// Checksum returns a checksum over the data payload of the packet.
func (d PacketData) Checksum() uint16 {
	return d.pk.buf.Checksum(d.pk.dataOffset())
}

// ChecksumAtOffset returns a checksum over the data payload of the packet
// starting from offset.
func (d PacketData) ChecksumAtOffset(offset int) uint16 {
	return d.pk.buf.Checksum(offset)
}

// Range represents a contiguous subportion of a PacketBuffer.
type Range struct {
	pk     *PacketBuffer
	offset int
	length int
}

// Size returns the number of bytes in r.
func (r Range) Size() int {
	return r.length
}

// SubRange returns a new Range starting at off bytes of r. It returns an empty
// range if off is out-of-bounds.
func (r Range) SubRange(off int) Range {
	if off > r.length {
		return Range{pk: r.pk}
	}
	return Range{
		pk:     r.pk,
		offset: r.offset + off,
		length: r.length - off,
	}
}

// Capped returns a new Range with the same starting point of r and length
// capped at max.
func (r Range) Capped(max int) Range {
	if r.length <= max {
		return r
	}
	return Range{
		pk:     r.pk,
		offset: r.offset,
		length: max,
	}
}

// ToSlice returns a caller-owned copy of data in r.
func (r Range) ToSlice() []byte {
	if r.length == 0 {
		return nil
	}
	all := make([]byte, 0, r.length)
	r.iterate(func(v *buffer.View) {
		all = append(all, v.AsSlice()...)
	})
	return all
}

// ToView returns a caller-owned copy of data in r.
func (r Range) ToView() *buffer.View {
	if r.length == 0 {
		return nil
	}
	newV := buffer.NewView(r.length)
	r.iterate(func(v *buffer.View) {
		newV.Write(v.AsSlice())
	})
	return newV
}

// iterate calls fn for each piece in r. fn is always called with a non-empty
// slice.
func (r Range) iterate(fn func(*buffer.View)) {
	r.pk.buf.SubApply(r.offset, r.length, fn)
}

// PayloadSince returns a caller-owned view containing the payload starting from
// and including a particular header.
func PayloadSince(h PacketHeader) *buffer.View {
	offset := h.pk.headerOffset()
	for i := headerType(0); i < h.typ; i++ {
		offset += h.pk.headers[i].length
	}
	return Range{
		pk:     h.pk,
		offset: offset,
		length: int(h.pk.buf.Size()) - offset,
	}.ToView()
}

// BufferSince returns a caller-owned view containing the packet payload
// starting from and including a particular header.
func BufferSince(h PacketHeader) buffer.Buffer {
	offset := h.pk.headerOffset()
	for i := headerType(0); i < h.typ; i++ {
		offset += h.pk.headers[i].length
	}
	clone := h.pk.buf.Clone()
	clone.TrimFront(int64(offset))
	return clone
}