In my last blog post, I showed how to build the ArchLinux kernel with Rust
support enabled and also packaging the correct build metadata to support
out-of-tree kernel modules.
In the meantime I gave a presentation about it in the Rust Zürisee Meetup and
kept the AUR package more or less up to date.
Also some people got interested in enabling it for the official kernel build
for ArchLinux but hit some issues and the change got reverted again.
Building with the distro rust-bindgen
With the release of Linux 6.11, instead of a pinned version there should be an
actual minimum version for the Rust toolchain to build the Linux kernel with
Rust support. See the git pull request for 6.11 and the rust-version-policy
on https://rust-for-linux.com.
Using the distro packaged Rust toolchain should make it easier to be included
in the official repositories some day, since using rustup to pin the version
leads to issues when building in a clean chroot environment.
So in December 2024 I started with using rust-bindgen from the Arch
repositories: https://github.com/rnestler/archpkg-linux-rust/pull/11. This
lead into a roadblock when I noticed, that rust-bindgen 0.71.1 breaks the build
of Linux 6.11 with Rust 1.78, while it worked just fine with rust-bindgen
0.70.1.
In January 2025, when updating to to the 6.12 version of the kernel I upgraded
the PKGBUILD to use Rust 1.83 and the current ArchLinux version of
rust-bindgen: https://github.com/rnestler/archpkg-linux-rust/pull/12. This
seemed to work just fine.
Do to unclarities how to ensure that the Rust version used to build the kernel
is the same that will be used to build out-of-tree modules, I decided to still
pin the Rust version used and only support building the kernel with rustup.
Building with the whole distro Rust toolchain
In February a nice pull-request hit my archpkg-linux-rust repo:
https://github.com/rnestler/archpkg-linux-rust/pull/13. The author of it
fixed a bug in the PKGBUILD and adapted it as well to build with rustc and
rust-src from the repos.
Since I didn't want to break building the package with rustup I decided to
support both ways. This is made easy since we can just depend on rust and
rust-src which are both also provided by the rustup package:
$ pacman -Qi rustup
Name : rustup
Version : 1.27.1-1
Description : The Rust toolchain installer
Architecture : x86_64
URL : https://github.com/rust-lang/rustup.rs
Licenses : MIT Apache-2.0
Groups : None
Provides : rust cargo rust-nightly cargo-nightly rustfmt rust-src lib32-rust-libs rust-musl rust-wasm rust-analyzer
...
If rustup is installed we ensure in the PKGBUILD that the rust-src
component is also available:
if rustup --help >/dev/null 2>&1; then
echo "Installing rust-src"
rustup component add rust-src
fi
To ease debugging if something in the toolchain isn't working we call the
rustavailable kernel make target, which will output if something is missing:
echo "Verifying that Rust support is available"
make LLVM=1 rustavailable
Since one of my main goals was to support building out-of-tree Rust kernel
modules, I wanted to store the rust toolchain used, so that a user which has
rustup available can simply use the same Rust version to compile a kernel
module. This turned out to be a bit tricker than expected.
Just doing the most straightforward thing:
rustc --version | cut -d' ' -f 2 > "$srcdir/rust-toolchain"
failed with the following error:
error: in /home/roughl/projects/archpkg/linux-rust/src/rust-toolchain: empty toolchain override file detected. Please remove it, or else specify the desired toolchain properties in the file
The reason is, that the shell redirection of the output into a file
(>"$srcdir/rust-toolchain"), will first create the file empty, then start
rustc. Now the rustc binstub from rustup will see that there is a
rust-toolchain file present when doing the rustc --version call, but since
it is empty it complains.
To fix this issue we need to first determine the version and then pipe it into a file:
rust_version=$(rustc --version | cut -d' ' -f 2)
echo "$rust_version" > "$srcdir/rust-toolchain"
Looking forward
Since we can now build the Linux kernel with Rust support using the distro
packages it should be possible to build it in a clean chroot environment and
thus there shouldn't be much in the way of Rust support landing in official
kernel packages.