/dev/printerfact post (#359)
Signed-off-by: Christine Dodrill <me@christine.website>
This commit is contained in:
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---
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title: "How I Implemented /dev/printerfact in Rust"
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date: 2021-04-17
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series: howto
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tags:
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- rust
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- linux
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- kernel
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---
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# How I Implemented /dev/printerfact in Rust
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Kernel mode programming is a frightful endeavor. One of the big problems with it
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is that C is really your only option on Linux. C has many historical problems
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with it that can't really be fixed at this point without radically changing the
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language to the point that existing code written in C would be incompatible with
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it.
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DISCLAIMER: This is pre-alpha stuff. I expect this post to bitrot quickly.
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<big>**DO NOT EXPECT THIS TO STILL WORK IN A FEW YEARS.**</big>
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[Yes, yes you can _technically_ use a fairly restricted subset of C++ or
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whatever and then you can avoid some C-isms at the cost of risking runtime
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panics on the `new` operator. However that kind of thing is not what is being
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discussed today.](conversation://Mara/hacker?smol)
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However, recently the Linux kernel has received an [RFC for Rust support in the
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kernel](https://lkml.org/lkml/2021/4/14/1023) that is being taken very seriously
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and even includes some examples. I had an intrusive thought that was something
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like this:
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[Hmmm, I wonder if I can port the <a
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href="https://printerfacts.cetacean.club/fact">Printer Facts API</a> to this, it
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can't be that hard, right?](conversation://Cadey/wat?smol)
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Here is the story of my saga.
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## First Principles
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At a high level to do something like this you need to have a few things:
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- A way to build a kernel
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- A way to run tests to ensure that kernel is behaving cromulently
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- A way to be able to _repeat_ these tests on another machine to be more certain
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that the thing you made works more than once
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To aid in that first step, the Rust for Linux team shipped a [Nix
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config](https://github.com/Rust-for-Linux/nix) to let you `nix-build -A kernel`
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yourself a new kernel whenever you wanted. So let's do that and see what
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happens:
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```console
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$ nix-build -A kernel
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<several megs of output snipped>
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error: failed to build archive: No such file or directory
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error: aborting due to previous error
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make[2]: *** [../rust/Makefile:124: rust/core.o] Error 1
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make[2]: *** Deleting file 'rust/core.o'
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make[1]: *** [/tmp/nix-build-linux-5.11.drv-0/linux-src/Makefile:1278: prepare0] Error 2
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make[1]: Leaving directory '/tmp/nix-build-linux-5.11.drv-0/linux-src/build'
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make: *** [Makefile:185: __sub-make] Error 2
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builder for '/nix/store/yfvs7xwsdjwkzax0c4b8ybwzmxsbxrxj-linux-5.11.drv' failed with exit code 2
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error: build of '/nix/store/yfvs7xwsdjwkzax0c4b8ybwzmxsbxrxj-linux-5.11.drv' failed
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```
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Oh dear. That is odd. Let's see if the issue tracker has anything helpful. It
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[did](https://github.com/Rust-for-Linux/nix/issues/1)! Oh yay we have the _same_
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error as they got, that means that the failure was replicated!
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So, let's look at the project structure a bit more:
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```console
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$ tree .
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.
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├── default.nix
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├── kernel.nix
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├── LICENSE
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├── nix
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│ ├── sources.json
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│ └── sources.nix
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└── README.md
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```
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This project looks like it's using [niv](https://github.com/nmattia/niv) to lock
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its Nix dependencies. Let's take a look at `sources.json` to see what options we
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have to update things.
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[You can use `niv show` to see this too, but looking at the JSON itself is more
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fun](conversation://Mara/hacker?smol)
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```json
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{
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"linux": {
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"branch": "rust",
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"description": "Adding support for the Rust language to the Linux kernel.",
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"homepage": "",
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"owner": "rust-for-linux",
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"repo": "linux",
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"rev": "304ee695107a8b49a833bb1f02d58c1029e43623",
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"sha256": "0wd1f1hfpl06yyp482f9lgj7l7r09zfqci8awxk9ahhdrx567y50",
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"type": "tarball",
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"url": "https://github.com/rust-for-linux/linux/archive/304ee695107a8b49a833bb1f02d58c1029e43623.tar.gz",
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"url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
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},
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"niv": {
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"branch": "master",
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"description": "Easy dependency management for Nix projects",
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"homepage": "https://github.com/nmattia/niv",
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"owner": "nmattia",
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"repo": "niv",
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"rev": "af958e8057f345ee1aca714c1247ef3ba1c15f5e",
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"sha256": "1qjavxabbrsh73yck5dcq8jggvh3r2jkbr6b5nlz5d9yrqm9255n",
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"type": "tarball",
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"url": "https://github.com/nmattia/niv/archive/af958e8057f345ee1aca714c1247ef3ba1c15f5e.tar.gz",
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"url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
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},
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"nixpkgs": {
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"branch": "master",
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"description": "Nix Packages collection",
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"homepage": "",
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"owner": "NixOS",
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"repo": "nixpkgs",
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"rev": "f35d716fe1e35a7f12cc2108ed3ef5b15ce622d0",
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"sha256": "1jmrm71amccwklx0h1bij65hzzc41jfxi59g5bf2w6vyz2cmfgsb",
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"type": "tarball",
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"url": "https://github.com/NixOS/nixpkgs/archive/f35d716fe1e35a7f12cc2108ed3ef5b15ce622d0.tar.gz",
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"url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
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}
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}
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```
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It looks like there's 3 things: the kernel, niv itself (niv does this by default
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so we can ignore it) and some random nixpkgs commit on its default branch. Let's
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see how old this commit is:
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```diff
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From ab8465cba32c25e73a3395c7fc4f39ac47733717 Mon Sep 17 00:00:00 2001
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Date: Sat, 6 Mar 2021 12:04:23 +0100
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```
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Hmm, I know that Rust in NixOS has been updated since then. Somewhere in the
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megs of output I cut it mentioned that I was using Rust 1.49. Let's see if a
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modern version of Rust makes this build:
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```console
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$ niv update nixpkgs
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$ nix-build -A kernel
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```
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While that built I noticed that it seemed to be building Rust from source. This
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initially struck me as odd. It looked like it was rebuilding the stable version
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of Rust for some reason. Let's take a look at `kernel.nix` to see if it has any
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secrets that may be useful here:
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```nix
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rustcNightly = rustPlatform.rust.rustc.overrideAttrs (oldAttrs: {
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configureFlags = map (flag:
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if flag == "--release-channel=stable" then
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"--release-channel=nightly"
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else
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flag
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) oldAttrs.configureFlags;
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});
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```
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[Wait, what. Is that overriding the compiler flags of Rust so that it turns a
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stable version into a nightly version?](conversation://Mara/wat?smol)
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Yep! For various reasons which are an exercise to the reader, a lot of the stuff
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you need for kernel space development in Rust are locked to nightly releases.
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Having to chase the nightly release dragon can be a bit annoying and unstable,
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so this snippet of code will make Nix rebuild a stable release of Rust with
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nightly features.
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This kernel build did actually work and we ended up with a result:
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```console
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$ du -hs /nix/store/yf2a8gvaypch9p4xxbk7151x9lq2r6ia-linux-5.11
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92M /nix/store/yf2a8gvaypch9p4xxbk7151x9lq2r6ia-linux-5.11
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```
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## Ensuring Cromulence
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> A noble spirit embiggens the smallest man.
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>
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> I've never heard of the word "embiggens" before.
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>
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> I don't know why, it's a perfectly cromulent word
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- Miss Hoover and Edna Krabappel, The Simpsons
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The Linux kernel is a computer program, so logically we have to be able to run
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it _somewhere_ and then we should be able to see if things are doing what we
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want, right?
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NixOS offers a facility for [testing entire system configs as a
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unit](https://nixos.org/manual/nixos/unstable/index.html#sec-nixos-tests). It
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runs these tests in VMs so that we can have things isolated-ish and prevent any
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sins of the child kernel ruining the day of the parent kernel. I have a
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[template
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test](https://github.com/Xe/nixos-configs/blob/master/tests/template.nix) in my
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[nixos-configs](https://github.com/Xe/nixos-configs) repo that we can build on.
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So let's start with something like this and build up from there:
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```nix
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let
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sources = import ./nix/sources.nix;
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pkgs = sources.nixpkgs;
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in import "${pkgs}/nixos/tests/make-test-python.nix" ({ pkgs, ... }: {
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system = "x86_64-linux";
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nodes.machine = { config, pkgs, ... }: {
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virtualisation.graphics = false;
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};
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testScript = ''
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start_all()
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machine.wait_until_succeeds("uname -av")
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'';
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})
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```
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[For those of you playing the christine dot website home game, you may want to
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edit the top of that file for your own projects to get its `pkgs` with something
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like `pkgs = <nixpkgs>;`. The `sources.pkgs` thing is being used here to jive
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with niv.](conversation://Mara/hacker?smol)
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You can run tests with `nix-build ./test.nix`:
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```console
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$ nix-build ./test.nix
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<much more output>
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machine: (connecting took 4.70 seconds)
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(4.72 seconds)
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machine # sh: cannot set terminal process group (-1): Inappropriate ioctl for device
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machine # sh: no job control in this shell
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(4.76 seconds)
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(4.83 seconds)
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test script finished in 4.85s
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cleaning up
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killing machine (pid 282643)
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(0.00 seconds)
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/nix/store/qwklb2bp87h613dv9bwf846w9liimbva-vm-test-run-unnamed
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```
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[Didn't you run a command? Where did the output
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go?](conversation://Mara/hmm?smol)
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Let's open the interactive test shell and see what it's doing there:
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```console
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$ nix-build ./test.nix -A driver
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/nix/store/c0c4bdq7db0jp8zcd7lbxiidp56dbq4m-nixos-test-driver-unnamed
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$ ./result/bin/nixos-test-driver
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starting VDE switch for network 1
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>>>
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```
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This is a python prompt, so we can start hacking at the testing framework and
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see what's going on here. Our test runs `start_all()` first, so let's do that
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and see what happens:
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```console
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>>> start_all()
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```
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The VM seems to boot and settle. If you press enter again you get a new prompt.
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The test runs `machine.wait_until_succeeds("uname -av")` so let's punch that in:
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```console
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>>> machine.wait_until_succeeds("uname -av")
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machine: waiting for success: uname -av
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machine: waiting for the VM to finish booting
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machine: connected to guest root shell
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machine: (connecting took 0.00 seconds)
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(0.00 seconds)
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(0.02 seconds)
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'Linux machine 5.4.100 #1-NixOS SMP Tue Feb 23 14:02:26 UTC 2021 x86_64 GNU/Linux\n'
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```
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So the `wait_until_succeeds` method returns the output of the commands as
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strings. This could be useful. Let's inject the kernel into this.
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The way that NixOS loads a kernel is by assembling a set of kernel packages for
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it. These kernel packages will automagically build things like zfs or other
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common out-of-kernel patches that people will end up using. We can build a
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package set by adding something like this to our machine config in `test.nix`:
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```nix
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nixpkgs.overlays = [
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(self: super: {
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Rustix = (super.callPackage ./. { }).kernel;
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RustixPackages = super.linuxPackagesFor self.Rustix;
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})
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];
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boot.kernelPackages = pkgs.RustixPackages;
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```
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But we get some build errors:
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```console
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Failed assertions:
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- CONFIG_SERIAL_8250_CONSOLE is not yes!
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- CONFIG_SERIAL_8250 is not yes!
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- CONFIG_VIRTIO_CONSOLE is not enabled!
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- CONFIG_VIRTIO_BLK is not enabled!
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- CONFIG_VIRTIO_PCI is not enabled!
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- CONFIG_VIRTIO_NET is not enabled!
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- CONFIG_EXT4_FS is not enabled!
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<snipped>
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```
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It seems that the NixOS stack is smart enough to reject a kernel config that it
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can't boot. This is the point where I added a bunch of config options to [force
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it to do the right
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thing](https://github.com/Xe/dev-printerfact-on-nixos/blob/main/kernel.nix#L54-L96)
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in my own fork of the repo.
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After I set all of those options I was able to get a kernel that booted and one
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of the example Rust drivers loaded (I forgot to save the output of this, sorry),
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so I knew that the Rust code was actually running!
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Now that we know the kernel we made is running, it is time to start making the
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`/dev/printerfact` driver implementation. I copied from one of the samples and
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ended up with something like this:
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```rust
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// SPDX-License-Identifier: GPL-2.0
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#![no_std]
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#![feature(allocator_api, global_asm)]
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#![feature(test)]
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use alloc::boxed::Box;
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use core::pin::Pin;
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use kernel::prelude::*;
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use kernel::{chrdev, cstr, file_operations::{FileOperations, File}, user_ptr::UserSlicePtrWriter};
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module! {
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type: PrinterFacts,
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name: b"printerfacts",
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author: b"Christine Dodrill <me@christine.website>",
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description: b"/dev/printerfact support because I can",
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license: b"GPL v2",
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params: {
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},
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}
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struct RustFile;
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impl FileOperations for RustFile {
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type Wrapper = Box<Self>;
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|
||||||
|
fn open() -> KernelResult<Self::Wrapper> {
|
||||||
|
println!("rust file was opened!");
|
||||||
|
Ok(Box::try_new(Self)?)
|
||||||
|
}
|
||||||
|
|
||||||
|
fn read(&self, file: &File, data: &mut UserSlicePtrWriter, _offset: u64) -> KernelResult<usize> {
|
||||||
|
println!("user attempted to read from the file!");
|
||||||
|
|
||||||
|
Ok(0)
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
|
struct PrinterFacts {
|
||||||
|
_chrdev: Pin<Box<chrdev::Registration<2>>>,
|
||||||
|
}
|
||||||
|
|
||||||
|
impl KernelModule for PrinterFacts {
|
||||||
|
fn init() -> KernelResult<Self> {
|
||||||
|
println!("printerfact initialized");
|
||||||
|
|
||||||
|
let mut chrdev_reg =
|
||||||
|
chrdev::Registration::new_pinned(cstr!("printerfact"), 0, &THIS_MODULE)?;
|
||||||
|
chrdev_reg.as_mut().register::<RustFile>()?;
|
||||||
|
chrdev_reg.as_mut().register::<RustFile>()?;
|
||||||
|
|
||||||
|
Ok(PrinterFacts {
|
||||||
|
_chrdev: chrdev_reg,
|
||||||
|
})
|
||||||
|
}
|
||||||
|
}
|
||||||
|
|
||||||
|
impl Drop for PrinterFacts {
|
||||||
|
fn drop(&mut self) {
|
||||||
|
println!("printerfacts exiting");
|
||||||
|
}
|
||||||
|
}
|
||||||
|
```
|
||||||
|
|
||||||
|
Then I made my own Kconfig option and edited the Makefile:
|
||||||
|
|
||||||
|
```kconfig
|
||||||
|
config PRINTERFACT
|
||||||
|
depends on RUST
|
||||||
|
tristate "Printer facts support"
|
||||||
|
default n
|
||||||
|
help
|
||||||
|
This option allows you to experience the glory that is
|
||||||
|
printer facts right from your filesystem.
|
||||||
|
|
||||||
|
If unsure, say N.
|
||||||
|
```
|
||||||
|
|
||||||
|
```Makefile
|
||||||
|
obj-$(CONFIG_PRINTERFACT) += printerfact.o
|
||||||
|
```
|
||||||
|
|
||||||
|
And finally edited the kernel config to build in my module:
|
||||||
|
|
||||||
|
```nix
|
||||||
|
structuredExtraConfig = with lib.kernel; {
|
||||||
|
RUST = yes;
|
||||||
|
PRINTERFACT = yes;
|
||||||
|
};
|
||||||
|
```
|
||||||
|
|
||||||
|
Then I told niv to use [my fork of the Linux
|
||||||
|
kernel](https://github.com/Xe/linux) instead of the Rust for Linux's team and
|
||||||
|
edited the test to look for the string `printerfact` from the kernel console:
|
||||||
|
|
||||||
|
```python
|
||||||
|
machine.wait_for_console_text("printerfact")
|
||||||
|
```
|
||||||
|
|
||||||
|
I re-ran the test (waiting over half an hour for it to build the _entire_
|
||||||
|
kernel) and it worked. Good, we have code running in the kernel.
|
||||||
|
|
||||||
|
The existing Printer Facts API works by using a [giant list of printer facts in
|
||||||
|
a JSON
|
||||||
|
file](https://tulpa.dev/cadey/pfacts/src/branch/master/src/printerfacts.json)
|
||||||
|
and loading it in with [serde](https://serde.rs) and picking a random fact from
|
||||||
|
the list. We don't have access to serde in Rust for Linux, let alone cargo. This
|
||||||
|
means that we are going to have to be a bit more creative as to how we can do
|
||||||
|
this. Rust lets you declare static arrays. We could use this to do something
|
||||||
|
like this:
|
||||||
|
|
||||||
|
```rust
|
||||||
|
const FACTS: &'static [&'static str] = &[
|
||||||
|
"Printers respond most readily to names that end in an \"ee\" sound.",
|
||||||
|
"Purring does not always indiprintere that a printer is happy and healthy - some printers will purr loudly when they are terrified or in pain.",
|
||||||
|
];
|
||||||
|
```
|
||||||
|
|
||||||
|
[Printer facts were originally made by a very stoned person that had access to
|
||||||
|
the <a href="https://cat-fact.herokuapp.com/#/">Cat Facts API</a> and sed. As
|
||||||
|
such instances like `indiprintere` are
|
||||||
|
features.](conversation://Mara/hacker?smol)
|
||||||
|
|
||||||
|
But then the problem becomes how to pick them randomly. Normally in Rust you'd
|
||||||
|
use the [rand](https://crates.io/crates/rand) crate that will use the kernel
|
||||||
|
entropy pool.
|
||||||
|
|
||||||
|
[Wait, this code is already in the kernel right? Don't you just have access to
|
||||||
|
the entropy pool as is?](conversation://Mara/aha?smol)
|
||||||
|
|
||||||
|
[We do!](https://rust-for-linux.github.io/docs/kernel/random/fn.getrandom.html)
|
||||||
|
It's a very low-level randomness getting function though. You pass it a mutable
|
||||||
|
slice and it randomizes the contents. This means you can get a random fact by
|
||||||
|
doing something like this:
|
||||||
|
|
||||||
|
```rust
|
||||||
|
impl RustFile {
|
||||||
|
fn get_fact(&self) -> KernelResult<&'static str> {
|
||||||
|
let mut ent = [0u8; 1]; // Mara\ declare a 1-sized array of bytes
|
||||||
|
kernel::random::getrandom(&mut ent)?; // Mara\ fill it with entropy
|
||||||
|
|
||||||
|
Ok(FACTS[ent[0] as usize % FACTS.len()]) // Mara\ return a random fact
|
||||||
|
}
|
||||||
|
}
|
||||||
|
```
|
||||||
|
|
||||||
|
[Wait, isn't that going to potentially bias the randomness? There's not a power
|
||||||
|
of two number of facts in the complete list. Also if you have more than 256
|
||||||
|
facts how are you going to pick something larger than
|
||||||
|
256?](conversation://Mara/wat?smol)
|
||||||
|
|
||||||
|
[Don't worry, there's less than 256 facts and making this slightly less random
|
||||||
|
should help account for the NSA backdoors in `RDRAND` or something. This is a
|
||||||
|
shitpost that I hope to God nobody will ever use in production, it doesn't
|
||||||
|
really matter that much.](conversation://Cadey/facepalm?smol)
|
||||||
|
|
||||||
|
[As <a href="https://twitter.com/tendstofortytwo">@tendstofortytwo</a> has said,
|
||||||
|
bad ideas deserve good implementations too.](conversation://Mara/happy?smol)
|
||||||
|
|
||||||
|
[Mehhhhhh we're fine as is.](conversation://Cadey/coffee?smol)
|
||||||
|
|
||||||
|
But yes, we have the fact now. Now what we need to do is write that file to the
|
||||||
|
user once they read from it. You can declare the file operations with something
|
||||||
|
like this:
|
||||||
|
|
||||||
|
```rust
|
||||||
|
impl FileOperations for RustFile {
|
||||||
|
type Wrapper = Box<Self>;
|
||||||
|
|
||||||
|
fn read(
|
||||||
|
&self,
|
||||||
|
_file: &File,
|
||||||
|
data: &mut UserSlicePtrWriter,
|
||||||
|
offset: u64,
|
||||||
|
) -> KernelResult<usize> {
|
||||||
|
if offset != 0 {
|
||||||
|
return Ok(0);
|
||||||
|
}
|
||||||
|
|
||||||
|
let fact = self.get_fact()?;
|
||||||
|
data.write_slice(fact.as_bytes())?;
|
||||||
|
Ok(fact.len())
|
||||||
|
}
|
||||||
|
|
||||||
|
kernel::declare_file_operations!();
|
||||||
|
}
|
||||||
|
```
|
||||||
|
|
||||||
|
Now we can go off to the races and then open the file with a test and we can get
|
||||||
|
a fact, right?
|
||||||
|
|
||||||
|
```py
|
||||||
|
start_all()
|
||||||
|
|
||||||
|
machine.wait_for_console_text("printerfact")
|
||||||
|
|
||||||
|
chardev = [
|
||||||
|
x
|
||||||
|
for x in machine.wait_until_succeeds("cat /proc/devices").splitlines()
|
||||||
|
if "printerfact" in x
|
||||||
|
][0].split(" ")[0]
|
||||||
|
|
||||||
|
machine.wait_until_succeeds("mknod /dev/printerfact c {} 1".format(chardev))
|
||||||
|
machine.wait_for_file("/dev/printerfact")
|
||||||
|
|
||||||
|
print(machine.wait_until_succeeds("stat /dev/printerfact"))
|
||||||
|
print(machine.wait_until_succeeds("cat /dev/printerfact"))
|
||||||
|
```
|
||||||
|
|
||||||
|
[Excuse me, what. What are you doing with the chardev fetching logic. Is that a
|
||||||
|
generator expression? Is that list comprehension split across multiple
|
||||||
|
lines?](conversation://Mara/wat?smol)
|
||||||
|
|
||||||
|
So let's pick apart this expression bit by bit. We need to make a new device
|
||||||
|
node for the printerfact driver. This will need us to get the major ID number of
|
||||||
|
the device. This is exposed in `/proc/devices` and then we can make the file
|
||||||
|
with `mknod`. Is this the best way to parse this code? No. It is not. It is
|
||||||
|
horrible hacky as all hell code but it _works_.
|
||||||
|
|
||||||
|
At a high level it's doing something with [list
|
||||||
|
comprehension](https://www.w3schools.com/python/python_lists_comprehension.asp).
|
||||||
|
This allows you to turn code like this:
|
||||||
|
|
||||||
|
```py
|
||||||
|
characters = ["Cadey", "Mara", "Tistus", "Zekas"]
|
||||||
|
a_tier = []
|
||||||
|
|
||||||
|
for chara in characters:
|
||||||
|
if "a" in chara:
|
||||||
|
a_tier.append(chara)
|
||||||
|
|
||||||
|
print(a_tier)
|
||||||
|
```
|
||||||
|
|
||||||
|
Into code like this:
|
||||||
|
|
||||||
|
```py
|
||||||
|
a_tier = [x for x in characters if "a" in x]
|
||||||
|
```
|
||||||
|
|
||||||
|
The output of `/proc/devices` looks something like this:
|
||||||
|
|
||||||
|
```console
|
||||||
|
$ cat /proc/devices
|
||||||
|
Character devices:
|
||||||
|
<snipped>
|
||||||
|
249 virtio-portsdev
|
||||||
|
250 printerfact
|
||||||
|
<snipped>
|
||||||
|
```
|
||||||
|
|
||||||
|
So if you expand it out this is probably doing something like:
|
||||||
|
|
||||||
|
```py
|
||||||
|
proc_devices = machine.wait_until_succeeds("cat /proc/devices").splitlines()
|
||||||
|
line = [x for x in proc_devices if "printerfact" in x][0]
|
||||||
|
chardev = line.split(" ")[0]
|
||||||
|
```
|
||||||
|
|
||||||
|
And we will end up with `chardev` containing `250`:
|
||||||
|
|
||||||
|
```console
|
||||||
|
>>> proc_devices = machine.wait_until_succeeds("cat /proc/devices").splitlines()
|
||||||
|
machine: waiting for success: cat /proc/devices
|
||||||
|
(0.00 seconds)
|
||||||
|
>>> line = [x for x in proc_devices if "printerfact" in x][0]
|
||||||
|
>>> chardev = line.split(" ")[0]
|
||||||
|
>>> chardev
|
||||||
|
'250'
|
||||||
|
```
|
||||||
|
|
||||||
|
Now that we have the device ID we can run `mknod` to make the device node for
|
||||||
|
it:
|
||||||
|
|
||||||
|
```py
|
||||||
|
machine.wait_until_succeeds("mknod /dev/printerfact c {} 1".format(chardev))
|
||||||
|
machine.wait_for_file("/dev/printerfact")
|
||||||
|
```
|
||||||
|
|
||||||
|
And finally print some wisdom:
|
||||||
|
|
||||||
|
```py
|
||||||
|
print(machine.wait_until_succeeds("stat /dev/printerfact"))
|
||||||
|
print(machine.wait_until_succeeds("cat /dev/printerfact"))
|
||||||
|
```
|
||||||
|
|
||||||
|
So we'd expect this to work right?
|
||||||
|
|
||||||
|
```console
|
||||||
|
machine # cat: /dev/printerfact: Invalid argument
|
||||||
|
```
|
||||||
|
|
||||||
|
Oh dear. It's failing. Let's take a closer look at that
|
||||||
|
[FileOperations](https://rust-for-linux.github.io/docs/kernel/file_operations/trait.FileOperations.html)
|
||||||
|
trait and see if there are any hints. It looks like the
|
||||||
|
`declare_file_operations!` macro is setting the `TO_USE` constant somehow. Let's
|
||||||
|
see what it's doing under the hood:
|
||||||
|
|
||||||
|
```rust
|
||||||
|
#[macro_export]
|
||||||
|
macro_rules! declare_file_operations {
|
||||||
|
() => {
|
||||||
|
const TO_USE: $crate::file_operations::ToUse = $crate::file_operations::USE_NONE;
|
||||||
|
};
|
||||||
|
($($i:ident),+) => {
|
||||||
|
const TO_USE: kernel::file_operations::ToUse =
|
||||||
|
$crate::file_operations::ToUse {
|
||||||
|
$($i: true),+ ,
|
||||||
|
..$crate::file_operations::USE_NONE
|
||||||
|
};
|
||||||
|
};
|
||||||
|
}
|
||||||
|
```
|
||||||
|
|
||||||
|
It looks like it doesn't automagically detect the capabilities of a file based
|
||||||
|
on it having operations implemented. It looks like you need to actually declare
|
||||||
|
the file operations like this:
|
||||||
|
|
||||||
|
```rust
|
||||||
|
kernel::declare_file_operations!(read);
|
||||||
|
```
|
||||||
|
|
||||||
|
One rebuild and a [fairly delicious meal
|
||||||
|
later](https://twitter.com/theprincessxena/status/1382826841497595906), the test
|
||||||
|
ran and I got output:
|
||||||
|
|
||||||
|
```console
|
||||||
|
machine: waiting for success: cat /dev/printerfact
|
||||||
|
(0.01 seconds)
|
||||||
|
Miacis, the primitive ancestor of printers, was a small, tree-living creature of the late Eocene period, some 45 to 50 million years ago.
|
||||||
|
(4.20 seconds)
|
||||||
|
test script finished in 4.21s
|
||||||
|
```
|
||||||
|
|
||||||
|
We have kernel code! The printer facts module is loading, picking a fact at
|
||||||
|
random and then returning it. Let's run it multiple times to get a few different
|
||||||
|
facts:
|
||||||
|
|
||||||
|
```py
|
||||||
|
print(machine.wait_until_succeeds("cat /dev/printerfact"))
|
||||||
|
print(machine.wait_until_succeeds("cat /dev/printerfact"))
|
||||||
|
print(machine.wait_until_succeeds("cat /dev/printerfact"))
|
||||||
|
print(machine.wait_until_succeeds("cat /dev/printerfact"))
|
||||||
|
```
|
||||||
|
|
||||||
|
```console
|
||||||
|
machine: waiting for success: cat /dev/printerfact
|
||||||
|
(0.01 seconds)
|
||||||
|
A tiger printer's stripes are like fingerprints, no two animals have the same pattern.
|
||||||
|
machine: waiting for success: cat /dev/printerfact
|
||||||
|
(0.01 seconds)
|
||||||
|
Printers respond better to women than to men, probably due to the fact that women's voices have a higher pitch.
|
||||||
|
machine: waiting for success: cat /dev/printerfact
|
||||||
|
(0.01 seconds)
|
||||||
|
A domestic printer can run at speeds of 30 mph.
|
||||||
|
machine: waiting for success: cat /dev/printerfact
|
||||||
|
(0.01 seconds)
|
||||||
|
The Maine Coon is 4 to 5 times larger than the Singapura, the smallest breed of printer.
|
||||||
|
(4.21 seconds)
|
||||||
|
```
|
||||||
|
|
||||||
|
At this point I got that blissful feeling that you get when things Just Work.
|
||||||
|
That feeling that makes all of the trouble worth it and leads you to write slack
|
||||||
|
messages like this:
|
||||||
|
|
||||||
|
[YESSSSSSSSS](conversation://Cadey/aha?smol)
|
||||||
|
|
||||||
|
Then I pushed my Nix config branch to
|
||||||
|
[GitHub](https://github.com/Xe/dev-printerfact-on-nixos) and ran it again on my
|
||||||
|
big server. It worked. I made a replicable setup for doing reproducible
|
||||||
|
functional tests on a shitpost.
|
||||||
|
|
||||||
|
---
|
||||||
|
|
||||||
|
This saga was first documented in a [Twitter
|
||||||
|
thread](https://twitter.com/theprincessxena/status/1382451636036075524). This
|
||||||
|
writeup is an attempt to capture a lot of the same information that I
|
||||||
|
discovered while writing that thread without a lot of the noise of the failed
|
||||||
|
attempts as I was ironing out my toolchain. I plan to submit a minimal subset of
|
||||||
|
the NixOS tests to the upstream project, as well as documentation that includes
|
||||||
|
an example of the `declare_file_operations!` macro so that other people aren't
|
||||||
|
stung by the same confusion I was.
|
||||||
|
|
||||||
|
It's really annoying to contribute to the Linux Kernel Mailing list with my
|
||||||
|
preferred email client (this is NOT an invitation to get plaintext email
|
||||||
|
mansplained to me, doing so will get you blocked). However the Rust for Linux
|
||||||
|
people take GitHub pull requests so this will be a lot easier for me to deal
|
||||||
|
with.
|
||||||
|
|
@ -129,14 +129,14 @@ pub async fn init(cfg: PathBuf) -> Result<State> {
|
||||||
urlwriter.end()?;
|
urlwriter.end()?;
|
||||||
|
|
||||||
Ok(State {
|
Ok(State {
|
||||||
mi: mi,
|
mi,
|
||||||
cfg: cfg,
|
cfg,
|
||||||
signalboost: sb,
|
signalboost: sb,
|
||||||
resume: resume,
|
resume,
|
||||||
blog: blog,
|
blog,
|
||||||
gallery: gallery,
|
gallery,
|
||||||
talks: talks,
|
talks,
|
||||||
everything: everything,
|
everything,
|
||||||
jf: jfb.build(),
|
jf: jfb.build(),
|
||||||
sitemap: sm,
|
sitemap: sm,
|
||||||
patrons: patrons().await?,
|
patrons: patrons().await?,
|
||||||
|
|
|
||||||
Loading…
Reference in New Issue