Deterministic macOS DMGs

Deterministic macOS DMG Notes.

Working macOS DMGs are created in Linux by combining a recent clang, the Apple binutils (ld, ar, etc) and DMG authoring tools.

Apple uses clang extensively for development and has upstreamed the necessary functionality so that a vanilla clang can take advantage. It supports the use of -F, -target, -mmacosx-version-min, and --sysroot, which are all necessary when building for macOS.

Apple's version of binutils (called cctools) contains lots of functionality missing in the FSF's binutils. In addition to extra linker options for frameworks and sysroots, several other tools are needed as well such as install_name_tool, lipo, and nmedit. These do not build under linux, so they have been patched to do so. The work here was used as a starting point: mingwandroid/toolchain4.

In order to build a working toolchain, the following source packages are needed from Apple: cctools, dyld, and ld64.

These tools inject timestamps by default, which produce non-deterministic binaries. The ZERO_AR_DATE environment variable is used to disable that.

This version of cctools has been patched to use the current version of clang's headers and its libLTO.so rather than those from llvmgcc, as it was originally done in toolchain4.

To complicate things further, all builds must target an Apple SDK. These SDKs are free to download, but not redistributable. To obtain it, register for a developer account, then download the Xcode 7.3.1 dmg.

This file is several gigabytes in size, but only a single directory inside is needed:

Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk

This directory has been extracted and packaged into a tarball whose contents are identical with this one on Github whose sha256sum is ff5f65e4637ef68cd8412a5f451cafe1dd03068bb57d41ca3fc971fb58a67de3.

The gitian build process has been adapted to use this .tar.xz file. Previously, it used a .tar.gz version of the same content, which was extracted using contrib/macdeploy/extract-osx-sdk.sh (since removed). The method to extract the data on Linux from the .dmg file was elaborate and has also been removed from this description for brevity - consult an older version if you need to see those instructions / scripts.

The Gitian descriptors build 2 sets of files: Linux tools, then Apple binaries which are created using these tools. The build process has been designed to avoid including the SDK's files in Gitian's outputs. All interim tarballs are fully deterministic and may be freely redistributed.

genisoimage is used to create the initial DMG. It is not deterministic as-is, so it has been patched. A system genisoimage will work fine, but it will not be deterministic because the file-order will change between invocations. The patch can be seen here: theuni/osx-cross-depends. No effort was made to fix this cleanly, so it likely leaks memory badly. But it's only used for a single invocation, so that's no real concern.

genisoimage cannot compress DMGs, so afterwards, the 'dmg' tool from the libdmg-hfsplus project is used to compress it. There are several bugs in this tool and its maintainer has seemingly abandoned the project. It has been forked and is available (with fixes) here: theuni/libdmg-hfsplus.

The 'dmg' tool has the ability to create DMGs from scratch as well, but this functionality is broken. Only the compression feature is currently used. Ideally, the creation could be fixed and genisoimage would no longer be necessary.

Background images and other features can be added to DMG files by inserting a .DS_Store before creation. This is generated by the script contrib/macdeploy/custom_dsstore.py.

As of OS X 10.9 Mavericks, using an Apple-blessed key to sign binaries is a requirement in order to satisfy the new Gatekeeper requirements. Because this private key cannot be shared, we'll have to be a bit creative in order for the build process to remain somewhat deterministic. Here's how it works:

  • Builders use Gitian to create an unsigned release. This outputs an unsigned dmg which users may choose to bless and run. It also outputs an unsigned app structure in the form of a tarball, which also contains all of the tools that have been previously (deterministically) built in order to create a final dmg.
  • The Apple keyholder uses this unsigned app to create a detached signature, using the script that is also included there. Detached signatures are available from this repository.
  • Builders feed the unsigned app + detached signature back into Gitian. It uses the pre-built tools to recombine the pieces into a deterministic dmg.