Guix

Guix Reduces Bootstrap Seed by 50%

Jan Nieuwenhuizen — 8-a de Oktobro 2019

We are delighted to announce that the first reduction by 50% of the Guix bootstrap binaries has now been officially released!

This is a very important step because the ~250MB seed of binary code was 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 and beautifully explained by Carl Dong in his Bitcoin Build System Security talk.

It is therefore equally important that we continue towards our final goal: A Full Source bootstrap; removing all unauditable binary seeds.

Guix’ Rigorous Regular Bootstrap

GNU Guix takes a rigorous approach to bootstrapping. Bootstrapping in this context refers to how the distribution gets built from nothing.

The GNU system is primarily made of C code, with glibc at its core. The GNU build system itself assumes the availability of a Bourne shell and command-line tools provided by the Core GNU Utilities and Awk, Grep, Make, Sed, and some others.

The build environment of a package is a container that contains nothing but the package’s declared inputs. To be able to build anything at all in this container, Guix needs pre-built binaries of Guile, GCC, Binutils, glibc, and the other tools mentioned above.

So there is an obvious chicken-and-egg problem: How does the first package get built? How does the first compiler get compiled?

                            gcc-final
                                ^
                                |
                          cross-gcc-boot
                                ^
                                |
                        gcc-boot0 (5.5.0)
                                ^
                                |
                 binutils-boot0, libstdc++-boot0
                                ^
                                |
           diffutils-boot0, findutils-boot0, file-boot0
                                ^
                                |
                           make-boot0
                                ^
                                |
                                *
        bootstrap-binutils, bootstrap-gcc, bootstrap-glibc (~130MB)
      bootstrap-bash, bootstrap-coreutils&co, bootstrap-guile (~120MB)

full graph

The answer to this starts with bootstrap binaries. The first package that gets built with these bootstrap binaries is make, next are diffutils, findutils, and file. Eventually a gcc-final is built: the compiler used to build regular packages.

i686/x86_64 Reduced Binary Seed bootstrap

The Guix development branch we just merged introduces a reduced binary seed bootstrap for x86_64 and i686, where the bottom of the dependency graph looks like this:

                         gcc-mesboot (4.9.4)
                                ^
                                |
                        glibc-mesboot (2.16.0)
                                ^
                                |
                         gcc-mesboot1 (4.7.4)
                                ^
                                |
                        binutils-mesboot (2.20.1a)
                                ^
                                |
                         gcc-mesboot0 (2.95.3)
                                ^
                                |
                         glibc-mesboot0 (2.2.5)
                                ^
                                |
                        gcc-core-mesboot (2.95.3)
                                ^
                                |
    make-mesboot0, diffutils-mesboot, binutils-mesboot0 (2.20.1a)
                                ^
                                |
                            tcc-boot
                                ^
                                |
                            tcc-boot0
                                ^
                                |
                            mes-boot
                                ^
                                |
                                *
              bootstrap-mescc-tools, bootstrap-mes (~10MB)
      bootstrap-bash, bootstrap-coreutils&co, bootstrap-guile (~120MB)

full graph

The new Reduced Binary Seed bootstrap removes Binutils, GCC, and glibc and replaces them by GNU Mes and MesCC Tools. This reduces the trusted binary seed by ~120MB - half of it!

As a user, it means your package manager has a formal description of how to build all your applications, in a reproducible way, starting from nothing but this ~120MB seed. It means you can rebuild any of those software artifacts locally without trusting a single binary provider.

For comparison, traditional distros often have an informally specified bootstrap story, usually relying on much bigger binary seeds. We estimate those seeds to weigh in at ~550MB (the size of debootstrap --arch=i386 --include=build-essential,dpkg-dev,debhelper,gcc,libc6-dev,make,texinfo bullseye ./bullseye-chroot http://deb.debian.org/debian, with bullseye-chroot/var/cache/apt/archives removed) in the case of Debian—ignoring cycles that show up higher in the graph.

These bootstrap binaries can now be re-created by doing

    guix build bootstrap-binaries

Work started three years ago with a simple LISP-1.5 interpreter.

A year later, Mes 0.5 had become a tiny Scheme interpreter written in simple subset of C that came with a simple C compiler in Scheme. And yes, these were mutual self-hosting.

The next step was to find a path towards compiling Guix’s default GCC (5.5.0). Sadly, bootstrapping GCC compilers has been becoming increasingly difficult over the years. We looked at GCC 1.42: not easy to bootstrap (100,000 LOC) and it depends on Bison. Reluctantly, we started looking for non-GNU alternatives 8cc, pcc, cc500 but finally settled on TinyCC. TinyCC (TCC) can compile GCC (4.7.4) which is currently the most recent release of GCC that can be built without a C++ compiler.

Another year later, Mes 0.13 has grown its own tiny C library and compiles a heavily patched and simplified TCC. This looked very promising and we suggested for TinyCC to help our bootstrapping effort by moving towards a simplified C subset. Instead we were encouraged to make MesCC a full blown C99 compliant compiler. That felt as a setback but it gave us the perspective of removing TCC from the bootstrap later on. Using Nyacc, the amazing parser framework with C99 parser by Matt Wette, has even made that a feasible perspective.

It took only half a year to mature into Mes 0.19 so that building TinyCC (25,000 LOC) now only takes ~8min instead of the initial 5h.

With a bootstrapped TCC we tried building some versions of GCC (1.4, 2.6.3, 2.95.3, 3.0, 3.1, 3.2.3, 3.4.0, 3.4.6, 4.1.1, 4.1.2) to try to build some versions of glibc (1.06.4, 1.09.1, 2.0.1. 2.1.3, 2.3, 2.3.6, 2.2.5, 2.12.2, 2.15, 2.16.0, 2.28) using slightly less versions of Binutils (1.9, 2.5.1, 2.5.2, 2.6, 2.7, 2.10.1, 2.14, 2.14a, 2.15a, 2.17a, 2.18a, 2.20.1a, 2.23.2, 2.25, 2.28). There were many interesting dependencies, tradeoffs, patching of generated Autotools outputs, especially if you are using your own tiny C library and headers.

Typically, newer versions of the tool chain fix all kinds of bugs in the build system and C code compliance, which is great. However, simultaneously new features are introduced or dependencies are added that are not necessary for bootstrapping, increasing the bootstrap hurdle. Sometimes, newer tools are more strict or old configure scripts do not recognise newer tool versions.

Also, can you spot the triplets of tool versions that combine into integral versions of the tool chain? ;-)

Scheme-only Bootstrap

Our next target will be another reduction by ~50%; the Scheme-only bootstrap will replace the Bash, Coreutils, etc. binaries by Gash and Gash Core Utils.

Gash is a work-in-progress implementation of a POSIX shell in Scheme, that is already capable-enough to interpret Autoconf-generated configure scripts. It can run on Guile but it is designed to also run on Mes, meaning that we can use it early on during bootstrap.

We are excited that the Nlnet Foundation is now sponsoring this work!

Creating a GNU Tool Chain Bootstrap Story

The Reduced Binary Seed bootstrap starts by building ancient GNU software, notably GCC (2.95.3), glibc (2.2.5).

This amazing achievement is mirrored only by its terrible clumsiness. Is this really how we want to secure the bootstrap of our GNU system?

                         gcc-mesboot (4.6.4)
                                ^
                                |
                            tcc-boot
                                ^
                                |
                            mes-boot
                                ^
                                |
                                *

Maybe if we could go straight from TinyCC to GCC (4.6.4) we need no longer depend on an ancient GNU tool chain and have a somewhat more modern and more maintainable bootstrap path.

Now that we have shown it can be done, we think it is time for GNU tool chain developers to step in and help create a better version of our tool chain bootstrap story.

Towards a Full Source Bootstrap

We expect many interesting challenges before we approach this lofty target.

The stage0 project by Jeremiah Orians starts everything from ~512 bytes; virtually nothing. Have a look at this incredible project if you haven’t already done so.

Jeremiah is also leading the Mes-M2 effort that is about bootstrapping Mes from stage0. The Mes Scheme interpreter is being rewritten in an even more simple subset of C, without preprocessor macros even. That C-like language is called M2-Planet, after its transpiler.

About Bootstrappable Builds and GNU Mes

Software is bootstrappable when it does not depend on a binary seed that cannot be built from source. Software that is not bootstrappable - even if it is free software - is a serious security risk for a variety of reasons. The Bootstrappable Builds project aims to reduce the number and size of binary seeds to a bare minimum.

GNU Mes is closely related to the Bootstrappable Builds project. Mes aims to create an entirely source-based bootstrapping path for the Guix System and other interested GNU/Linux distributions. The goal is to start from a minimal, easily inspectable binary (which should be readable as source) and bootstrap into something close to R6RS Scheme.

Currently, Mes consists of a mutual self-hosting scheme interpreter and C compiler. It also implements a C library. Mes, the scheme interpreter, is written in about 5,000 lines of code of simple C. MesCC, the C compiler, is written in scheme. Together, Mes and MesCC can compile a lightly patched TinyCC that is self-hosting. Using this TinyCC and the Mes C library, it is possible to bootstrap the entire Guix System for i686-linux and x86_64-linux.

About GNU Guix

GNU Guix is a transactional package manager and an advanced distribution of the GNU system that respects user freedom. Guix can be used on top of any system running the kernel Linux, or it can be used as a standalone operating system distribution for i686, x86_64, ARMv7, and AArch64 machines.

In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. When used as a standalone GNU/Linux distribution, Guix offers a declarative, stateless approach to operating system configuration management. Guix is highly customizable and hackable through Guile programming interfaces and extensions to the Scheme language.