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// Copyright (c) 2022, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'dart:convert';
import 'dart:io';
import 'dart:math';
import 'dart:typed_data';
import 'package:dart2native/macho.dart';
import 'package:dart2native/macho_parser.dart';
const String kSnapshotSegmentName = "__CUSTOM";
const String kSnapshotSectionName = "__dart_app_snap";
const int kMinimumSegmentSize = 0x4000;
// Since arm64 macOS has 16K pages, which is larger than the 4K pages on x64
// macOS, we use this larger page size to ensure the MachO file is aligned
// properly on all architectures.
const int kSegmentAlignment = 0x4000;
int align(int size, int base) {
final int over = size % base;
if (over != 0) {
return size + (base - over);
return size;
// Utility for aligning parts of MachO headers to the defined sizes.
int vmSizeAlign(int size) {
return align(max(size, kMinimumSegmentSize), kSegmentAlignment);
// Returns value + amount only if the original value is within the bounds
// defined by [withinStart, withinStart + withinSize).
Uint32 addIfWithin(
Uint32 value, Uint64 amount, Uint64 withinStart, Uint64 withinSize) {
final intWithinStart = withinStart.asInt();
final intWithinSize = withinSize.asInt();
if (value >= intWithinStart && value < (intWithinStart + intWithinSize)) {
return (value.asUint64() + amount).asUint32();
} else {
return value;
// Trims a bytestring that an arbitrary number of null characters on the end of
// it.
String trimmedBytestring(Uint8List bytestring) {
return String.fromCharCodes(bytestring.takeWhile((value) => value != 0));
// Simplifies casting so we get null values back instead of exceptions.
T? cast<T>(x) => x is T ? x : null;
// Inserts a segment definition into a MachOFile. This does NOT insert the
// actual segment into the file. It only inserts the definition of that segment
// into the MachO header.
// In addition to simply specifying the definition for the segment, this
// function also moves the existing __LINKEDIT segment to the end of the header
// definition as is required by the MachO specification (or at least MacOS's
// implementation of it). In doing so there are several offsets in the original
// __LINKEDIT segment that must be updated to point to their new location
// because the __LINKEDIT segment and sections are now in a different
// place. This function takes care of those shifts as well.
// Returns the original, unmodified, __LINKEDIT segment.
Future<MachOSegmentCommand64> insertSegmentDefinition(MachOFile file,
File segment, String segmentName, String sectionName) async {
// Load in the data to be inserted.
final segmentData = await segment.readAsBytes();
// Find the existing __LINKEDIT segment
final linkedit = cast<MachOSegmentCommand64>(file.commands
.where((segment) =>
segment.asType() is MachOSegmentCommand64 &&
MachOConstants.SEG_LINKEDIT ==
trimmedBytestring((segment as MachOSegmentCommand64).segname))
final linkeditIndex = file.commands.indexWhere((segment) =>
segment.asType() is MachOSegmentCommand64 &&
MachOConstants.SEG_LINKEDIT ==
trimmedBytestring((segment as MachOSegmentCommand64).segname));
if (linkedit == null) {
throw FormatException(
"Could not find a __LINKEDIT section in the specified binary.");
} else {
// Create the new segment.
final Uint8List segname = Uint8List(16);
segname.setRange(0, segmentName.length, ascii.encode(segmentName));
segname.fillRange(segmentName.length, 16, 0);
final Uint64 vmaddr = linkedit.vmaddr;
final Uint64 vmsize = Uint64(vmSizeAlign(segmentData.length));
final Uint64 fileoff = linkedit.fileoff;
final Uint64 filesize = vmsize;
final Int32 maxprot = MachOConstants.VM_PROT_READ;
final Int32 initprot = maxprot;
final Uint32 nsects = Uint32(1);
final Uint8List sectname = Uint8List(16);
sectname.setRange(0, sectionName.length, ascii.encode(sectionName));
sectname.fillRange(sectionName.length, 16, 0);
final Uint64 addr = vmaddr;
final Uint64 size = Uint64(segmentData.length);
final Uint32 offset = fileoff.asUint32();
final Uint32 flags = MachOConstants.S_REGULAR;
final Uint32 zero = Uint32(0);
final loadCommandDefinitionSize = 4 * 2;
final sectionDefinitionSize = 16 * 2 + 8 * 2 + 4 * 8;
final segmentDefinitionSize = 16 + 8 * 4 + 4 * 4;
final commandSize = loadCommandDefinitionSize +
segmentDefinitionSize +
final loadCommand =
MachOLoadCommand(MachOConstants.LC_SEGMENT_64, Uint32(commandSize));
final section = MachOSection64(sectname, segname, addr, size, offset, zero,
zero, zero, flags, zero, zero, zero);
final segment = MachOSegmentCommand64(Uint32(commandSize), segname, vmaddr,
vmsize, fileoff, filesize, maxprot, initprot, nsects, zero, [section]);
// Setup the new linkedit command.
final shiftedLinkeditVmaddr = linkedit.vmaddr + segment.vmsize;
final shiftedLinkeditFileoff = linkedit.fileoff + segment.filesize;
final shiftedLinkedit = MachOSegmentCommand64(
// Shift all of the related commands that need to reference the new file
// position of the linkedit segment.
for (var i = 0; i < file.commands.length; i++) {
final command = file.commands[i];
final offsetAmount = segment.filesize;
final withinStart = linkedit.fileoff;
final withinSize = linkedit.filesize;
// For the specific command that we need to adjust, we need to move the
// commands' various offsets forward by the new segment's size in the file
// (segment.filesize). However, we need to ensure that when we move the
// offset forward, we exclude cases where the offset was originally
// outside of the linkedit segment (i.e. offset < linkedit.fileoff or
// offset >= linkedit.fileoff + linkedit.filesize). The DRY-ing function
// addIfWithin takes care of that repeated logic.
if (command is MachODyldInfoCommand) {
file.commands[i] = MachODyldInfoCommand(
command.rebase_off, offsetAmount, withinStart, withinSize),
command.bind_off, offsetAmount, withinStart, withinSize),
command.weak_bind_off, offsetAmount, withinStart, withinSize),
command.lazy_bind_off, offsetAmount, withinStart, withinSize),
command.export_off, offsetAmount, withinStart, withinSize),
} else if (command is MachOSymtabCommand) {
file.commands[i] = MachOSymtabCommand(
addIfWithin(command.symoff, offsetAmount, withinStart, withinSize),
addIfWithin(command.stroff, offsetAmount, withinStart, withinSize),
} else if (command is MachODysymtabCommand) {
file.commands[i] = MachODysymtabCommand(
addIfWithin(command.tocoff, offsetAmount, withinStart, withinSize),
command.modtaboff, offsetAmount, withinStart, withinSize),
command.extrefsymoff, offsetAmount, withinStart, withinSize),
command.indirectsymoff, offsetAmount, withinStart, withinSize),
command.extreloff, offsetAmount, withinStart, withinSize),
command.locreloff, offsetAmount, withinStart, withinSize),
} else if (command is MachOLinkeditDataCommand) {
file.commands[i] = MachOLinkeditDataCommand(
addIfWithin(command.dataoff, offsetAmount, withinStart, withinSize),
// Now we need to build the new header from these modified pieces.
file.header = MachOHeader(
file.header!.ncmds + Uint32(1),
file.header!.sizeofcmds + loadCommand.cmdsize,
file.commands[linkeditIndex] = shiftedLinkedit;
file.commands.insert(linkeditIndex, segment);
return linkedit;
// Pipe from one file stream into another. We do this in chunks to avoid
// excessive memory load.
Future<int> pipeStream(RandomAccessFile from, RandomAccessFile to,
{int? numToWrite, int chunkSize = 1 << 30}) async {
int numWritten = 0;
final int fileLength = from.lengthSync();
while (from.positionSync() != fileLength) {
final int availableBytes = fileLength - from.positionSync();
final int numToRead = numToWrite == null
? min(availableBytes, chunkSize)
: min(numToWrite - numWritten, min(availableBytes, chunkSize));
final buffer = await;
await to.writeFrom(buffer);
numWritten += numToRead;
if (numToWrite != null && numWritten >= numToWrite) {
return numWritten;
class _MacOSVersion {
final int? _major;
final int? _minor;
static final _regexp = RegExp(r'Version (?<major>\d+).(?<minor>\d+)');
static const _parseFailure = 'Could not determine macOS version';
const _MacOSVersion._internal(this._major, this._minor);
static const _unknown = _MacOSVersion._internal(null, null);
factory _MacOSVersion() {
if (!Platform.isMacOS) return _unknown;
final match =
_regexp.matchAsPrefix(Platform.operatingSystemVersion) as RegExpMatch?;
if (match == null) return _unknown;
final minor = int.tryParse(match.namedGroup('minor')!);
final major = int.tryParse(match.namedGroup('major')!);
return _MacOSVersion._internal(major, minor);
bool get isValid => _major != null;
int get major => _major ?? (throw _parseFailure);
int get minor => _minor ?? (throw _parseFailure);
// Writes an "appended" dart runtime + script snapshot file in a format
// compatible with MachO executables.
Future writeAppendedMachOExecutable(
String dartaotruntimePath, String payloadPath, String outputPath) async {
File originalExecutableFile = File(dartaotruntimePath);
MachOFile machOFile = MachOFile();
await machOFile.loadFromFile(originalExecutableFile);
// Insert the new segment that contains our snapshot data.
File newSegmentFile = File(payloadPath);
// Note that these two values MUST match the ones in
// runtime/bin/, which looks specifically for the snapshot in
// this segment/section.
final linkeditCommand = await insertSegmentDefinition(
machOFile, newSegmentFile, kSnapshotSegmentName, kSnapshotSectionName);
// Write out the new executable, with the same contents except the new header.
File outputFile = File(outputPath);
RandomAccessFile stream = await FileMode.write);
// Write the MachO header.
final int headerBytesWritten = stream.positionSync();
RandomAccessFile newSegmentFileStream = await;
RandomAccessFile originalFileStream = await;
await originalFileStream.setPosition(headerBytesWritten);
// Write the unchanged data from the original file.
await pipeStream(originalFileStream, stream,
numToWrite: linkeditCommand.fileoff.asInt() - headerBytesWritten);
// Write the inserted section data, ensuring that the data is padded to the
// segment size.
await pipeStream(newSegmentFileStream, stream);
final int newSegmentLength = newSegmentFileStream.lengthSync();
final int alignedSegmentSize = vmSizeAlign(newSegmentLength);
await stream.writeFrom(List.filled(alignedSegmentSize - newSegmentLength, 0));
// Copy the rest of the file from the original to the new one.
await pipeStream(originalFileStream, stream);
await stream.close();
if (machOFile.hasCodeSignature) {
if (!Platform.isMacOS) {
throw 'Cannot sign MachO binary on non-macOS platform';
// After writing the modified file, we perform ad-hoc signing (no identity)
// to ensure that any LC_CODE_SIGNATURE block has the correct CD hashes.
// This is necessary for platforms where signature verification is always on
// (e.g., OS X on M1).
// We use the `-f` flag to force signature overwriting as the official
// Dart binaries (including dartaotruntime) are fully signed.
final args = ['-f', '-s', '-', outputPath];
// If running on macOS >=11.0, then the linker-signed option flag can be
// used to create a signature that does not need to be force overridden.
final version = _MacOSVersion();
if (version.isValid && version.major >= 11) {
final signingProcess =
await'codesign', ['-o', 'linker-signed', ...args]);
if (signingProcess.exitCode == 0) {
print('Failed to add a linker signed signature, '
'adding a regular signature instead.');
// If that fails or we're running on an older or undetermined version of
// macOS, we fall back to signing without the linker-signed option flag.
// Thus, to sign the binary, the developer must force signature overwriting.
final signingProcess = await'codesign', args);
if (signingProcess.exitCode != 0) {
..write('Failed to replace the dartaotruntime signature, ')
..write('subcommand terminated with exit code ')
if (signingProcess.stdout.isNotEmpty) {
..writeln('Subcommand stdout:')
if (signingProcess.stderr.isNotEmpty) {
..writeln('Subcommand stderr:')
throw 'Could not sign the new executable';