^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) ===================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) Reproducible builds
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3) ===================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) It is generally desirable that building the same source code with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6) the same set of tools is reproducible, i.e. the output is always
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) exactly the same. This makes it possible to verify that the build
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) infrastructure for a binary distribution or embedded system has not
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) been subverted. This can also make it easier to verify that a source
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) or tool change does not make any difference to the resulting binaries.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) The `Reproducible Builds project`_ has more information about this
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) general topic. This document covers the various reasons why building
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) the kernel may be unreproducible, and how to avoid them.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) Timestamps
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) ----------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) The kernel embeds timestamps in three places:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) * The version string exposed by ``uname()`` and included in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) ``/proc/version``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) * File timestamps in the embedded initramfs
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) * If enabled via ``CONFIG_IKHEADERS``, file timestamps of kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) headers embedded in the kernel or respective module,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) exposed via ``/sys/kernel/kheaders.tar.xz``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) By default the timestamp is the current time and in the case of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) ``kheaders`` the various files' modification times. This must
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) be overridden using the `KBUILD_BUILD_TIMESTAMP`_ variable.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) If you are building from a git commit, you could use its commit date.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) and enables warnings if they are used. If you incorporate external
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) code that does use these, you must override the timestamp they
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) correspond to by setting the `SOURCE_DATE_EPOCH`_ environment
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) variable.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) User, host
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) ----------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44) The kernel embeds the building user and host names in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) ``/proc/version``. These must be overridden using the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46) `KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables. If you are
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47) building from a git commit, you could use its committer address.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49) Absolute filenames
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50) ------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) When the kernel is built out-of-tree, debug information may include
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53) absolute filenames for the source files. This must be overridden by
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) Depending on the compiler used, the ``__FILE__`` macro may also expand
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57) to an absolute filename in an out-of-tree build. Kbuild automatically
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) uses the ``-fmacro-prefix-map`` option to prevent this, if it is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) supported.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) The Reproducible Builds web site has more information about these
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62) `prefix-map options`_.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) Generated files in source packages
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65) ----------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) The build processes for some programs under the ``tools/``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) subdirectory do not completely support out-of-tree builds. This may
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) cause a later source package build using e.g. ``make rpm-pkg`` to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) include generated files. You should ensure the source tree is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) pristine by running ``make mrproper`` or ``git clean -d -f -x`` before
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) building a source package.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74) Module signing
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75) --------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) If you enable ``CONFIG_MODULE_SIG_ALL``, the default behaviour is to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78) generate a different temporary key for each build, resulting in the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) modules being unreproducible. However, including a signing key with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) your source would presumably defeat the purpose of signing modules.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) One approach to this is to divide up the build process so that the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) unreproducible parts can be treated as sources:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85) 1. Generate a persistent signing key. Add the certificate for the key
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86) to the kernel source.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) 2. Set the ``CONFIG_SYSTEM_TRUSTED_KEYS`` symbol to include the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) signing key's certificate, set ``CONFIG_MODULE_SIG_KEY`` to an
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) empty string, and disable ``CONFIG_MODULE_SIG_ALL``.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91) Build the kernel and modules.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) 3. Create detached signatures for the modules, and publish them as
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) sources.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) 4. Perform a second build that attaches the module signatures. It
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97) can either rebuild the modules or use the output of step 2.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) Structure randomisation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) -----------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) If you enable ``CONFIG_GCC_PLUGIN_RANDSTRUCT``, you will need to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103) pre-generate the random seed in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) ``scripts/gcc-plugins/randomize_layout_seed.h`` so the same value
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105) is used in rebuilds.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) Debug info conflicts
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) --------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) This is not a problem of unreproducibility, but of generated files
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) being *too* reproducible.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) Once you set all the necessary variables for a reproducible build, a
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) vDSO's debug information may be identical even for different kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) versions. This can result in file conflicts between debug information
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116) packages for the different kernel versions.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118) To avoid this, you can make the vDSO different for different
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) kernel versions by including an arbitrary string of "salt" in it.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120) This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) .. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) .. _KCFLAGS: kbuild.html#kcflags
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125) .. _prefix-map options: https://reproducible-builds.org/docs/build-path/
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126) .. _Reproducible Builds project: https://reproducible-builds.org/
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127) .. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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