===================
Reproducible builds
===================

It is generally desirable that building the same source code with
the same set of tools is reproducible, i.e. the output is always
exactly the same.  This makes it possible to verify that the build
infrastructure for a binary distribution or embedded system has not
been subverted.  This can also make it easier to verify that a source
or tool change does not make any difference to the resulting binaries.

The `Reproducible Builds project`_ has more information about this
general topic.  This document covers the various reasons why building
the kernel may be unreproducible, and how to avoid them.

Timestamps
----------

The kernel embeds timestamps in three places:

* The version string exposed by ``uname()`` and included in
  ``/proc/version``

* File timestamps in the embedded initramfs

* If enabled via ``CONFIG_IKHEADERS``, file timestamps of kernel
  headers embedded in the kernel or respective module,
  exposed via ``/sys/kernel/kheaders.tar.xz``

By default the timestamp is the current time and in the case of
``kheaders`` the various files' modification times. This must
be overridden using the `KBUILD_BUILD_TIMESTAMP`_ variable.
If you are building from a git commit, you could use its commit date.

The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
and enables warnings if they are used.  If you incorporate external
code that does use these, you must override the timestamp they
correspond to by setting the `SOURCE_DATE_EPOCH`_ environment
variable.

User, host
----------

The kernel embeds the building user and host names in
``/proc/version``.  These must be overridden using the
`KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables.  If you are
building from a git commit, you could use its committer address.

Absolute filenames
------------------

When the kernel is built out-of-tree, debug information may include
absolute filenames for the source files.  This must be overridden by
including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.

Depending on the compiler used, the ``__FILE__`` macro may also expand
to an absolute filename in an out-of-tree build.  Kbuild automatically
uses the ``-fmacro-prefix-map`` option to prevent this, if it is
supported.

The Reproducible Builds web site has more information about these
`prefix-map options`_.

Generated files in source packages
----------------------------------

The build processes for some programs under the ``tools/``
subdirectory do not completely support out-of-tree builds.  This may
cause a later source package build using e.g. ``make rpm-pkg`` to
include generated files.  You should ensure the source tree is
pristine by running ``make mrproper`` or ``git clean -d -f -x`` before
building a source package.

Module signing
--------------

If you enable ``CONFIG_MODULE_SIG_ALL``, the default behaviour is to
generate a different temporary key for each build, resulting in the
modules being unreproducible.  However, including a signing key with
your source would presumably defeat the purpose of signing modules.

One approach to this is to divide up the build process so that the
unreproducible parts can be treated as sources:

1. Generate a persistent signing key.  Add the certificate for the key
   to the kernel source.

2. Set the ``CONFIG_SYSTEM_TRUSTED_KEYS`` symbol to include the
   signing key's certificate, set ``CONFIG_MODULE_SIG_KEY`` to an
   empty string, and disable ``CONFIG_MODULE_SIG_ALL``.
   Build the kernel and modules.

3. Create detached signatures for the modules, and publish them as
   sources.

4. Perform a second build that attaches the module signatures.  It
   can either rebuild the modules or use the output of step 2.

Structure randomisation
-----------------------

If you enable ``CONFIG_RANDSTRUCT``, you will need to pre-generate
the random seed in ``scripts/basic/randstruct.seed`` so the same
value is used by each build. See ``scripts/gen-randstruct-seed.sh``
for details.

Debug info conflicts
--------------------

This is not a problem of unreproducibility, but of generated files
being *too* reproducible.

Once you set all the necessary variables for a reproducible build, a
vDSO's debug information may be identical even for different kernel
versions.  This can result in file conflicts between debug information
packages for the different kernel versions.

To avoid this, you can make the vDSO different for different
kernel versions by including an arbitrary string of "salt" in it.
This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``.

.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
.. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host
.. _KCFLAGS: kbuild.html#kcflags
.. _prefix-map options: https://reproducible-builds.org/docs/build-path/
.. _Reproducible Builds project: https://reproducible-builds.org/
.. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/
