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SubjectRe: [PATCH v8 27/31] Kbuild: add Rust support
On Wednesday, August 17th, 2022 at 17:13, Miguel Ojeda <> wrote:

> Hi Arnd,
> On Wed, Aug 17, 2022 at 4:40 PM Arnd Bergmann wrote:
> > Hi Miguel,
> >
> > I tried enabling rust support in the gcc builds I provide at
> >
> Thanks for giving it a go!
> > to make this more accessible, but it appears that the command line
> > options here are not portable:
> >
> > /home/arnd/cross/x86_64/gcc-12.1.0+rust-nolibc/x86_64-linux/bin/x86_64-linux-gccrs
> So you mean with GCC Rust, right? (i.e. we have "GCC builds" working,
> via compiling the Rust side with LLVM and linking with the GCC C side,
> but it is not intended for production or to be supported, even if we
> cover it in our CI, test it boots and loads modules etc.).
> Indeed, `gccrs` does not support `rustc` flags yet. I am not sure if
> the GCC Rust team will eventually provide a driver for those like
> clang does for e.g. `cl` -- I would hope they do, since many projects
> would benefit from it, but maybe they plan to start simply by
> modifying Cargo to call them as they need instead.

There is already a prototype of such a driver. It can be found at Unlike what the name suggests it is not cargo specific. It consists of two binaries. The first calls cargo, but tells it to use the second binary instead of a real rustc. This second part then translates all arguments to what gccrs expects. It is possible to directly invoke this second binary. For now it probably won't work for rust-for-linux though as it doesn't have all arguments that are used by rust-for-linux implemented.

> If they don't support it, we will have to map the flags on our side --
> it should not be a big problem. However, see below...
> > I guess nobody has tried this so far. Would you think that fixing this is only
> > a matter for fixing the build system to pass the correct flags depending on the
> > compiler, or is this broken in a more fundamental way?
> If you meant GCC Rust, then it is a bit too early for the compiler. As
> far as I now, they are working on compiling the `core` crate and
> supporting more stable language features. They are also researching
> the integration of the borrow checker, though we wouldn't need that
> for "only" compiling the kernel.
> Now, if they decided to focus on supporting Rust for Linux early on
> (which would be great), they would still need to work on the delta
> between what what they target now and what we use (which includes both
> stable and some unstable features), plus I assume infrastructure bits
> like the platform (target spec) support, the flags / `rustc` driver
> (though I would be happy to do as much as possible on our side to
> help), etc.
> (We privately talked about possible timelines for all that if they
> were to focus on Rust for Linux etc., but I let them comment or not on
> that... Cc'ing them! :)
> Cheers,
> Miguel

As alternative to GCC Rust there is also which uses libgccjit as backend for the official rust compiler rather than writing a full Rust frontend for GCC from scratch. With a bit of patching to force it to be used, I was able to compile all Rust samples with GCC using rustc_codegen_gcc. However it gives warnings of the following kind:

ld.lld: warning: rust/built-in.a(core.o):(.data.rel.local) is being placed in '.data.rel.local'

And hangs very early in the boot process. If I enable early logging, it prints up to "Booting the kernel." and then does nothing. This is probably because support for setting a different relocation model is not yet implemented. I opened for this.

There may be other issues, but rustc_codegen_gcc is probably going to be the easiest route towards a LLVM free rust-for-linux build. By the way note that rust-bindgen which we use for generating rust bindings from C headers depends on LLVM. See


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