1=================== 2Reproducible builds 3=================== 4 5It is generally desirable that building the same source code with 6the same set of tools is reproducible, i.e. the output is always 7exactly the same. This makes it possible to verify that the build 8infrastructure for a binary distribution or embedded system has not 9been subverted. This can also make it easier to verify that a source 10or tool change does not make any difference to the resulting binaries. 11 12The `Reproducible Builds project`_ has more information about this 13general topic. This document covers the various reasons why building 14the kernel may be unreproducible, and how to avoid them. 15 16Timestamps 17---------- 18 19The kernel embeds timestamps in three places: 20 21* The version string exposed by ``uname()`` and included in 22 ``/proc/version`` 23 24* File timestamps in the embedded initramfs 25 26* If enabled via ``CONFIG_IKHEADERS``, file timestamps of kernel 27 headers embedded in the kernel or respective module, 28 exposed via ``/sys/kernel/kheaders.tar.xz`` 29 30By default the timestamp is the current time and in the case of 31``kheaders`` the various files' modification times. This must 32be overridden using the `KBUILD_BUILD_TIMESTAMP`_ variable. 33If you are building from a git commit, you could use its commit date. 34 35The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros, 36and enables warnings if they are used. If you incorporate external 37code that does use these, you must override the timestamp they 38correspond to by setting the `SOURCE_DATE_EPOCH`_ environment 39variable. 40 41User, host 42---------- 43 44The kernel embeds the building user and host names in 45``/proc/version``. These must be overridden using the 46`KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables. If you are 47building from a git commit, you could use its committer address. 48 49Absolute filenames 50------------------ 51 52When the kernel is built out-of-tree, debug information may include 53absolute filenames for the source files. This must be overridden by 54including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable. 55 56Depending on the compiler used, the ``__FILE__`` macro may also expand 57to an absolute filename in an out-of-tree build. Kbuild automatically 58uses the ``-fmacro-prefix-map`` option to prevent this, if it is 59supported. 60 61The Reproducible Builds web site has more information about these 62`prefix-map options`_. 63 64Generated files in source packages 65---------------------------------- 66 67The build processes for some programs under the ``tools/`` 68subdirectory do not completely support out-of-tree builds. This may 69cause a later source package build using e.g. ``make rpm-pkg`` to 70include generated files. You should ensure the source tree is 71pristine by running ``make mrproper`` or ``git clean -d -f -x`` before 72building a source package. 73 74Module signing 75-------------- 76 77If you enable ``CONFIG_MODULE_SIG_ALL``, the default behaviour is to 78generate a different temporary key for each build, resulting in the 79modules being unreproducible. However, including a signing key with 80your source would presumably defeat the purpose of signing modules. 81 82One approach to this is to divide up the build process so that the 83unreproducible parts can be treated as sources: 84 851. Generate a persistent signing key. Add the certificate for the key 86 to the kernel source. 87 882. Set the ``CONFIG_SYSTEM_TRUSTED_KEYS`` symbol to include the 89 signing key's certificate, set ``CONFIG_MODULE_SIG_KEY`` to an 90 empty string, and disable ``CONFIG_MODULE_SIG_ALL``. 91 Build the kernel and modules. 92 933. Create detached signatures for the modules, and publish them as 94 sources. 95 964. Perform a second build that attaches the module signatures. It 97 can either rebuild the modules or use the output of step 2. 98 99Structure randomisation 100----------------------- 101 102If you enable ``CONFIG_RANDSTRUCT``, you will need to pre-generate 103the random seed in ``scripts/gcc-plugins/randomize_layout_seed.h`` 104so the same value is used in rebuilds. 105 106Debug info conflicts 107-------------------- 108 109This is not a problem of unreproducibility, but of generated files 110being *too* reproducible. 111 112Once you set all the necessary variables for a reproducible build, a 113vDSO's debug information may be identical even for different kernel 114versions. This can result in file conflicts between debug information 115packages for the different kernel versions. 116 117To avoid this, you can make the vDSO different for different 118kernel versions by including an arbitrary string of "salt" in it. 119This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``. 120 121.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp 122.. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host 123.. _KCFLAGS: kbuild.html#kcflags 124.. _prefix-map options: https://reproducible-builds.org/docs/build-path/ 125.. _Reproducible Builds project: https://reproducible-builds.org/ 126.. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/ 127