Re: Building BCacheFS module with Rust support via DKMS package

Greetings.

Martin Steigerwald - 06.08.26, 19:07:33 CEST:
> I am building my own kernel with:
> 
> time eatmydata make LLVM=1 -j16 bindeb-pkg LOCALVERSION=-t14g5
> 
> I do have rustc and rust-src as of 1.95.0+dfsg1-2, bindgen as of
> 0.72.1-1+b2 on Devuan Ceres which is based on Debian Unstable aka Sid.
> 
> I am quite confident my kernel is build with rust support:
> 
> % grep _RUST /boot/config-7.1.6-t14g5
> CONFIG_RUSTC_VERSION=109500
> CONFIG_RUST_IS_AVAILABLE=y
> CONFIG_RUSTC_LLVM_VERSION=210108
> CONFIG_RUSTC_LLVM_MAJOR_VERSION=21
> CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE=y
> CONFIG_RUSTC_HAS_SPAN_FILE=y
> CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES=y
> CONFIG_RUSTC_HAS_FILE_WITH_NUL=y
> CONFIG_RUSTC_HAS_FILE_AS_C_STR=y
> CONFIG_RUST=y
> CONFIG_RUSTC_VERSION_TEXT=
>   "rustc 1.95.0 (59807616e 2026-04-14) (built from a source tarball)"
> CONFIG_HAVE_RUST=y
> CONFIG_HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC=y
[…]
> CONFIG_RUST_OVERFLOW_CHECKS=y
> # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
> CONFIG_RUST_INLINE_HELPERS=y
> 
> I install linux-image and linux-header packages.
> 
> Yet with bcachefs-kernel-dkms up to 1:1.39.0 I see:
> 
> built without Rust support; this will be required in the near future -
> ensure a compatible Rust toolchain (rustc + bindgen + rust-src) is
> available at module build time
> 
> Would bcachefs-kernel-dkms use LLVM automatically? AFAIR I did not
> manage to have a kernel with Rust support using GCC which I used
> previously.

It does.

Also rust source files are being built into object files.

Also things like "rust/libcore.rmeta" are build.

Yet not included into linux-headers package.

So for some reason the Debian build does not include the necessary Rust 
files within the linux-headers.

This is why Rust build is disabled during DKMS module build as indicated 
by "make.log":

bcachefs: building without Rust — missing the kernel's prebuilt Rust 
stdlib (/usr/src/linux-headers-7.1.7-t14g5/rust/libcore.rmeta); the kernel 
was built/installed without its rust/ artifacts

I leave it at that for now.

Best,
-- 
Martin