.. SPDX-License-Identifier: CC-BY-SA-2.0-UK

*******************
Reproducible Builds
*******************

================
How we define it
================

The Yocto Project defines reproducibility as where a given input build
configuration will give the same binary output regardless of when it is built
(now or in 5 years time), regardless of the path on the filesystem the build is
run in, and regardless of the distro and tools on the underlying host system the
build is running on.

==============
Why it matters
==============

The project aligns with the `Reproducible Builds project
<https://reproducible-builds.org/>`_, which shares information about why
reproducibility matters. The primary focus of the project is the ability to
detect security issues being introduced. However, from a Yocto Project
perspective, it is also hugely important that our builds are deterministic. When
you build a given input set of metadata, we expect you to get consistent output.
This has always been a key focus but, :yocto_docs:`since release 3.1 ("dunfell")
</ref-manual/migration-3.1.html#reproducible-builds-now-enabled-by-default>`,
it is now true down to the binary level including timestamps.

For example, at some point in the future life of a product, you find that you
need to rebuild to add a security fix. If this happens, only the components that
have been modified should change at the binary level. This would lead to much
easier and clearer bounds on where validation is needed.

This also gives an additional benefit to the project builds themselves, our hash
equivalence for :ref:`Shared State <overview-manual/concepts:Shared State>`
object reuse works much more effectively when the binary output remains the
same.

.. note::

   We strongly advise you to make sure your project builds reproducibly
   before finalizing your production images. It would be too late if you
   only address this issue when the first updates are required.

===================
How we implement it
===================

There are many different aspects to build reproducibility, but some particular
things we do within the build system to ensure reproducibility include:

-  Adding mappings to the compiler options to ensure debug filepaths are mapped
   to consistent target compatible paths. This is done through the
   :term:`DEBUG_PREFIX_MAP` variable which sets the ``-fmacro-prefix-map`` and
   ``-fdebug-prefix-map`` compiler options correctly to map to target paths.
-  Being explicit about recipe dependencies and their configuration (no floating
   configure options or host dependencies creeping in). In particular this means
   making sure :term:`PACKAGECONFIG` coverage covers configure options which may
   otherwise try and auto-detect host dependencies.
-  Using recipe specific sysroots to isolate recipes so they only see their
   dependencies. These are visible as ``recipe-sysroot`` and
   ``recipe-sysroot-native`` directories within the :term:`WORKDIR` of a given
   recipe and are populated only with the dependencies a recipe has.
-  Build images from a reduced package set: only packages from recipes the image
   depends upon.
-  Filtering the tools available from the host's ``PATH`` to only a specific set
   of tools, set using the :term:`HOSTTOOLS` variable.

.. note::

   Because of an open bug in GCC, using ``DISTRO_FEATURES:append = " lto"`` or
   adding ``-flto`` (Link Time Optimization) to ``CFLAGS`` makes the resulting
   binary non-reproducible, in that it depends on the full absolute build path
   to ``recipe-sysroot-native``, so installing the Yocto Project in a different
   directory results in a different binary.

   This issue is addressed by
   :yocto_bugs:`bug 14481 -  Programs built with -flto are not reproducible</show_bug.cgi?id=14481>`.

=========================================
Can we prove the project is reproducible?
=========================================

Yes, we can prove it and we regularly test this on the Autobuilder. At the
time of writing (release 3.3, "hardknott"), :term:`OpenEmbedded-Core (OE-Core)`
is 100% reproducible for all its recipes (i.e. world builds) apart from the Go
language and Ruby documentation packages. Unfortunately, the current
implementation of the Go language has fundamental reproducibility problems as
it always depends upon the paths it is built in.

.. note::

   Only BitBake and :term:`OpenEmbedded-Core (OE-Core)`, which is the ``meta``
   layer in Poky, guarantee complete reproducibility. The moment you add
   another layer, this warranty is voided, because of additional configuration
   files, ``bbappend`` files, overridden classes, etc.

To run our automated selftest, as we use in our CI on the Autobuilder, you can
run::

   oe-selftest -r reproducible.ReproducibleTests.test_reproducible_builds

This defaults to including a ``world`` build so, if other layers are added, it would
also run the tests for recipes in the additional layers. The first build will be
run using :ref:`Shared State <overview-manual/concepts:Shared State>` if
available, the second build explicitly disables
:ref:`Shared State <overview-manual/concepts:Shared State>` and builds on the
specific host the build is running on. This means we can test reproducibility
builds between different host distributions over time on the Autobuilder.

If ``OEQA_DEBUGGING_SAVED_OUTPUT`` is set, any differing packages will be saved
here. The test is also able to run the ``diffoscope`` command on the output to
generate HTML files showing the differences between the packages, to aid
debugging. On the Autobuilder, these appear under
https://autobuilder.yocto.io/pub/repro-fail/ in the form ``oe-reproducible +
<date> + <random ID>``, e.g. ``oe-reproducible-20200202-1lm8o1th``.

The project's current reproducibility status can be seen at
:yocto_home:`/reproducible-build-results/`

You can also check the reproducibility status on supported host distributions:

-  CentOS: :yocto_ab:`/typhoon/#/builders/reproducible-centos`
-  Debian: :yocto_ab:`/typhoon/#/builders/reproducible-debian`
-  Fedora: :yocto_ab:`/typhoon/#/builders/reproducible-fedora`
-  Ubuntu: :yocto_ab:`/typhoon/#/builders/reproducible-ubuntu`

===============================
Can I test my layer or recipes?
===============================

Once again, you can run a ``world`` test using the
:ref:`oe-selftest <ref-manual/release-process:Testing and Quality Assurance>`
command provided above. This functionality is implemented
in :oe_git:`meta/lib/oeqa/selftest/cases/reproducible.py
</openembedded-core/tree/meta/lib/oeqa/selftest/cases/reproducible.py>`.

You could subclass the test and change ``targets`` to a different target.

You may also change ``sstate_targets`` which would allow you to "pre-cache" some
set of recipes before the test, meaning they are excluded from reproducibility
testing. As a practical example, you could set ``sstate_targets`` to
``core-image-sato``, then setting ``targets`` to ``core-image-sato-sdk`` would
run reproducibility tests only on the targets belonging only to ``core-image-sato-sdk``.