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Andrew Geissler09036742021-06-25 14:25:14 -05001.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
2
3*******************
4Reproducible Builds
5*******************
6
7================
8How we define it
9================
10
11The Yocto Project defines reproducibility as where a given input build
12configuration will give the same binary output regardless of when it is built
13(now or in 5 years time), regardless of the path on the filesystem the build is
14run in, and regardless of the distro and tools on the underlying host system the
15build is running on.
16
17==============
18Why it matters
19==============
20
21The project aligns with the `Reproducible Builds project
22<https://reproducible-builds.org/>`_, which shares information about why
23reproducibility matters. The primary focus of the project is the ability to
24detect security issues being introduced. However, from a Yocto Project
25perspective, it is also hugely important that our builds are deterministic. When
26you build a given input set of metadata, we expect you to get consistent output.
27This has always been a key focus but, :yocto_docs:`since release 3.1 ("dunfell")
28</ref-manual/migration-3.1.html#reproducible-builds-now-enabled-by-default>`,
29it is now true down to the binary level including timestamps.
30
31For example, at some point in the future life of a product, you find that you
32need to rebuild to add a security fix. If this happens, only the components that
33have been modified should change at the binary level. This would lead to much
34easier and clearer bounds on where validation is needed.
35
36This also gives an additional benefit to the project builds themselves, our hash
37equivalence for :ref:`Shared State <overview-manual/concepts:Shared State>`
38object reuse works much more effectively when the binary output remains the
39same.
40
41.. note::
42
43 We strongly advise you to make sure your project builds reproducibly
44 before finalizing your production images. It would be too late if you
45 only address this issue when the first updates are required.
46
47===================
48How we implement it
49===================
50
51There are many different aspects to build reproducibility, but some particular
52things we do within the build system to ensure reproducibility include:
53
54- Adding mappings to the compiler options to ensure debug filepaths are mapped
55 to consistent target compatible paths. This is done through the
56 ``DEBUG_PREFIX_MAP`` variable which sets the ``-fmacro-prefix-map`` and
57 ``-fdebug-prefix-map`` compiler options correctly to map to target paths.
58- Being explicit about recipe dependencies and their configuration (no floating
59 configure options or host dependencies creeping in). In particular this means
60 making sure :term:`PACKAGECONFIG` coverage covers configure options which may
61 otherwise try and auto-detect host dependencies.
62- Using recipe specific sysroots to isolate recipes so they only see their
63 dependencies. These are visible as ``recipe-sysroot`` and
64 ``recipe-sysroot-native`` directories within the :term:`WORKDIR` of a given
65 recipe and are populated only with the dependencies a recipe has.
66- Build images from a reduced package set: only packages from recipes the image
67 depends upon.
68- Filtering the tools available from the host's ``PATH`` to only a specific set
69 of tools, set using the :term:`HOSTTOOLS` variable.
70
Patrick Williams213cb262021-08-07 19:21:33 -050071.. note::
72
Patrick Williams0ca19cc2021-08-16 14:03:13 -050073 Because of an open bug in GCC, using ``DISTRO_FEATURES:append = " lto"`` or
Patrick Williams213cb262021-08-07 19:21:33 -050074 adding ``-flto`` (Link Time Optimization) to ``CFLAGS`` makes the resulting
75 binary non-reproducible, in that it depends on the full absolute build path
76 to ``recipe-sysroot-native``, so installing the Yocto Project in a different
77 directory results in a different binary.
78
79 This issue is addressed by
80 :yocto_bugs:`bug 14481 - Programs built with -flto are not reproducible</show_bug.cgi?id=14481>`.
81
Andrew Geissler09036742021-06-25 14:25:14 -050082=========================================
83Can we prove the project is reproducible?
84=========================================
85
86Yes, we can prove it and we regularly test this on the Autobuilder. At the
87time of writing (release 3.3, "hardknott"), :term:`OpenEmbedded-Core (OE-Core)`
88is 100% reproducible for all its recipes (i.e. world builds) apart from the Go
89language and Ruby documentation packages. Unfortunately, the current
90implementation of the Go language has fundamental reproducibility problems as
91it always depends upon the paths it is built in.
92
93.. note::
94
95 Only BitBake and :term:`OpenEmbedded-Core (OE-Core)`, which is the ``meta``
96 layer in Poky, guarantee complete reproducibility. The moment you add
97 another layer, this warranty is voided, because of additional configuration
98 files, ``bbappend`` files, overridden classes, etc.
99
100To run our automated selftest, as we use in our CI on the Autobuilder, you can
101run::
102
103 oe-selftest -r reproducible.ReproducibleTests.test_reproducible_builds
104
105This defaults to including a ``world`` build so, if other layers are added, it would
106also run the tests for recipes in the additional layers. The first build will be
107run using :ref:`Shared State <overview-manual/concepts:Shared State>` if
108available, the second build explicitly disables
109:ref:`Shared State <overview-manual/concepts:Shared State>` and builds on the
110specific host the build is running on. This means we can test reproducibility
111builds between different host distributions over time on the Autobuilder.
112
113If ``OEQA_DEBUGGING_SAVED_OUTPUT`` is set, any differing packages will be saved
114here. The test is also able to run the ``diffoscope`` command on the output to
115generate HTML files showing the differences between the packages, to aid
116debugging. On the Autobuilder, these appear under
117https://autobuilder.yocto.io/pub/repro-fail/ in the form ``oe-reproducible +
118<date> + <random ID>``, e.g. ``oe-reproducible-20200202-1lm8o1th``.
119
120The project's current reproducibility status can be seen at
121:yocto_home:`/reproducible-build-results/`
122
123You can also check the reproducibility status on supported host distributions:
124
125- CentOS: :yocto_ab:`/typhoon/#/builders/reproducible-centos`
126- Debian: :yocto_ab:`/typhoon/#/builders/reproducible-debian`
127- Fedora: :yocto_ab:`/typhoon/#/builders/reproducible-fedora`
128- Ubuntu: :yocto_ab:`/typhoon/#/builders/reproducible-ubuntu`
129
130===============================
131Can I test my layer or recipes?
132===============================
133
134Once again, you can run a ``world`` test using the
135:ref:`oe-selftest <ref-manual/release-process:Testing and Quality Assurance>`
136command provided above. This functionality is implemented
137in :oe_git:`meta/lib/oeqa/selftest/cases/reproducible.py
138</openembedded-core/tree/meta/lib/oeqa/selftest/cases/reproducible.py>`.
139
140You could subclass the test and change ``targets`` to a different target.
141
142You may also change ``sstate_targets`` which would allow you to "pre-cache" some
143set of recipes before the test, meaning they are excluded from reproducibility
144testing. As a practical example, you could set ``sstate_targets`` to
145``core-image-sato``, then setting ``targets`` to ``core-image-sato-sdk`` would
146run reproducibility tests only on the targets belonging only to ``core-image-sato-sdk``.