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  1. Deterministic OS X Dmg Notes.
  2. Working OS X DMGs are created in Linux by combining a recent clang,
  3. the Apple binutils (ld, ar, etc) and DMG authoring tools.
  4. Apple uses clang extensively for development and has upstreamed the necessary
  5. functionality so that a vanilla clang can take advantage. It supports the use
  6. of -F, -target, -mmacosx-version-min, and --sysroot, which are all necessary
  7. when building for OS X.
  8. Apple's version of binutils (called cctools) contains lots of functionality
  9. missing in the FSF's binutils. In addition to extra linker options for
  10. frameworks and sysroots, several other tools are needed as well such as
  11. install_name_tool, lipo, and nmedit. These do not build under linux, so they
  12. have been patched to do so. The work here was used as a starting point:
  13. [mingwandroid/toolchain4](
  14. In order to build a working toolchain, the following source packages are needed
  15. from Apple: cctools, dyld, and ld64.
  16. These tools inject timestamps by default, which produce non-deterministic
  17. binaries. The ZERO_AR_DATE environment variable is used to disable that.
  18. This version of cctools has been patched to use the current version of clang's
  19. headers and its rather than those from llvmgcc, as it was
  20. originally done in toolchain4.
  21. To complicate things further, all builds must target an Apple SDK. These SDKs
  22. are free to download, but not redistributable.
  23. To obtain it, register for a developer account, then download the [Xcode 7.3.1 dmg](
  24. This file is several gigabytes in size, but only a single directory inside is
  25. needed:
  26. ```
  28. ```
  29. Unfortunately, the usual linux tools (7zip, hpmount, loopback mount) are incapable of opening this file.
  30. To create a tarball suitable for Gitian input, there are two options:
  31. Using Mac OS X, you can mount the dmg, and then create it with:
  32. ```
  33. $ hdiutil attach Xcode_7.3.1.dmg
  34. $ tar -C /Volumes/Xcode/ -czf MacOSX10.11.sdk.tar.gz MacOSX10.11.sdk
  35. ```
  36. Alternatively, you can use 7zip and SleuthKit to extract the files one by one.
  37. The script contrib/macdeploy/ automates this. First ensure
  38. the dmg file is in the current directory, and then run the script. You may wish
  39. to delete the intermediate 5.hfs file and MacOSX10.11.sdk (the directory) when
  40. you've confirmed the extraction succeeded.
  41. ```bash
  42. apt-get install p7zip-full sleuthkit
  43. contrib/macdeploy/
  44. rm -rf 5.hfs MacOSX10.11.sdk
  45. ```
  46. The Gitian descriptors build 2 sets of files: Linux tools, then Apple binaries
  47. which are created using these tools. The build process has been designed to
  48. avoid including the SDK's files in Gitian's outputs. All interim tarballs are
  49. fully deterministic and may be freely redistributed.
  50. genisoimage is used to create the initial DMG. It is not deterministic as-is,
  51. so it has been patched. A system genisoimage will work fine, but it will not
  52. be deterministic because the file-order will change between invocations.
  53. The patch can be seen here: [theuni/osx-cross-depends](
  54. No effort was made to fix this cleanly, so it likely leaks memory badly. But
  55. it's only used for a single invocation, so that's no real concern.
  56. genisoimage cannot compress DMGs, so afterwards, the 'dmg' tool from the
  57. libdmg-hfsplus project is used to compress it. There are several bugs in this
  58. tool and its maintainer has seemingly abandoned the project. It has been forked
  59. and is available (with fixes) here: [theuni/libdmg-hfsplus](
  60. The 'dmg' tool has the ability to create DMGs from scratch as well, but this
  61. functionality is broken. Only the compression feature is currently used.
  62. Ideally, the creation could be fixed and genisoimage would no longer be necessary.
  63. Background images and other features can be added to DMG files by inserting a
  64. .DS_Store before creation. This is generated by the script
  65. contrib/macdeploy/
  66. As of OS X Mavericks (10.9), using an Apple-blessed key to sign binaries is a
  67. requirement in order to satisfy the new Gatekeeper requirements. Because this
  68. private key cannot be shared, we'll have to be a bit creative in order for the
  69. build process to remain somewhat deterministic. Here's how it works:
  70. - Builders use Gitian to create an unsigned release. This outputs an unsigned
  71. dmg which users may choose to bless and run. It also outputs an unsigned app
  72. structure in the form of a tarball, which also contains all of the tools
  73. that have been previously (deterministically) built in order to create a
  74. final dmg.
  75. - The Apple keyholder uses this unsigned app to create a detached signature,
  76. using the script that is also included there. Detached signatures are available from this [repository](
  77. - Builders feed the unsigned app + detached signature back into Gitian. It
  78. uses the pre-built tools to recombine the pieces into a deterministic dmg.