Part 2 - A Minimal Rust Kernel

Changes made before starting part 2 of the tutorial

Before starting part two I noticed that there were two issues I had to look at.

Unsafe no_mangle

When attempting to build with the thumbv7em-none-eabihf target it was getting an error saying that #[no_mangle] needed to be wrapped in an unsafe block. The original guide made no mention of this, however after a quick google search I was able to find the relevant RFC to consider some attributes unsafe here. I looked at the linked PR and found that the change was made to nightly as recently as Oct 31 2024 and since I was on a nightly version from the end of December 2024 it means it affected me.

I then made the change to the code like so

// main.rs
#![no_std]
#![no_main]
use core::panic::PanicInfo;

#[unsafe(no_mangle)]
pub extern "C" fn _start() -> ! {
    loop {}
}

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

Rust Analyzer still referencing the std library

Rust analyzer was giving false-positive errors as it still references the std library despite the use of #![no_std]. In order to fix this I would need to tell rust analyzer to not check on all targets. Since I use the Helix modal editor for dev work I can create a local languages.toml file with any changes that are relevant for the projects.

After creating .helix/languages.toml in the base of my repo and adding the following contents rust analyzer no longer reported errors based on using the std library.

[language-server.rust-analyzer.config.check]
allTargets = false
targets = "aarch64-apple-darwin"

Actually working on Part 2

Thankfully while working on part two there were no issues involving my Nix setup or using a newer version. I only had to add the llvm-tools-preview to my rust-toolchain.toml file so that the feature was added as a part of my dev shell and then add cargo-bootimage to the list of Nix packages that should be available as well.

Thoughts

• I had a look at the later versions of the bootloader library in case trying to use it would be worth it but as of right now it seems too large a task. Will possibly look at it later when I have more experience with OS dev.