| Message ID | 20190914112420.qift4wjk7llxg57x@decadent.org.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] Documentation: kbuild: Add document about reproducible builds | expand |
On Sat, Sep 14, 2019 at 8:24 PM Ben Hutchings <ben@decadent.org.uk> 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@decadent.org.uk> Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com> 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/
On Sun, 15 Sep 2019 12:24:49 +0900 Masahiro Yamada <yamada.masahiro@socionext.com> wrote: > On Sat, Sep 14, 2019 at 8:24 PM Ben Hutchings <ben@decadent.org.uk> 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@decadent.org.uk> > > > Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com> I have applied this, thanks. jon
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/
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@decadent.org.uk> --- 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