Compiling an out of tree rust module

This chapter will be less Rust focused, and more an introduction on how to compile Out-of-tree kernel modules in general. The examples were compiled with a net-next kernel based on v6.8-rc1.

Module code

The easiest way to get started is to copy over a module to your directory, e.g. copy over samples/rust/rust_minimal.rs and try compiling it. You can read up more information about modules in Rust in the blog entry Porting my first phy driver or look at the commits on github.

Rust available?

The second test is to make sure that Rust is available for your compilation:

export PATH="$HOME/.cargo/bin:$PATH"
export LIBCLANG_PATH=/usr/lib/llvm-16/lib
export LLVM=1

$ KSRC=~/code/kernel/net-next
$ make -C $KSRC rustavailable

make: Entering directory '~/code/kernel/net-next'
Rust is available!
make: Leaving directory '~/code/kernel/net-next'

If it does not show up, make sure to check out the blog entry on Getting started.

Now, you can compile your external Out-of-tree module with the following command:

$ make -C $KSRC M=$(pwd) rustavailable

Makefile

Now, to make you life easier, the next step is creating a Makefile.

chrysh@bentobox ~/code/rust/quarto_rs/src (git)-[main] % cat Makefile
# SPDX-License-Identifier: GPL-2.0

# obj-$(CONFIG_SAMPLE_RUST_QUARTO)		+= rust_quarto.o
obj-m += rust_quarto.o
KERNEL_SRC := ~/code/kernel/net-next
SRC := $(shell pwd)

all:
	$(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules

modules_install:
	$(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install

Don’t forget to check out a few environment variables before you compile:

$ export PATH="$(HOME)/.cargo/bin:$(PATH)"
$ export LIBCLANG_PATH=/usr/lib/llvm-16/lib
$ export LLVM=1

Now, when you enter make, you module gets compiled

% make
make -C ~/code/kernel/net-next M=/home/chrysh/code/rust/quarto_rs/src modules
make[1]: Entering directory '/home/chrysh/code/kernel/net-next'
  RUSTC [M] /home/chrysh/code/rust/quarto_rs/src/rust_quarto.o
  MODPOST /home/chrysh/code/rust/quarto_rs/src/Module.symvers
  LD [M]  /home/chrysh/code/rust/quarto_rs/src/rust_quarto.ko
make[1]: Leaving directory '/home/chrysh/code/kernel/net-next'

References

You can find this code in the branch out_of_tree on github.