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statefile.go
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statefile.go
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// Copyright 2018 Google Inc.
//
// 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 statefile defines the state file data stream.
//
// This package currently does not include any details regarding the state
// encoding itself, only details regarding state metadata and data layout.
//
// The file format is defined as follows.
//
// /------------------------------------------------------\
// | header (8-bytes) |
// +------------------------------------------------------+
// | metadata length (8-bytes) |
// +------------------------------------------------------+
// | metadata |
// +------------------------------------------------------+
// | data |
// \------------------------------------------------------/
//
// First, it includes a 8-byte magic header which is the following
// sequence of bytes [0x67, 0x56, 0x69, 0x73, 0x6f, 0x72, 0x53, 0x46]
//
// This header is followed by an 8-byte length N (big endian), and an
// ASCII-encoded JSON map that is exactly N bytes long.
//
// This map includes only strings for keys and strings for values. Keys in the
// map that begin with "_" are for internal use only. They may be read, but may
// not be provided by the user. In the future, this metadata may contain some
// information relating to the state encoding itself.
//
// After the map, the remainder of the file is the state data.
package statefile
import (
"bytes"
"crypto/hmac"
"crypto/sha256"
"encoding/json"
"fmt"
"hash"
"io"
"strings"
"time"
"gvisor.googlesource.com/gvisor/pkg/binary"
"gvisor.googlesource.com/gvisor/pkg/compressio"
"gvisor.googlesource.com/gvisor/pkg/hashio"
)
// keySize is the AES-256 key length.
const keySize = 32
// compressionChunkSize is the chunk size for compression.
const compressionChunkSize = 1024 * 1024
// maxMetadataSize is the size limit of metadata section.
const maxMetadataSize = 16 * 1024 * 1024
// magicHeader is the byte sequence beginning each file.
var magicHeader = []byte("\x67\x56\x69\x73\x6f\x72\x53\x46")
// ErrBadMagic is returned if the header does not match.
var ErrBadMagic = fmt.Errorf("bad magic header")
// ErrMetadataMissing is returned if the state file is missing mandatory metadata.
var ErrMetadataMissing = fmt.Errorf("missing metadata")
// ErrInvalidMetadataLength is returned if the metadata length is too large.
var ErrInvalidMetadataLength = fmt.Errorf("metadata length invalid, maximum size is %d", maxMetadataSize)
// ErrMetadataInvalid is returned if passed metadata is invalid.
var ErrMetadataInvalid = fmt.Errorf("metadata invalid, can't start with _")
// NewWriter returns a state data writer for a statefile.
//
// Note that the returned WriteCloser must be closed.
func NewWriter(w io.Writer, key []byte, metadata map[string]string, compressionLevel int) (io.WriteCloser, error) {
if metadata == nil {
metadata = make(map[string]string)
}
for k := range metadata {
if strings.HasPrefix(k, "_") {
return nil, ErrMetadataInvalid
}
}
// Create our HMAC function.
h := hmac.New(sha256.New, key)
mw := io.MultiWriter(w, h)
// First, write the header.
if _, err := mw.Write(magicHeader); err != nil {
return nil, err
}
// Generate a timestamp, for convenience only.
metadata["_timestamp"] = time.Now().UTC().String()
defer delete(metadata, "_timestamp")
// Write the metadata.
b, err := json.Marshal(metadata)
if err != nil {
return nil, err
}
if len(b) > maxMetadataSize {
return nil, ErrInvalidMetadataLength
}
// Metadata length.
if err := binary.WriteUint64(mw, binary.BigEndian, uint64(len(b))); err != nil {
return nil, err
}
// Metadata bytes; io.MultiWriter will return a short write error if
// any of the writers returns < n.
if _, err := mw.Write(b); err != nil {
return nil, err
}
// Write the current hash.
cur := h.Sum(nil)
for done := 0; done < len(cur); {
n, err := mw.Write(cur[done:])
done += n
if err != nil {
return nil, err
}
}
w = hashio.NewWriter(w, h)
// Wrap in compression.
return compressio.NewWriter(w, compressionChunkSize, compressionLevel)
}
// MetadataUnsafe reads out the metadata from a state file without verifying any
// HMAC. This function shouldn't be called for untrusted input files.
func MetadataUnsafe(r io.Reader) (map[string]string, error) {
return metadata(r, nil)
}
// metadata validates the magic header and reads out the metadata from a state
// data stream.
func metadata(r io.Reader, h hash.Hash) (map[string]string, error) {
if h != nil {
r = io.TeeReader(r, h)
}
// Read and validate magic header.
b := make([]byte, len(magicHeader))
if _, err := r.Read(b); err != nil {
return nil, err
}
if !bytes.Equal(b, magicHeader) {
return nil, ErrBadMagic
}
// Read and validate metadata.
b, err := func() (b []byte, err error) {
defer func() {
if r := recover(); r != nil {
b = nil
err = fmt.Errorf("%v", r)
}
}()
metadataLen, err := binary.ReadUint64(r, binary.BigEndian)
if err != nil {
return nil, err
}
if metadataLen > maxMetadataSize {
return nil, ErrInvalidMetadataLength
}
b = make([]byte, int(metadataLen))
if _, err := io.ReadFull(r, b); err != nil {
return nil, err
}
return b, nil
}()
if err != nil {
return nil, err
}
if h != nil {
// Check the hash prior to decoding.
cur := h.Sum(nil)
buf := make([]byte, len(cur))
if _, err := io.ReadFull(r, buf); err != nil {
return nil, err
}
if !hmac.Equal(cur, buf) {
return nil, hashio.ErrHashMismatch
}
}
// Decode the metadata.
metadata := make(map[string]string)
if err := json.Unmarshal(b, &metadata); err != nil {
return nil, err
}
return metadata, nil
}
// NewReader returns a reader for a statefile.
func NewReader(r io.Reader, key []byte) (io.Reader, map[string]string, error) {
// Read the metadata with the hash.
h := hmac.New(sha256.New, key)
metadata, err := metadata(r, h)
if err != nil {
return nil, nil, err
}
r = hashio.NewReader(r, h)
// Wrap in compression.
rc, err := compressio.NewReader(r)
if err != nil {
return nil, nil, err
}
return rc, metadata, nil
}