Rewrite init in Rust

[mtjhrc]

Currently our custom init is written in C, I believe it it would ease further development to rewrite it in Rust.

Unanswered questions we need to investigate:

• do we still keep statically linking it? - probably yes
  • is musl libc good enough for us?
  • alternatively is there really no way to link against glibc statically?
  • alternatively can we just go no-std, no-libc route via https://github.com/bytecodealliance/rustix
    • similar to nix crate but can make raw syscalls
    • this should also shrink the init binary size, not sure if we care that much though
• how is Rust support for FreeBSD guests?
  • will we make sure upstream libkrun init supports FreeBSD?

[mtjhrc]

Assigned to @jakecorrenti since I believe he started working on this

[nohajc]

I can answer the FreeBSD part. The aarch64 variant is a tier 3 target which makes it more difficult to cross-compile.

https://doc.rust-lang.org/nightly/rustc/platform-support.html#tier-3

Currently, you need Rust nightly to make it somewhat reasonable.

Example: https://github.com/nohajc/anylinuxfs/blob/main/build-app.sh#L39-L45

[nohajc]

Just want to mention there is a language with arguably better cross-compilation story, excellent C interop and producing lean binaries. Zig.

Not sure if the goal here is to just replace C with something easier to write or have everything in one language.

Anyway, I was actually considering Zig for the guest part of FreeBSD tests so I can avoid the Rust nightly issue. Although to be fair, Zig hasn't reached 1.0 yet, so it might not be considered stable either.

[mtjhrc]

Not sure if the goal here is to just replace C with something easier to write or have everything in one language.

For me "something easier to write" is the main goal, "have everything in one language" is also nice, but more like a secondary goal. Using something like Zig introduces a huge dependency for us, which I'd like to avoid. I've never used Zig, but it looked cool (less safety oriented than Rust though...)

[nohajc]

It's somewhere between C and Rust safety-wise. I don't have much experience with it myself. It just crossed my mind.

It's probably still suitable for early adopters only. If you're looking for stable language editions there's no such thing currently.

Although there are projects which use it as a C cross-toolchain instead of gcc or llvm. It can serve as a really smart wrapper for clang/llvm. It has some pretty nice tricks up its sleeve like bundling all existing libcs for any platform you can think of. So you can do simpler builds without any sysroot for example.