nixpkgs bootstrap intro

March 24, 2022

This post is an informal walk through bootstrap setup of nixpkgs. It’s quite long and has many facets we look at here. If you don’t understand some bits of it don’t worry: it’s both a complicated topic and I did a bad job at explaining it.

Initially I tried to cram everything into a single post and got largest post ever. Once I realized it became unreadable I moved out details on glibc into a separate hacking-on-glibc post. Now this post is “only” on par with other larges posts /o\.

Good luck :)

Intro

nixpkgs is a package repository for NixOS linux distribution. nixpkgs can also be used outside NixOS on other linux (and non-linux!) distributions.

Each nixpkgs package is built in a container environment where build process sees only explicitly specified dependencies (via mount namespace). That way we get more deterministic (and ideally fully reproducible) build environment and result when ran on another machine.

Such a setup makes it trivial to notice missing required dependencies. I frequently write a nixpkgs recipe for a package before trying to package it on another distributions :)

Precise dependencies sound great in theory, but how does such a system deal with circular dependencies and bootstrap dependencies? For example gcc depends on some C compiler presence. How do they get satisfied?

Bootstrap binaries

nixpkgs solves it by providing a set of pre-built seed binaries called bootstrap-files (or bootstrapTools). These binaries were at some previous point built on an already working nixpkgs system. One can also build them manually.

Seed binaries don’t change until someone decides to refresh them. Specifically they don’t get rebuil on each nixpkgs commit. In theory bootstrap-files could be left untouched forever at least for existing target systems. In practice bugs do occasinally happen in bootstrap-files and we need to fix them. It’s also useful to have some up-to-date baseline when building other fresh packages: building gcc-11 with gcc-8 (seed binary) is a lot simpler than building gcc-11 with gcc-3.

To get the idea what it takes to bet a set of bootstrap files let’s build fresh set of them ourselves. We’ll use default definition in make-bootstrap-tools.nix:

$ nix build -f ./pkgs/stdenv/linux/make-bootstrap-tools.nix bootstrapFiles

We can even cross-compile bootstrap files for a differen (potentially not yet supported) target:

$ nix build -f ./pkgs/stdenv/linux/make-bootstrap-tools.nix bootstrapFiles --argstr crossSystem powerpc64le-linux

Is it big? Inspecting the final result:

$ readlink result
/nix/store/3xq6in2gn3z3cvjjf51fyn53bg3k4nh6-bootstrap-tools.tar.xz

$ LANG=C ls -lh /nix/store/3xq6in2gn3z3cvjjf51fyn53bg3k4nh6-bootstrap-tools.tar.xz
-r--r--r-- 2 root root 21M Jan  1  1970 /nix/store/3xq6in2gn3z3cvjjf51fyn53bg3k4nh6-bootstrap-tools.tar.xz

Archive size is 21MB. This size is smaller than compressed nixpkgs tree (~26MB today). Let’s peek at things that hide inside:

$ tar --list -f /nix/store/3xq6in2gn3z3cvjjf51fyn53bg3k4nh6-bootstrap-tools.tar.xz
./bin/
./bin/[
./bin/ar
...
./bin/yes
./include/
./include/c++/
./include/c++/10.3.0/
./include/c++/10.3.0/algorithm
...
./include-glibc/
./include-glibc/a.out.h
./include-glibc/aio.h
...
./include-glibc/xen/privcmd.h
./lib/
./lib/crt1.o
./lib/crti.o
./lib/crtn.o
./lib/gcc/
./lib/gcc/x86_64-unknown-linux-gnu/
./lib/gcc/x86_64-unknown-linux-gnu/10.3.0/
...
./lib/gcc/x86_64-unknown-linux-gnu/10.3.0/libgcov.a
./lib/ld-2.33.so
./lib/ld-linux-x86-64.so.2
./lib/libbfd-2.35.2.so
./lib/libbfd.la
...
./lib/libz.so.1.2.11
./libexec/
./libexec/gcc/
./libexec/gcc/x86_64-unknown-linux-gnu/
./libexec/gcc/x86_64-unknown-linux-gnu/10.3.0/
./libexec/gcc/x86_64-unknown-linux-gnu/10.3.0/cc1
...

The contents (once again) are defined by https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/linux/make-bootstrap-tools.nix. It tells ut that the following packages are present in the final tarball:

Just 21 package! Most are very cross-compiler friendly. Some of packages have reduced functionality not needed for simplest build requirements:

  coreutilsMinimal = coreutils.override (args: { aclSupport = false; attrSupport = false; /*...*/ })
  tarMinimal = gnutar.override { acl = null; };
  busyboxMinimal = busybox.override { useMusl = true; enableStatic = true; /*...*/ }
  bootGCC = gcc.cc.override { enableLTO = false; };
  bootBinutils = binutils.bintools.override { withAllTargets = false; gold = false; enableShared = false; /*...*/ }

The tarball generation process is literally copying build files to make self-contained archive:

  build = stdenv.mkDerivation {
    name = "stdenv-bootstrap-tools";

    buildCommand = ''
        set -x
        mkdir -p $out/bin $out/lib $out/libexec

        # Copy what we need of Glibc.
        cp -d ${libc.out}/lib/ld*.so* $out/lib
        cp -d ${libc.out}/lib/libc*.so* $out/lib
        # ...
        cp -d ${coreutilsMinimal.out}/bin/* $out/bin
        (cd $out/bin && rm vdir dir sha*sum pinky factor pathchk runcon shuf who whoami shred users)
        cp ${bash.out}/bin/bash $out/bin
        cp ${findutils.out}/bin/find $out/bin
        cp ${findutils.out}/bin/xargs $out/bin
        cp -d ${diffutils.out}/bin/* $out/bin
        # ...
        nuke-refs $out/bin/*
        nuke-refs $out/lib/*
        # ...
    '';
    //...

Once these bootstrap binaries are built they are referred explicitly as a fetchurl{} “source” tarball input: https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/linux/bootstrap-files/i686.nix

There is a caveat: we can’t run these binaries as is if we just unpack the tarball. I’ll try anyway:

$ mkdir /tmp/b
$ cd /tmp/b
$ tar xf /nix/store/3xq6in2gn3z3cvjjf51fyn53bg3k4nh6-bootstrap-tools.tar.xz
$ ls bin/bash
bin/bash
$ unshare --user --map-root-user chroot . /bin/bash
chroot: failed to run command ‘/bin/bash’: No such file or directory

It happens because binaries intentionally hardcode invalid absolute paths to dynamic linker:

$ lddtree bin/bash
bash => bin/bash (interpreter => /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-glibc-2.33-108/lib/ld-linux-x86-64.so.2)
    libdl.so.2 => not found
    libc.so.6 => not found

These invalid paths are meant to be relocated at install time: patchelf binary patching tool is used for that in https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/linux/bootstrap-tools/scripts/unpack-bootstrap-tools.sh

Note that patchelf is also a dynamically linked binary. How can we run it against broken interpreter? You might have already noticed it in unpack-bootstrap-tools.sh above: dynamic loader is called by absolute path and it’s search paths are overridden by LD_LIBRARY_PATH variable.

Trying running bash using the same trick:

$ PS1='foo> ' unshare --user --map-root-user chroot . /lib/ld-linux-x86-64.so.2 --library-path /lib /bin/bash

foo> echo /*
/bin /include /include-glibc /lib /libexec

foo> ls /
bash: ls: command not found
foo> LD_LIBRARY_PATH=/lib /lib/ld-linux-x86-64.so.2 /bin/ls
bin  include  include-glibc  lib  libexec

Seems to work :)

I used --library-path /lib to step aside the complications of mixing LD_LIBRARY_PATH value for host’s chroot command. But once in a chroot LD_LIBRARY_PATH=/lib does the trick as well.

Another way to make chroot to Just Work without LD_LIBRARY_PATH= is to fake relocation with this funny symlink:

$ ln -s ../../../lib nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-glibc-2.33-108/lib
$ unshare --user --map-root-user chroot . /bin/bash
$ export PATH=/bin PS1='minimal> '
minimal> ls /
bin  include  include-glibc  lib  libexec  nix
minimal>  gcc --version
gcc (GCC) 10.3.0

minimal> echo 'int main(){}' > a.c
minimal> gcc a.c -o a
minimal> ./a
bash: ./a: No such file or directory

Now we are able to run gcc (and also bash).

The environment is able to compile simple executables. Why do compiled binaries still fail to run? That is is an exercise for the reader :) Hint: it can be hacked with yet another symlink. Try to find which one. hacking-on-glibc post also provides another way of solving it.

stdenv

So how do we bootstrap fresh nixpkgs out of outdated gcc and glibc from bootstrap-tools?

To provide minimal build environment for most packages nixpkgs has a special package: stdenv. Let’s inspect it’s contents:

$ nix-shell --pure -p stdenv

[nix-shell]$ echo $PATH | tr ':' $'\n' | ~/bin/unnix
/<<NIX>>/bash-interactive-5.1-p12/bin
/<<NIX>>/patchelf-0.14.3/bin
/<<NIX>>/gcc-wrapper-10.3.0/bin
/<<NIX>>/gcc-10.3.0/bin
/<<NIX>>/glibc-2.33-108-bin/bin
/<<NIX>>/coreutils-9.0/bin
/<<NIX>>/binutils-wrapper-2.35.2/bin
/<<NIX>>/binutils-2.35.2/bin
/<<NIX>>/coreutils-9.0/bin
/<<NIX>>/findutils-4.8.0/bin
/<<NIX>>/diffutils-3.8/bin
/<<NIX>>/gnused-4.8/bin
/<<NIX>>/gnugrep-3.7/bin
/<<NIX>>/gawk-5.1.1/bin
/<<NIX>>/gnutar-1.34/bin
/<<NIX>>/gzip-1.11/bin
/<<NIX>>/bzip2-1.0.6.0.2-bin/bin
/<<NIX>>/gnumake-4.3/bin
/<<NIX>>/bash-5.1-p12/bin
/<<NIX>>/patch-2.7.6/bin
/<<NIX>>/xz-5.2.5-bin/bin

The list of binaries is suspiciously close to our bootstrap-tools tarball. That is not a coincidence: if it’s good enough for most packages it should be enough for gcc.

In theory we could just use bootstrap-tools to define stdenv and use it to build things like xorg. However such a scheme would complicate updating gcc (and all other stdenv packages): (once again) bootstrap archive does not get updated frequently. We would need to refresh bootstrap tarballs routinely just to get a gcc update.

To solve an update problem nixpkgs uses a level of indirection: first nixpkgs defines bootstrap-stdenv as bootstrap-tools and then builds stdenv out of bootstrap-stdenv. The rest of nixpkgs uses only stdenv and avoids bootstrap-stdenv.

Thus our example xorg chain of build-time dependencies is: bootstrap-tools -> bootstrap-stdenv -> stdenv -> xorg.

Simple, eh?

Let’s look at a stdenv.mkDerivation function normally used to define packages. We define a one-liner foo package that prints contents of it’s build environment at build time:

$ nix build --impure --expr 'with import <nixpkgs> {}; stdenv.mkDerivation { name = "foo"; unpackPhase = "echo $CC; $CC -v; exit 1"; }' -L

foo> unpacking sources
foo> gcc
foo> Using built-in specs.
foo> COLLECT_GCC=/nix/store/2dv93bbc06c7zg866qid73j3r36zz3jx-gcc-10.3.0/bin/gcc
foo> COLLECT_LTO_WRAPPER=/nix/store/2dv93bbc06c7zg866qid73j3r36zz3jx-gcc-10.3.0/libexec/gcc/x86_64-unknown-linux-gnu/10.3.0/lto-wrapper
foo> Target: x86_64-unknown-linux-gnu
foo> Configured with:
foo> Thread model: posix
foo> Supported LTO compression algorithms: zlib
foo> gcc version 10.3.0 (GCC)
...

As a small detour there are also other popular stdenvs, like an LLVM-based one:

$ nix build --impure --expr 'with import <nixpkgs> {}; pkgsLLVM.stdenv.mkDerivation { name = "foo"; unpackPhase = "echo $CC; $CC -v; exit 1"; }' -L

foo-x86_64-unknown-linux-gnu> unpacking sources
foo-x86_64-unknown-linux-gnu> x86_64-unknown-linux-gnu-clang
foo-x86_64-unknown-linux-gnu> clang version 11.1.0
foo-x86_64-unknown-linux-gnu> Target: x86_64-unknown-linux-gnu
foo-x86_64-unknown-linux-gnu> Thread model: posix
foo-x86_64-unknown-linux-gnu> InstalledDir: /nix/store/y61l0kbqfchdk39i319ycrfblc6zz3s8-clang-11.1.0/bin
...

nixpkgs provides many toolchains via various stdenvs. To name a few (assuming x86_64-linux system):

Finding out how those interact to one another (which stdenv is defined in terms of which) is an exercise for the reader :)

stdenv tower

So how exactly do we ascend from not-quite-working bootstrapTools to stdenv? What is hiding behind the arrow in “bootstrap-stdenv -> stdenv” part above?

The precise answer is hidden in stdenv.nix.

The main take away from there is that there are multiple stages of stdenv:

bootstrap-stdenv -> ? -> ?? -> ??? -> … -> stdenv.

Instead of trying to figure out what each stage does exactly out of definition above let’s debug it and see what we can find out.

Let’s inject a prev attribute to each intermediate instance of stdenv and walk along that chain. Here is the full local patch:

--- a/pkgs/stdenv/generic/default.nix
+++ b/pkgs/stdenv/generic/default.nix
@@ -54,0 +55,3 @@ argsStdenv@{ name ? "stdenv", preHook ? "", initialPath
+
+, # debug reference to previous stdenv
+  prev ? {}
@@ -145,0 +149,2 @@ let
+      inherit prev;
+
--- a/pkgs/stdenv/linux/default.nix
+++ b/pkgs/stdenv/linux/default.nix
@@ -106,0 +107,2 @@ let
+        prev = prevStage.stdenv;
+
@@ -437,0 +440,2 @@ in
+
+      prev = prevStage.stdenv;

I left out almost all of patch context on purpose. It’s not very readable as a diff. Now we can access all stdenv iterations via stdenv.prev.

First, let’s find out how many hops are there from bootstrap to final stdenv:

nixpkgs $ nix repl .
nix-repl> stdenv
«derivation /nix/store/s6l15yfxq567as8wdw7cfvy6c3p9wscw-stdenv-linux.drv»
nix-repl> stdenv.prev
«derivation /nix/store/q3dsgi310i9r5b1pgdym2fnvlxbkxls2-bootstrap-stage4-stdenv-linux.drv»
nix-repl> stdenv.prev.prev
«derivation /nix/store/b5hwd6s4b2xq2l6nidhklypn69vhlfap-bootstrap-stage3-stdenv-linux.drv»
nix-repl> stdenv.prev.prev.prev
«derivation /nix/store/bkbn1y2593jqclaq2kjcx5iambylsafq-bootstrap-stage2-stdenv-linux.drv»
nix-repl> stdenv.prev.prev.prev.prev
«derivation /nix/store/1wklspdwi7c03jrqjbh2f6ymmpqshcif-bootstrap-stage1-stdenv-linux.drv»
nix-repl> stdenv.prev.prev.prev.prev.prev
«derivation /nix/store/bvkp4zp1kidp9wfk2f46yyy73y3n38mx-bootstrap-stage0-stdenv-linux.drv»
nix-repl> stdenv.prev.prev.prev.prev.prev.prev
error: attribute 'stdenv' missing

5(!) intermediate steps! Let’s check out compiler version of the intial one and the one right after:

$ nix build --impure --expr 'with import ./. {}; stdenv.prev.prev.prev.prev.prev.mkDerivation { name = "foo"; unpackPhase = "$CC --version; exit 1"; }' -L
foo> unpacking sources
foo> /nix/store/hbppa2cjx9929jrv796fpni2m06j3fzw-bootstrap-stage0-stdenv-linux/setup: line 1358: --version: command not found

The very first (or zeroth) bootstrap stdenv does not even provide basic “$CC” variable. It’s not really a usable stdenv just yet. We can also guess it from it’s empty definition:

  ({}: {
    __raw = true;

    gcc-unwrapped = null;
    binutils = null;
    coreutils = null;
    gnugrep = null;
  })

It’s only a default case that will gradually grow a piece on each step. Let’s look at the next one:

$ nix build --impure --expr 'with import ./. {}; stdenv.prev.prev.prev.prev.mkDerivation { name = "foo"; unpackPhase = "$CC --version; exit 1"; }' -L

foo> unpacking sources
foo> gcc (GCC) 8.3.0
...

The next stdenv provides build environment based on gcc-8.3.0 (which is way older than default nixpkgs gcc-10.3.0 version).

Now we have a tool to check what is the actual difference between all these stdenv iterations! I usually use NIX_DEBUG=1 variable to look at what nixpkgs injects in the search paths and default options of gcc:

nix build --impure --expr 'with import ./. {}; stdenv.prev.prev.prev.prev.mkDerivation { name = "foo"; unpackPhase = "NIX_DEBUG=1 $CC --version; exit 1"; }' -L

foo> unpacking sources
foo> HARDENING: disabled flags: pie
foo> HARDENING: Is active (not completely disabled with "all" flag)
foo> HARDENING: enabling fortify
foo> HARDENING: enabling stackprotector
foo> HARDENING: enabling strictoverflow
foo> HARDENING: enabling format
foo> HARDENING: enabling pic
foo> extra flags before to /nix/store/i3ibpx67yncp4w4mpkf5pwvjjsd0aqln-bootstrap-tools/bin/gcc:
foo>   -O2
foo>   -D_FORTIFY_SOURCE=2
foo>   -fstack-protector-strong
foo>   --param
foo>   ssp-buffer-size=4
foo>   -fno-strict-overflow
foo>   -Wformat
foo>   -Wformat-security
foo>   -Werror=format-security
foo>   -fPIC
foo> original flags to /nix/store/i3ibpx67yncp4w4mpkf5pwvjjsd0aqln-bootstrap-tools/bin/gcc:
foo>   --version
foo> extra flags after to /nix/store/i3ibpx67yncp4w4mpkf5pwvjjsd0aqln-bootstrap-tools/bin/gcc:
foo>   -B/nix/store/39k40hf9z4wr5wac5xbnznza1ym2f8kz-bootstrap-stage0-glibc-bootstrap/lib/
foo>   -idirafter
foo>   /nix/store/39k40hf9z4wr5wac5xbnznza1ym2f8kz-bootstrap-stage0-glibc-bootstrap/include
foo>   -idirafter
foo>   /nix/store/i3ibpx67yncp4w4mpkf5pwvjjsd0aqln-bootstrap-tools/lib/gcc/x86_64-unknown-linux-gnu/8.3.0/include-fixed
foo>   -B/nix/store/i3ibpx67yncp4w4mpkf5pwvjjsd0aqln-bootstrap-tools/lib
foo>   -B/nix/store/8wmhf2pbmx0vbs60yk6x9w5lm0zrqjlz-bootstrap-stage1-gcc-wrapper-/bin/
foo>   -frandom-seed=dhfkc7mzra
foo> gcc (GCC) 8.3.0
foo> Copyright (C) 2018 Free Software Foundation, Inc.
foo> This is free software; see the source for copying conditions.  There is NO
foo> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Here the important parts are the paths specified with -L flag (library lookup paths) and by -B flag (objects and tools for compiler itself). Both should be familiar by now from “hello world” dissection section at hacking-on-glibc post.

Also note that nixpkgs builds files with default -O2 optimization level until specified otherwise (also note a few warning options on top enabled by default).

Let’s check out our final stdenv:

$ nix build --impure --expr 'with import ./. {}; stdenv.mkDerivation { name = "foo"; unpackPhase = "NIX_DEBUG=1 $CC --version; exit 1"; }' -L

foo> unpacking sources
foo> HARDENING: disabled flags: pie
foo> HARDENING: Is active (not completely disabled with "all" flag)
foo> HARDENING: enabling pic
foo> HARDENING: enabling format
foo> HARDENING: enabling stackprotector
foo> HARDENING: enabling fortify
foo> HARDENING: enabling strictoverflow
foo> extra flags before to /nix/store/j5pl47x8yplyfpbbgvcrzjwbm08n9rvi-gcc-12.0.0/bin/gcc:
foo>   -fPIC
foo>   -Wformat
foo>   -Wformat-security
foo>   -Werror=format-security
foo>   -fstack-protector-strong
foo>   --param
foo>   ssp-buffer-size=4
foo>   -O2
foo>   -D_FORTIFY_SOURCE=2
foo>   -fno-strict-overflow
foo> original flags to /nix/store/j5pl47x8yplyfpbbgvcrzjwbm08n9rvi-gcc-12.0.0/bin/gcc:
foo>   --version
foo> extra flags after to /nix/store/j5pl47x8yplyfpbbgvcrzjwbm08n9rvi-gcc-12.0.0/bin/gcc:
foo>   -B/nix/store/km6a4zxn29liy6l2xq441p2yap1ka1j4-glibc-2.35/lib/
foo>   -idirafter
foo>   /nix/store/6xn0firi6hlz6x161drdj0p0jzcrrfla-glibc-2.35-dev/include
foo>   -idirafter
foo>   /nix/store/j5pl47x8yplyfpbbgvcrzjwbm08n9rvi-gcc-12.0.0/lib/gcc/x86_64-unknown-linux-gnu/12.0.1/include-fixed
foo>   -B/nix/store/fi0acb9a2fscg7afnwjmglj55rqwj8kj-gcc-12.0.0-lib/lib
foo>   -B/nix/store/v9s3sv5c4rr9r067qw66c1iq5i0ffsvc-gcc-wrapper-12.0.0/bin/
foo>   -frandom-seed=rw1fgisg1r
foo> gcc (GCC) 12.0.1 20220213 (experimental)
foo> Copyright (C) 2022 Free Software Foundation, Inc.
foo> This is free software; see the source for copying conditions.  There is NO
foo> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Note how all the -B and -L paths flipped from bootstrap variants of the package (like -B/nix/store/39k40hf9z4wr5wac5xbnznza1ym2f8kz-bootstrap-stage0-glibc-bootstrap/lib/) to nice final package names (like -B/nix/store/km6a4zxn29liy6l2xq441p2yap1ka1j4-glibc-2.35/lib/).

Such a long stdenv tower reaches it’s main goal: bootstrap-tools must not be used in final stdenv directly or as runtime depends. stdenv must only be defined by source packages from nixpkgs.

Ideally changing bootstrap-tools archive should not affect contents of final stdenv at all. In practice it happens only when content-addressed mode is enabled: https://nixos.wiki/wiki/Ca-derivations.

To achieve independence from bootstrap binaries nixpkgs gradually substitutes parts of bootstrap-stdenv from bootstrap-tools to packages defined by nixpkgs expressions:

Sounds simple? Heh, not really. I probably got a few details above wrong. I was still not sure what are the exact dependencies that are being rebuilt.

Let’s first look at the final stdenv’s references after it’s fully built:

$ nix-store --query --graph $(nix-build -A stdenv) | dot -Tsvg > stdenv-final-runtime.svg

Rendered svg: stdenv-final graph

Note that there are no (runtime) references to bootstrap-tools packages.

Let’s look at stage4 for comparison:

$ nix-store --query --graph $(nix-build -A stdenv.prev) | dot -Tsvg > stdenv-stage4-runtime.svg

Rendered svg: stdenv-stage4 graph

Note how patchelf and binutils still use glibc which was built using bootstrap-tools compiler.

The above graphs don’t show build-time dependencies. Ideally stdenv-final should not directly depend on anything related to boostrap-tools. We can get the graph by looking at the derivation instead of final store path:

$ nix-store --query --graph $(nix-instantiate -A stdenv) | dot -Tsvg > stdenv-drv-final-runtime.svg

Rendered svg: stdenv-drv graph

It’s not really readable: there are too many mostly irrelevant minor details like patches and source tarballs. Can we rearrange nodes explicitly as they are pulled in into each stage?

The simplest (but somewhat incomplete) seems to be the use of nix-diff derivation differ:

$ nix store diff-closures $(nix-instantiate -A stdenv.prev) $(nix-instantiate -A stdenv)

acl: ∅ → 2.3.1, +109.0 KiB
attr: ∅ → 2.5.1, +78.9 KiB
bash: ∅ → 5.1-p16, +1519.2 KiB
binutils: +6390.2 KiB
binutils-wrapper: +9.9 KiB
bootstrap: ε → ∅, -129542.0 KiB
bootstrap-stage0-glibc: ε → ∅
bootstrap-stage4-gcc-wrapper: 12.0.0 → ∅, -47.5 KiB
bootstrap-stage4-stdenv: ε → ∅, -41.7 KiB
busybox: ε → ∅, -117.2 KiB
bzip2: ∅ → 1.0.6.0.2, +147.6 KiB
coreutils: ∅ → 9.0, +1769.5 KiB
diffutils: ∅ → 3.8, +1450.5 KiB
ed: ∅ → 1.18, +134.7 KiB
expand-response: -17.0 KiB
findutils: ∅ → 4.9.0, +1472.3 KiB
gawk: ∅ → 5.1.1, +2374.2 KiB
gcc-wrapper: ∅ → 12.0.0, +47.5 KiB
gnugrep: ∅ → 3.7, +773.2 KiB
gnumake: ∅ → 4.3, +1391.1 KiB
gnused: ∅ → 4.8, +725.0 KiB
gnutar: ∅ → 1.34, +2834.7 KiB
gzip: ∅ → 1.11, +152.0 KiB
patch: ∅ → 2.7.6, +222.3 KiB
pcre: ∅ → 8.45, +514.5 KiB
stdenv: ∅ → ε, +42.1 KiB
zlib: -121.4 KiB

Here we see exact list of packages that differ in the whole tree between stdenv and it’s immediate predecessor.

Or we can look at just compiler wrapper difference of stdenv (let’s try nix-diff for a change):

$ nix-diff $(nix-instantiate -A stdenv.prev.cc) $(nix-instantiate -A stdenv.cc) --line-oriented | cat

- /nix/store/d0ivnqxcmjdg9ihdl4ww9a0c79pyl0nd-bootstrap-stage4-gcc-wrapper-10.3.0.drv:{out}
+ /nix/store/45fs8hhm6afg0m0p2d635zhjh608bqsj-gcc-wrapper-10.3.0.drv:{out}
• The set of input derivation names do not match:
    - bootstrap-stage0-stdenv-linux
    + bash-5.1-p12
    + bootstrap-stage4-stdenv-linux
    + coreutils-9.0
    + gnugrep-3.7
• The input derivation named `binutils-wrapper-2.35.2` differs
  - /nix/store/gcyaki78ksxg9s211y4zr4ppnrq1jwlm-binutils-wrapper-2.35.2.drv:{out}
  + /nix/store/9g4r35k465xg4p02c5krbxqnwvihpmbx-binutils-wrapper-2.35.2.drv:{out}
  • The set of outputs do not match:
      + {info}
      + {man}
  • The set of input source names do not match:
      + gnu-binutils-strip-wrapper.sh
  • The set of input derivation names do not match:
      - bootstrap-stage1-stdenv-linux
      + bash-5.1-p12
      + bootstrap-stage4-stdenv-linux
      + coreutils-9.0
      + gnugrep-3.7
  • The set of input derivations named `binutils-2.35.2` do not match
      - /nix/store/l343vxcc5ik63ccrggff3js03y1l9154-binutils-2.35.2.drv:{out}
      + /nix/store/vfzsbipvxkbj9m4yh5is0shfxn3p7b2m-binutils-2.35.2.drv:{info,man,out}
  • The input derivation named `expand-response-params` differs
    - /nix/store/5y71wc7khvy8m2qh6vvrvwx31dhk68r7-expand-response-params.drv:{out}
    + /nix/store/w07x2k4wi8xvmh2nyxvx50nw5pxaga6y-expand-response-params.drv:{out}
    • The set of input derivation names do not match:
        - bootstrap-stage1-stdenv-linux
        + bootstrap-stage3-stdenv-linux
    • The environments do not match:
        stdenv=''
        - /nix/store/7wafj75gbf8kr9i3isdajc5vlm0r8jjp-bootstrap-stage1-stdenv-linux
        + /nix/store/n89rpfsfs317j2qbm57905qzzq0amyhy-bootstrap-stage3-stdenv-linux
    ''
  • Skipping environment comparison
• Skipping environment comparison

The above helps getting some intuition on what packages change from one stdenv to another.

This still does not show crucial details of where do those -B / -L options come from into the gcc-wrapper. And why they matter at all.

option stacking

In contrast to FHS distributions nix explicitly allows and encourages previous versions of software to co-exist with newer ones.

In our case of stdenv-stage2 just rebuilds glibc. On FHS system we would update glibc inplace and would rely on it’s backwards compatibility to avoid system breakage right after such an update. It is practical for simle use cases but sometimes this causes complications. For example it’s hard to downgrade glibc once you have rebuilt a few dependencies (say, gcc) against a newer version. And inplace glibc update can cause issues with already running executables that lazily load nss resolver libraries.

Simplistically nixpkgs sidesteps the problem by effectively building multiple separate worlds against different libcs (libc usually come with stdenv update or by using non-default stdenv, like pkgsMusl.stdenv).

For our bootstrap case we somehow need to transition:

One of the ways to do it is:

The “redirect gcc” part is a tricky but too much: all it needs is the override of default -B / -L / -Wl,-dynamic-linker, set of flags mentioned in hacking-on-glibc post.

We need to watch for option order if we already specify our toolchain explicitly. Let’s look at the following example artificial:

# prepare library copies to see the effect:

$ mkdir -p a b
$ cp /<<NIX>>/glibc-2.33-108/lib/crt1.o a/
$ cp /<<NIX>>/glibc-2.33-108/lib/crt1.o b/
$ cp /<<NIX>>/glibc-2.33-108/lib/libc.so a/
$ cp /<<NIX>>/glibc-2.33-108/lib/libc.so.6 a/
$ cp /<<NIX>>/glibc-2.33-108/lib/libc.so b/
$ cp /<<NIX>>/glibc-2.33-108/lib/libc.so.6 b/
$ cp /glibc-2.33-108/lib/ld-linux-x86-64.so.2 a/
$ cp /glibc-2.33-108/lib/ld-linux-x86-64.so.2 b/

$ tree
.
├── a
│   ├── crt1.o
│   ├── ld-linux-x86-64.so.2
│   ├── libc.so
│   └── libc.so.6
├── a.c
├── b
│   ├── crt1.o
│   ├── ld-linux-x86-64.so.2
│   ├── libc.so
│   └── libc.so.6

Above I placed idential copies of glibc into a new directory (suppose we built slightly newer version of glibc) and then pointed gcc there.

Quiz time: try to take some time and guess what the following command would print:

$ LANG=C gcc hello.c -o c -Wl,--verbose \
    -La -Lb \
    -Bb -Ba \
    -Wl,--dynamic-linker=$PWD/a/ld-linux-x86-64.so.2 -Wl,--dynamic-linker=$PWD/b/ld-linux-x86-64.so.2 \
    |& fgrep succeeded | unnix | uniq

Note that -L options go in a-then-b order, -B options go in b-then-a order and dynamic-linker again goes in a-then-b.

Guess which files get picked from which directory.

Here is the result:

$ gcc a.c -o c -Wl,--verbose -La -Lb -Bb -Ba -Wl,--dynamic-linker=$PWD/a/ld-linux-x86-64.so.2 -Wl,--dynamic-linker=$PWD/b/ld-linux-x86-64.so.2 |& fgrep succeeded | unnix | uniq
attempt to open b/crt1.o succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/crti.o succeeded
attempt to open /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/crtbegin.o succeeded
attempt to open /tmp/cctmOQBK.o succeeded
attempt to open /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/libgcc.a succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/libgcc_s.so succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/libgcc_s.so.1 succeeded
attempt to open /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/libgcc.a succeeded
attempt to open a/libc.so succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/libc.so.6 succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/libc_nonshared.a succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/ld-linux-x86-64.so.2 succeeded
attempt to open /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/libgcc.a succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/libgcc_s.so succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/libgcc_s.so.1 succeeded
attempt to open /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/libgcc.a succeeded
attempt to open /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/crtend.o succeeded
attempt to open /<<NIX>>/glibc-2.33-108/lib/crtn.o succeeded

Surprisingly (or not so surprisingly) crt1.o came from -Bb (first option in the list), libc.so came from -La (also first option in the list). But dynamic linker was … ignored?

Yeah. Note that gcc is already a thick wrapper in nixpkgs. Let’s expand it with NIX_DEBUG=1:

$ NIX_DEBUG=1 gcc a.c -o c -La -Lb -Bb -Ba -Wl,--dynamic-linker=$PWD/a/ld-linux-x86-64.so.2 -Wl,--dynamic-linker=$PWD/b/ld-linux-x86-64.so.2 |& unnix

HARDENING: disabled flags: pie pic format stackprotector fortify strictoverflow
extra flags before to /<<NIX>>/gcc-11.2.0/bin/gcc:
  -Wl\,-dynamic-linker=/<<NIX>>/glibc-2.33-108/lib/ld-linux-x86-64.so.2
original flags to /<<NIX>>/gcc-11.2.0/bin/gcc:
  a.c
  -o
  c
  -La
  -Lb
  -Bb
  -Ba
  -Wl\,--dynamic-linker=/tmp/zz/a/ld-linux-x86-64.so.2
  -Wl\,--dynamic-linker=/tmp/zz/b/ld-linux-x86-64.so.2
extra flags after to /<<NIX>>/gcc-11.2.0/bin/gcc:
  -B/<<NIX>>/gcc-11.2.0-lib/lib
  -B/<<NIX>>/glibc-2.33-108/lib/
  -idirafter
  /<<NIX>>/glibc-2.33-108-dev/include
  -idirafter
  /<<NIX>>/gcc-11.2.0/lib/gcc/x86_64-unknown-linux-gnu/11.2.0/include-fixed
  -B/<<NIX>>/gcc-wrapper-11.2.0/bin/
  -L/<<NIX>>/glibc-2.33-108/lib
  -L/<<NIX>>/gcc-11.2.0-lib/lib
HARDENING: disabled flags: relro pie bindnow
extra flags before to /<<NIX>>/binutils-2.35.2/bin/ld:
  ''
...

Note that gcc already injects -Wl,-dynamic-linker=/<>/glibc-2.33-108/lib/ld-linux-x86-64.so.2 as the very first parameter (even before our options). As a result it gets picked first. Inability to override the dynamic linker looks like minor a bug of nixpkgs wrapper. I think wrapper should consistently treat all libc overrides. It’s unsafe to mix different parts of glibc (we already saw SIGSEGV on hello.c above).

If you guessed this output right you already know a lot more than me on this topic :)

Given that option order matters a lot nixpkgs needs to make sure that overrides work as expected at least most of the time: if we override -L option for glibc, then -B option override should be present and should follow order specified by -L (and not the other way around).

I found out about these details only because nixpkgs was actually getting the option order wrong until https://github.com/NixOS/nixpkgs/pull/158047/commits/649ebfbed65189d7d62e4f2fe0e491552308a6f1 was applied.

For quite a while nixpkgs was using crt1.o from wrong glibc which made stdenv slightly contaminated by bootstrap-tools. It used to work because crt1.o contents did not change for many glibc releases. Until glibc-2.34. Then we started getting all sorts of linkage failures at bootstrap:

    expand-response-params> ld: /nix/store/p4s4jf7aq6v6z9iazll1aiqwb34aqxq9-bootstrap-tools/lib/crt1.o: in function `_start':
    expand-response-params> /build/glibc-2.27/csu/../sysdeps/x86_64/start.S:101: undefined reference to `__libc_csu_fini'
    expand-response-params> ld: /build/glibc-2.27/csu/../sysdeps/x86_64/start.S:102: undefined reference to `__libc_csu_init'
    expand-response-params> collect2: error: ld returned 1 exit status

Once understood the fix (or workaround) was trivial.

There are many ways to avoid the mix-up in future. The simplest would be to never pass more than one glibc via -B / -L and always disable defaul search paths. But that’s for another time :)

Have fun!