jakstys.lt/content/log/2024/zig-reproduced-without-binaries.md

---
title: "Zig Reproduced Without Binaries"
date: 2024-11-12T22:21:48+02:00
slug: zig-reproduced-without-binaries
draft: true
---

I decided to bootstrap zig without using binaries that are [checked in the
repository](https://github.com/ziglang/zig/blob/0.13.0/stage1/zig1.wasm) and
see if the resulting `zig1.wasm` in the latest zig release (0.13.0) is the same
the one bootstrapped without those binaries.

TLDR: `zig1.wasm` of the official 0.13.0 and our-hard-bootstrapped one are the
same. Whoof, Zig is clean from [this famous attack vector][2], or at least
there is nothing hiding in `zig1.wasm` that hasn't been in the checked-in
sources:

```
$ sha256sum code/zig{,2}/stage1/zig1.wasm
127909fb8c9610ce3f296d8a48014546c0f85055115002fb3aba4d865dcdbb27  code/zig/stage1/zig1.wasm
127909fb8c9610ce3f296d8a48014546c0f85055115002fb3aba4d865dcdbb27  code/zig2/stage1/zig1.wasm
```

Many, many thanks to [Hilton Chain][1] for reasons I that will become clear
later.

# Official zig1.wasm

Steps to acquire the official incarnation of `zig1.wasm` are straightforward:
download zig, build `zig3` using the official instructions, use it to
`update-zig1`:

```
git clone https://github.com/ziglang/zig; cd zig
git checkout 0.13.0
mkdir build; pushd build
  cmake ..
  make -j install
popd
build/stage3/bin/zig build update-zig1
```

Which results in an updated `code/zig/stage1/zig1.wasm`:

```
$ git diff --stat
 stage1/zig1.wasm | Bin 2675178 -> 2800926 bytes
 1 file changed, 0 insertions(+), 0 deletions(-)
```

We will be comparing this `zig1.wasm` to the one bootstrapped in the next
section.

# Binary-free zig1.wasm

Builting zig 0.13.0 without binaries is tricky, because to build zig 0.13.0, we
need a `zig1.wasm`, which has been checked in and continuously updated since
[late 2022](https://github.com/ziglang/zig/pull/13560):

```
commit 20d86d9c63476b6312b87dc5b0e4aa4822eb7717
Author: Andrew Kelley <andrew@ziglang.org>
Date:   2022-11-13T01:35:20+02:00

    add zig1.wasm.zst
    
    This commit adds a 637 KB binary file to the source repository. This
    commit does nothing else, so it should be replaced with a different
    commit before this branch is merged to avoid bloating the git
    repository.

 stage1/zig1.wasm.zst | Bin 0 -> 652012 bytes
 1 file changed, 0 insertions(+), 0 deletions(-)
```

[Andrew's motivation][3] is legit from a Zig developer's perspective. However,
checked-in binary blobs have trust issues, regardless of what we think about
the author.

The last commit that can[^1] be built without binary blobs is the parent of
this one:

```
commit 28514476ef8c824c3d189d98f23d0f8d23e496ea
Author: Andrew Kelley <andrew@ziglang.org>
Date:   2022-11-01T05:29:55+02:00

    remove `-fstage1` option

    After this commit, the self-hosted compiler does not offer the option to
    use stage1 as a backend anymore.
```

Once C++ implementation was removed, Zig is required to build Zig. This is a
cyclic dependency, which Zig Core team breaks by continuously checking in *a*
Zig implementation in wasm, the `zig1.wasm` file, which is used to build the
compiler.

Andrew suggests a motivated third-party to implement a [Zig
interpreter][zig-interpreter] in non-zig that could break this chain. While
that would be certainly be ideal, but nobody has built it yet 🤷.

The steps to build "trusted"[^3] zig are roughly:

1. Build zig from the C++ implementation of the commit above (with hacks and
   tricks to make it [actually compile][4]).
2. Use previous step to build the first Zig self-hosted.
3. Proceed to the next step. When the updated zig does not build, find creative
   ways to build it anyway (or, when really stuck, ask @mlugg).
4. Goto 2 for [45+ times][5].

After reaching `0.11.0-1894-gb92e30ff0b`, which is two `zig1.wasm` updates away
from 0.12.0, I received an email from Hilton Chain, titled `Thank you for the
work on bootstrapping Zig!`, where they took my PoC, [re-created all of it in
Guix DSL][6] and ran all the way to 0.13.0[^2]. This made me flabbergasted.

I audited their script to see if it really deletes `zig1.wasm` at every
checkout, ran it to produce `zig1.wasm` of `0.13.0` myself. Once I had
`zig1.wasm` of 0.13.0, I did the same as I did in the official `zig1.wasm`:
built zig3, used it to build `zig1.wasm`, and voilà, the hashes of the official
`zig1.wasm` and the one built by myself and Hilton match.

I am looking forward to Hilton landing his Zig work to Guix, so anyone can
audit the build script and reproduce this exercise by themselves with an
otherwise  [bootstrappable][7] system.

If anyone can trace origins of `zig1.wasm` and produce an identical version
themselves, perhaps it's not too bad to have it checked in?

[^1]: Not exactly. Some reverts and code movement is necessary. See the [`run`
    script][5] for details.

[^2]: Their work is on a branch in Guix repository, which has `zig` in the
    title. I will not link it here, as it will be removed when it lands, but it
    should be easy to find for determined readers before it does.

[^3]: We trust no-one except ourselves.

[1]: https://ultrarare.space/
[2]: https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_ReflectionsonTrustingTrust.pdf
[3]: https://ziglang.org/news/goodbye-cpp/
[4]: https://ziggit.dev/t/building-self-hosted-from-the-original-c-implementation/6607?u=motiejus
[5]: https://git.jakstys.lt/motiejus/zig-repro/src/commit/7f37da6e75cab9d4637b8173d713f91853c9ef54/run#L1032-L1076
[6]: https://issues.guix.gnu.org/74217
[7]: https://guix.gnu.org/en/blog/2023/the-full-source-bootstrap-building-from-source-all-the-way-down/

[zig-interpreter]: https://ziggit.dev/t/building-self-hosted-from-the-original-c-implementation/6607/2?u=motiejus