update_metadata.proto

// Copyright (c) 2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Update file format: A delta update file contains all the deltas needed
// to update a system from one specific version to another specific
// version. The update format is represented by this struct pseudocode:
// struct delta_update_file {
//   char magic[4] = "CrAU";
//   uint32 file_format_version = 1;
//   uint64 manifest_size;  // Size of protobuf DeltaArchiveManifest
//   // The Bzip2 compressed DeltaArchiveManifest
//   char manifest[];
//
//   // Data blobs for files, no specific format. The specific offset
//   // and length of each data blob is recorded in the DeltaArchiveManifest.
//   struct {
//     char data[];
//   } blobs[];
//
//   // These two are not signed:
//   uint64 signatures_message_size;
//   char signatures_message[];
//
// };

// The DeltaArchiveManifest protobuf is an ordered list of InstallOperation
// objects. These objects are stored in a linear array in the
// DeltaArchiveManifest. Each operation is applied in order by the client.

// The DeltaArchiveManifest also contains the initial and final
// checksums for the device.

// The client will perform each InstallOperation in order, beginning even
// before the entire delta file is downloaded (but after at least the
// protobuf is downloaded). The types of operations are explained:
// - REPLACE: Replace the dst_extents on the drive with the attached data,
//   zero padding out to block size.
// - REPLACE_BZ: bzip2-uncompress the attached data and write it into
//   dst_extents on the drive, zero padding to block size.
// - MOVE: Copy the data in src_extents to dst_extents. Extents may overlap,
//   so it may be desirable to read all src_extents data into memory before
//   writing it out.
// - BSDIFF: Read src_length bytes from src_extents into memory, perform
//   bspatch with attached data, write new data to dst_extents, zero padding
//   to block size.

package chromeos_update_engine;

// Data is packed into blocks on disk, always starting from the beginning
// of the block. If a file's data is too large for one block, it overflows
// into another block, which may or may not be the following block on the
// physical partition. An ordered list of extents is another
// representation of an ordered list of blocks. For example, a file stored
// in blocks 9, 10, 11, 2, 18, 12 (in that order) would be stored in
// extents { {9, 3}, {2, 1}, {18, 1}, {12, 1} } (in that order).
// In general, files are stored sequentially on disk, so it's more efficient
// to use extents to encode the block lists (this is effectively
// run-length encoding).
// A sentinel value (kuint64max) as the start block denotes a sparse-hole
// in a file whose block-length is specified by num_blocks.

// Signatures: Updates may be signed by the OS vendor. The client verifies
// an update's signature by hashing the entire download. The section of the
// download the contains the signature is at the end of the file, so when
// signing a file, only the part up to the signature part is signed.
// Then, the client looks inside the download's Signatures message for a
// Signature message that it knows how to handle. Generally, a client will
// only know how to handle one type of signature, but an update may contain
// many signatures to support many different types of client. Then client
// selects a Signature message and uses that, along with a known public key,
// to verify the download. The public key is expected to be part of the
// client.

message Extent {
  optional uint64 start_block = 1;
  optional uint64 num_blocks = 2;
}

message Signatures {
  message Signature {
    optional uint32 version = 1;
    optional string data = 2;
  }
  repeated Signature signatures = 1;
}

message DeltaArchiveManifest {
  message InstallOperation {
    enum Type {
      REPLACE = 0;  // Replace destination extents w/ attached data
      REPLACE_BZ = 1;  // Replace destination extents w/ attached bzipped data
      MOVE = 2;  // Move source extents to destination extents
      BSDIFF = 3;  // The data is a bsdiff binary diff
    }
    required Type type = 1;
    // The offset into the delta file (after the protobuf)
    // where the data (if any) is stored
    optional uint32 data_offset = 2;
    // The length of the data in the delta file
    optional uint32 data_length = 3;

    // Ordered list of extents that are read from (if any) and written to.
    repeated Extent src_extents = 4;
    // Byte length of src, not necessarily block aligned. It's only used for
    // BSDIFF, because we need to pass that external program the number
    // of bytes to read from the blocks we pass it.
    optional uint64 src_length = 5;

    repeated Extent dst_extents = 6;
    // byte length of dst, not necessarily block aligned. It's only used for
    // BSDIFF, because we need to fill in the rest of the last block
    // that bsdiff writes with '\0' bytes.
    optional uint64 dst_length = 7;
  }
  repeated InstallOperation install_operations = 1;
  repeated InstallOperation kernel_install_operations = 2;

  // (At time of writing) usually 4096
  optional uint32 block_size = 3 [default = 4096];

  // If signatures are present, the offset into the blobs, generally
  // tacked onto the end of the file. We use an offset rather than
  // a bool to allow for more flexibility in future file formats.
  // If this is absent, it means signatures aren't supported in this
  // file.
  optional uint64 signatures_offset = 4;
}