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Reproducible builds

We aim to make the outputs of linuxkit build reproducible, i.e. the build artefacts should be bit-by-bit identical copies if invoked with the same inputs and run with the same version of the linuxkit command. See this document on why this matters.

Note, we do not (yet) aim to make linuxkit pkg build builds reproducible.

Current status

Currently, the following output formats provide reproducible builds:

  • tar (Tested as part of the CI)
  • tar-kernel-initrd
  • docker
  • kernel+initrd (Tested as part of the CI)

Details

In general, linuxkit build lends itself for reproducible builds. LinuxKit packages, used during linuxkit build, are (signed) docker images. Packages are tagged with the content hash of the source code (and optionally release version) and are typically only updated if the source of the package changed (in which case the tag changes). For all intents and purposes, when pulled by tag, the contents of a packages should be bit-by-bit identical. Alternatively, the digest of the package, in which case, the pulled image will always be the same.

The first phase of the linuxkit build mostly untars and retars the images of the packages to produce an tar file of the root filesystem. This then serves as input for other output formats. During this first phase, there are a number of things to watch out for to generate reproducible builds:

  • Timestamps of generated files. The docker export command, as well as linuxkit build itself, creates a small number of files. The ModTime for these files needs to be clamped to a fixed date (otherwise the current time is used). Use the defaultModTime variable to set the ModTime of created files to a specific time.
  • Generated JSON files. linuxkit build generates a number of JSON files by marshalling Go struct variables. Examples are the OCI specification config.json and runtime.json files for containers. The default Go json.Marshal() function seems to do a reasonable good job in generating reproducible output from internal structures, including for JSON objects. However, during linuxkit build some of the OCI runtime spec fields are generated/modified and care must be taken to ensure consistent ordering. For JSON arrays (Go slices) it is best to sort them before Marshalling them.

Reproducible builds for the first phase of linuxkit build can be tested using -output tar and comparing the output of subsequent builds with tools like diff or the excellent diffoscope.

The second phase of linuxkit build converts the intermediary tar format into the desired output format. Making this phase reproducible depends on the tools used to generate the output.

Builds, which produce ISO formats should probably be converted to use go-diskfs before attempting to make them reproducible.

For ideas on how to make the builds for other output formats reproducible, see this page.

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