On Sat, Sep 14, 2019 at 8:24 PM Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote: > > In the Distribution Kernels track at Linux Plumbers Conference there > was some discussion around the difficulty of making kernel builds > reproducible. > > This is a solved problem, but the solutions don't appear to be > documented in one place. This document lists the issues I know about > and the settings needed to ensure reproducibility. > > Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> Acked-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> Thanks. > --- > v2: > * Delete extra "the" in the first paragraph. > * Rewrite paragraph about filenames to reflect that -fmacro-prefix-map > is used by default. > * Rewrite paragraph about source packages to be clearer. > > Documentation/kbuild/index.rst | 1 + > Documentation/kbuild/reproducible-builds.rst | 122 +++++++++++++++++++ > 2 files changed, 123 insertions(+) > create mode 100644 Documentation/kbuild/reproducible-builds.rst > > diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst > index e323a3f2cc81..0f144fad99a6 100644 > --- a/Documentation/kbuild/index.rst > +++ b/Documentation/kbuild/index.rst > @@ -18,6 +18,7 @@ Kernel Build System > headers_install > > issues > + reproducible-builds > > .. only:: subproject and html > > diff --git a/Documentation/kbuild/reproducible-builds.rst b/Documentation/kbuild/reproducible-builds.rst > new file mode 100644 > index 000000000000..ab92e98c89c8 > --- /dev/null > +++ b/Documentation/kbuild/reproducible-builds.rst > @@ -0,0 +1,122 @@ > +=================== > +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 a timestamp in two places: > + > +* The version string exposed by ``uname()`` and included in > + ``/proc/version`` > + > +* File timestamps in the embedded initramfs > + > +By default the timestamp is the current time. 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_GCC_PLUGIN_RANDSTRUCT``, you will need to > +pre-generate the random seed in > +``scripts/gcc-plgins/randomize_layout_seed.h`` so the same value > +is used in rebuilds. > + > +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/ -- Best Regards Masahiro Yamada
