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14.2 Подготовка к использованию двоичных файлов первоначальной загрузки

Dependency graph of the early bootstrap

The figure above shows the very beginning of the dependency graph of the distribution, corresponding to the package definitions of the (gnu packages bootstrap) module. A similar figure can be generated with guix graph (see Запуск guix graph), along the lines of:

guix graph -t derivation \
  -e '(@@ (gnu packages bootstrap) %bootstrap-gcc)' \
  | dot -Tps >

or, for the further Reduced Binary Seed bootstrap

guix graph -t derivation \
  -e '(@@ (gnu packages bootstrap) %bootstrap-mes)' \
  | dot -Tps >

At this level of detail, things are slightly complex. First, Guile itself consists of an ELF executable, along with many source and compiled Scheme files that are dynamically loaded when it runs. This gets stored in the guile-2.0.7.tar.xz tarball shown in this graph. This tarball is part of Guix’s “source” distribution, and gets inserted into the store with add-to-store (see Хранилище).

But how do we write a derivation that unpacks this tarball and adds it to the store? To solve this problem, the guile-bootstrap-2.0.drv derivation—the first one that gets built—uses bash as its builder, which runs, which in turn calls tar to unpack the tarball. Thus, bash, tar, xz, and mkdir are statically-linked binaries, also part of the Guix source distribution, whose sole purpose is to allow the Guile tarball to be unpacked.

Once guile-bootstrap-2.0.drv is built, we have a functioning Guile that can be used to run subsequent build programs. Its first task is to download tarballs containing the other pre-built binaries—this is what the .tar.xz.drv derivations do. Guix modules such as ftp-client.scm are used for this purpose. The module-import.drv derivations import those modules in a directory in the store, using the original layout. The module-import-compiled.drv derivations compile those modules, and write them in an output directory with the right layout. This corresponds to the #:modules argument of build-expression->derivation (see Деривации).

Finally, the various tarballs are unpacked by the derivations gcc-bootstrap-0.drv, glibc-bootstrap-0.drv, or bootstrap-mes-0.drv and bootstrap-mescc-tools-0.drv, at which point we have a working C tool chain.

Сборка инструментов сборки

Bootstrapping is complete when we have a full tool chain that does not depend on the pre-built bootstrap tools discussed above. This no-dependency requirement is verified by checking whether the files of the final tool chain contain references to the /gnu/store directories of the bootstrap inputs. The process that leads to this “final” tool chain is described by the package definitions found in the (gnu packages commencement) module.

The guix graph command allows us to “zoom out” compared to the graph above, by looking at the level of package objects instead of individual derivations—remember that a package may translate to several derivations, typically one derivation to download its source, one to build the Guile modules it needs, and one to actually build the package from source. The command:

guix graph -t bag \
  -e '(@@ (gnu packages commencement)
          glibc-final-with-bootstrap-bash)' | xdot -

displays the dependency graph leading to the “final” C library33, depicted below.

График зависимости ранних пакетов

Первый инструмент, который собирается с помощью двоичных файлов начальной загрузки, - это GNU Make—отмеченный make-boot0 выше—который является обязательным для всех следующих пакетов. После собираются Findutils и Diffutils.

Then come the first-stage Binutils and GCC, built as pseudo cross tools—i.e., with --target equal to --host. They are used to build libc. Thanks to this cross-build trick, this libc is guaranteed not to hold any reference to the initial tool chain.

From there the final Binutils and GCC (not shown above) are built. GCC uses ld from the final Binutils, and links programs against the just-built libc. This tool chain is used to build the other packages used by Guix and by the GNU Build System: Guile, Bash, Coreutils, etc.

И вуаля! Теперь у нас есть полный набор инструментов сборки, который ожидает система сборки GNU. Они находятся в переменной %final-input модуля (gnu packages commencement) и неявно используются любым пакетом, использующим gnu-build-system (see gnu-build-system).

Сборка двоичных файлов двоичной загрузки

Поскольку окончательный набор инструментов не зависит от двоичных файлов начальной загрузки, их редко требуется обновлять. Тем не менее, полезно иметь автоматический способ их создания, если произойдет обновление, и это то, что обеспечивает модуль (gnu packages make-bootstrap).

Следующая команда создает tar-архивы, содержащие двоичные файлы начальной загрузки (Binutils, GCC, glibc, для традиционной загрузки и linux-libre-headers, bootstrap-mescc-tools, bootstrap-mes для начальной загрузки Reduced Binary Seed, Guile и tarball содержащий смесь Coreutils и других основных инструментов командной строки):

tar архив начальной сборки guix

Сгенерированные тарболы - это те, на которые нужно ссылаться в модуле (gnu packages bootstrap), упомянутом в начале этого раздела.

Все еще здесь? Тогда, может быть, вы задались вопросом: когда мы достигнем фиксированной точки? Это интересный вопрос! Ответ неизвестен, но если вы хотите продолжить исследование (и располагаете значительными вычислительными ресурсами для этого), сообщите нам.

Сокращение набора Bootstrap Binaries

Our traditional bootstrap includes GCC, GNU Libc, Guile, etc. That’s a lot of binary code! Why is that a problem? It’s a problem because these big chunks of binary code are practically non-auditable, which makes it hard to establish what source code produced them. Every unauditable binary also leaves us vulnerable to compiler backdoors as described by Ken Thompson in the 1984 paper Reflections on Trusting Trust.

This is mitigated by the fact that our bootstrap binaries were generated from an earlier Guix revision. Nevertheless it lacks the level of transparency that we get in the rest of the package dependency graph, where Guix always gives us a source-to-binary mapping. Thus, our goal is to reduce the set of bootstrap binaries to the bare minimum.

The web site lists on-going projects to do that. One of these is about replacing the bootstrap GCC with a sequence of assemblers, interpreters, and compilers of increasing complexity, which could be built from source starting from a simple and auditable assembler.

Our first major achievement is the replacement of of GCC, the GNU C Library and Binutils by MesCC-Tools (a simple hex linker and macro assembler) and Mes (see GNU Mes Reference Manual in GNU Mes, a Scheme interpreter and C compiler in Scheme). Neither MesCC-Tools nor Mes can be fully bootstrapped yet and thus we inject them as binary seeds. We call this the Reduced Binary Seed bootstrap, as it has halved the size of our bootstrap binaries! Also, it has eliminated the C compiler binary; i686-linux and x86_64-linux Guix packages are now bootstrapped without any binary C compiler.

Work is ongoing to make MesCC-Tools and Mes fully bootstrappable and we are also looking at any other bootstrap binaries. Your help is welcome!



You may notice the glibc-intermediate label, suggesting that it is not quite final, but as a good approximation, we will consider it final.

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