159 lines
6.1 KiB
Markdown
159 lines
6.1 KiB
Markdown
---
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title: "Zig Reproduced Without Binaries"
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date: 2024-11-12T22:21:48+02:00
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slug: zig-reproduced-without-binaries
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---
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I decided to bootstrap Zig without using binaries that are [checked in the
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repository](https://github.com/ziglang/zig/blob/0.13.0/stage1/zig1.wasm) and
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answer if the resulting `zig1.wasm` in the latest Zig release (0.13.0) is the
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same the one bootstrapped without using those binaries.
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TLDR: yes, they are the same:
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```
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$ sha256sum code/zig{,2}/stage1/zig1.wasm
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127909fb8c9610ce3f296d8a48014546c0f85055115002fb3aba4d865dcdbb27 code/zig/stage1/zig1.wasm
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127909fb8c9610ce3f296d8a48014546c0f85055115002fb3aba4d865dcdbb27 code/zig2/stage1/zig1.wasm
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```
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I can now confidently say (and you can also check, you don't need to [trust
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me][2]) that there is nothing hiding in `zig1.wasm` that hasn't been
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checked-in as a source file.
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Many, many thanks to [Hilton Chain][1] for reasons I that will become clear
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later. The rest of this post walks through how I arrived to this claim.
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# Official zig1.wasm
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Steps to acquire the official incarnation of `zig1.wasm` are straightforward:
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download Zig, build `zig3` using the official instructions, use it to
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`update-zig1`:
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```
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git clone https://github.com/ziglang/zig; cd zig
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git checkout 0.13.0
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mkdir build; pushd build
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cmake ..
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make -j$(nproc) install
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popd
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build/stage3/bin/zig build update-zig1
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```
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Which results in an updated `code/zig/stage1/zig1.wasm`:
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```
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$ git diff --stat
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stage1/zig1.wasm | Bin 2675178 -> 2800926 bytes
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1 file changed, 0 insertions(+), 0 deletions(-)
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```
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We will be comparing this file to the one bootstrapped in the next section.
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# Binary-free zig1.wasm
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Building Zig 0.13.0 without binaries is tricky, because to build Zig 0.13.0, we
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need a `zig1.wasm`, which has been checked in and continuously updated since
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[late 2022](https://github.com/ziglang/zig/pull/13560):
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```
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commit 20d86d9c63476b6312b87dc5b0e4aa4822eb7717
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Author: Andrew Kelley <andrew@ziglang.org>
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Date: 2022-11-13T01:35:20+02:00
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add zig1.wasm.zst
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This commit adds a 637 KB binary file to the source repository. This
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commit does nothing else, so it should be replaced with a different
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commit before this branch is merged to avoid bloating the git
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repository.
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stage1/zig1.wasm.zst | Bin 0 -> 652012 bytes
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1 file changed, 0 insertions(+), 0 deletions(-)
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```
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[Andrew's motivation][3] is reasonable from a Zig developer's perspective.
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However, checked-in binary blobs have trust issues, regardless of what we think
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about the author.
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The last commit that can[^1] be built without using binary blobs is the parent
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of this one:
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```
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commit 28514476ef8c824c3d189d98f23d0f8d23e496ea
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Author: Andrew Kelley <andrew@ziglang.org>
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Date: 2022-11-01T05:29:55+02:00
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remove `-fstage1` option
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After this commit, the self-hosted compiler does not offer the option to
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use stage1 as a backend anymore.
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```
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After this, Zig is required to build Zig. This is a cyclic dependency, which
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Zig Core team breaks by continuously checking in *a* Zig compiler in
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wasm, the `zig1.wasm` file, which is used to build the compiler.
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Andrew suggests a motivated third-party to implement a [Zig
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interpreter][zig-interpreter] in non-Zig that could break this chain. While
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that would be certainly be ideal, nobody has built it yet 🤷.
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The steps to build "trusted"[^3] Zig are roughly:
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1. Build Zig from the C++ implementation of the commit above (with hacks and
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tricks to make it [actually compile][4]).
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2. Use previous step to build the first Zig self-hosted.
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3. Proceed to the next step. When the updated Zig does not build, find creative
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ways to build it anyway (or, when really stuck, ask @mlugg).
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4. Goto 2 for [45+ times][5].
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After reaching `0.11.0-1894-gb92e30ff0b`, which is two `zig1.wasm` updates away
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from 0.12.0, I received an email from Hilton Chain, titled `Thank you for the
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work on bootstrapping Zig!`, where they took my PoC, [re-created all of it in
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Guix DSL][6] and ran all the way to 0.13.0[^2]. This made me flabbergasted.
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I audited their script to see if it really deletes `zig1.wasm` at every
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checkout, ran it to produce `zig1.wasm` of `0.13.0` myself:
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```
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$ ./pre-inst-env guix build zig@0.13
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< ... a few hours ... >
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/gnu/store/mz95707dd7qmycpr1f0ndxhkmx3vdy1c-zig-0.13.0
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/gnu/store/kqwq8sjgwi561sp78vfi6xkgm9i3wysk-zig-0.13.0-zig1
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$ ls -l /gnu/store/kqwq8sjgwi561sp78vfi6xkgm9i3wysk-zig-0.13.0-zig1/bin/zig1.wasm
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-r--r--r-- 5 root root 2661492 Jan 1 1970 /gnu/store/kqwq8sjgwi561sp78vfi6xkgm9i3wysk-zig-0.13.0-zig1/bin/zig1.wasm
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```
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Once I had `zig1.wasm` of 0.13.0, I did the same as I did in the official
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`zig1.wasm`: built `zig3`, used it to build `zig1.wasm`, and voilà, the hashes of
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the official `zig1.wasm` and the one built here match.
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# Conclusions and open questions
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I am looking forward to Hilton landing this to Guix, so anyone can audit the
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build script and reproduce this exercise by themselves with an otherwise
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[bootstrappable][7] system. If you don't trust Guix, what and whom do you
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trust?
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If anyone can trace origins of `zig1.wasm` by producing an identical version
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themselves, perhaps it's not too bad to trust it and have it checked in?
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[^1]: Not exactly. Some reverts and code movement is necessary. See the [`run`
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script][5] for details.
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[^2]: Their work is on a branch in Guix repository, which has `zig` in the
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title. I will not link it here, as it will be removed when it lands, but it
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should be easy to find for determined readers before it does.
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[^3]: We trust no-one except ourselves and our little machine on our desk.
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[1]: https://ultrarare.space/
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[2]: https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_ReflectionsonTrustingTrust.pdf
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[3]: https://ziglang.org/news/goodbye-cpp/
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[4]: https://ziggit.dev/t/building-self-hosted-from-the-original-c-implementation/6607?u=motiejus
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[5]: https://git.jakstys.lt/motiejus/zig-repro/src/commit/7f37da6e75cab9d4637b8173d713f91853c9ef54/run#L1032-L1076
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[6]: https://issues.guix.gnu.org/74217
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[7]: https://guix.gnu.org/en/blog/2023/the-full-source-bootstrap-building-from-source-all-the-way-down/
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[zig-interpreter]: https://ziggit.dev/t/building-self-hosted-from-the-original-c-implementation/6607/2?u=motiejus
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