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diff --git a/documentation/test-manual/reproducible-builds.rst b/documentation/test-manual/reproducible-builds.rst new file mode 100644 index 0000000000..91f94a5c74 --- /dev/null +++ b/documentation/test-manual/reproducible-builds.rst @@ -0,0 +1,137 @@ +.. 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, :ref:`since release 3.1 ("dunfell") +<migration-guides/migration-3.1: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 +:ref:`overview-manual/concepts:Hash Equivalence` for +:ref:`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. + +========================================= +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. Different build targets +can be defined using the :term:`OEQA_REPRODUCIBLE_TEST_TARGET` variable in ``local.conf``. +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>` except for recipes defined in +the :term:`OEQA_REPRODUCIBLE_TEST_SSTATE_TARGETS` variable, 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``. |