97 lines
5.1 KiB
Markdown
97 lines
5.1 KiB
Markdown
Deterministic macOS DMG Notes.
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Working macOS DMGs are created in Linux by combining a recent clang,
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the Apple binutils (ld, ar, etc) and DMG authoring tools.
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Apple uses clang extensively for development and has upstreamed the necessary
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functionality so that a vanilla clang can take advantage. It supports the use
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of -F, -target, -mmacosx-version-min, and --sysroot, which are all necessary
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when building for macOS.
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Apple's version of binutils (called cctools) contains lots of functionality
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missing in the FSF's binutils. In addition to extra linker options for
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frameworks and sysroots, several other tools are needed as well such as
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install_name_tool, lipo, and nmedit. These do not build under linux, so they
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have been patched to do so. The work here was used as a starting point:
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[mingwandroid/toolchain4](https://github.com/mingwandroid/toolchain4).
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In order to build a working toolchain, the following source packages are needed
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from Apple: cctools, dyld, and ld64.
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These tools inject timestamps by default, which produce non-deterministic
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binaries. The ZERO_AR_DATE environment variable is used to disable that.
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This version of cctools has been patched to use the current version of clang's
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headers and its libLTO.so rather than those from llvmgcc, as it was
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originally done in toolchain4.
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To complicate things further, all builds must target an Apple SDK. These SDKs
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are free to download, but not redistributable.
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To obtain it, register for a developer account, then download the [Xcode 7.3.1 dmg](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_7.3.1/Xcode_7.3.1.dmg).
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This file is several gigabytes in size, but only a single directory inside is
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needed:
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```
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Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk
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```
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Unfortunately, the usual linux tools (7zip, hpmount, loopback mount) are incapable of opening this file.
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To create a tarball suitable for Gitian input, there are two options:
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Using macOS, you can mount the dmg, and then create it with:
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```
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$ hdiutil attach Xcode_7.3.1.dmg
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$ tar -C /Volumes/Xcode/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/ -czf MacOSX10.11.sdk.tar.gz MacOSX10.11.sdk
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```
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Alternatively, you can use 7zip and SleuthKit to extract the files one by one.
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The script contrib/macdeploy/extract-osx-sdk.sh automates this. First ensure
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the dmg file is in the current directory, and then run the script. You may wish
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to delete the intermediate 5.hfs file and MacOSX10.11.sdk (the directory) when
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you've confirmed the extraction succeeded.
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```bash
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apt-get install p7zip-full sleuthkit
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contrib/macdeploy/extract-osx-sdk.sh
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rm -rf 5.hfs MacOSX10.11.sdk
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```
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The Gitian descriptors build 2 sets of files: Linux tools, then Apple binaries
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which are created using these tools. The build process has been designed to
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avoid including the SDK's files in Gitian's outputs. All interim tarballs are
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fully deterministic and may be freely redistributed.
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genisoimage is used to create the initial DMG. It is not deterministic as-is,
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so it has been patched. A system genisoimage will work fine, but it will not
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be deterministic because the file-order will change between invocations.
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The patch can be seen here: [theuni/osx-cross-depends](https://raw.githubusercontent.com/theuni/osx-cross-depends/master/patches/cdrtools/genisoimage.diff).
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No effort was made to fix this cleanly, so it likely leaks memory badly. But
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it's only used for a single invocation, so that's no real concern.
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genisoimage cannot compress DMGs, so afterwards, the 'dmg' tool from the
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libdmg-hfsplus project is used to compress it. There are several bugs in this
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tool and its maintainer has seemingly abandoned the project. It has been forked
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and is available (with fixes) here: [theuni/libdmg-hfsplus](https://github.com/theuni/libdmg-hfsplus).
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The 'dmg' tool has the ability to create DMGs from scratch as well, but this
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functionality is broken. Only the compression feature is currently used.
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Ideally, the creation could be fixed and genisoimage would no longer be necessary.
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Background images and other features can be added to DMG files by inserting a
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.DS_Store before creation. This is generated by the script
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contrib/macdeploy/custom_dsstore.py.
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As of OS X 10.9 Mavericks, using an Apple-blessed key to sign binaries is a
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requirement in order to satisfy the new Gatekeeper requirements. Because this
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private key cannot be shared, we'll have to be a bit creative in order for the
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build process to remain somewhat deterministic. Here's how it works:
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- Builders use Gitian to create an unsigned release. This outputs an unsigned
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dmg which users may choose to bless and run. It also outputs an unsigned app
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structure in the form of a tarball, which also contains all of the tools
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that have been previously (deterministically) built in order to create a
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final dmg.
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- The Apple keyholder uses this unsigned app to create a detached signature,
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using the script that is also included there. Detached signatures are available from this [repository](https://github.com/bitcoin-core/bitcoin-detached-sigs).
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- Builders feed the unsigned app + detached signature back into Gitian. It
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uses the pre-built tools to recombine the pieces into a deterministic dmg.
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