Building a Linux Kernel Driver using Rust

In this example we're going to build a very basic Linux Kernel driver in Rust, compile a Linux kernel, and load the driver into it.

You will need QEMU installed and in your $PATH. If you have an AArch64 machine (Mac with Apple Silicon, Raspberry Pi 3 or newer, etc) you should use qemu-system-aarch64. If you have an x86-64 machine, you should use qemu-system-x86_64. You can use the 'wrong' one - it'll work just fine, but it will go much more slowly, and we have a lot of Linux kernel to build.

Task 1 - Fetch a Debian disk image

This example uses Debian Stable 'nocloud' disk image. Other disk images might work but this is the one we tested.

Make yourself a work area on your machine, and download either https://cdimage.debian.org/images/cloud/bookworm/20250316-2053/debian-12-nocloud-arm64-20250316-2053.qcow2 (AArch64 aka Arm64) or https://cdimage.debian.org/images/cloud/bookworm/20250316-2053/debian-12-nocloud-amd64-20250316-2053.qcow2 (x86-64 aka AMD64)

For example:

mkdir linux-rust-demo
cd linux-rust-demo
wget https://cdimage.debian.org/images/cloud/bookworm/20250316-2053/debian-12-nocloud-arm64-20250316-2053.qcow2

(Windows users can use their favourite tool to make the folder, and a web browser to download the disk image)

Task 1a - Fetch the BIOS (AArch64 only)

If you are going to use AArch64, you'll need a UEFI boot-loader because QEMU doesn't come with one (or at least, the QEMU in homebrew that I used didn't come with one).

Download it from https://gist.githubusercontent.com/theboreddev/5f79f86a0f163e4a1f9df919da5eea20/raw/f546faea68f4149c06cca88fa67ace07a3758268/QEMU_EFI-a096471-edk2-stable202011.tar.gz and unpack it.

wget https://gist.githubusercontent.com/theboreddev/5f79f86a0f163e4a1f9df919da5eea20/raw/f546faea68f4149c06cca88fa67ace07a3758268/QEMU_EFI-a096471-edk2-stable202011.tar.gz
tar xvf QEMU_EFI-a096471-edk2-stable202011.tar.gz

(Windows users, use your favourite tools for this)

Task 2 - Resize the disk image

If your downloads take a while, you may want to make a backup copy of the disk image, because as soon as you boot up the VM, the disk will be changed.

Then we're going to resize the disk image because it's too small (this applies to both the AArch64 one and the x86-64 one). We'll deal with making the partition larger a bit later once the VM has booted.

qemu-img resize debian-12-nocloud-arm64-20250316-2053.qcow2 +32G

Task 3 - Boot it up

We're now going to boot the VM.

For AArch64 on Apple Silicon macOS:

qemu-system-aarch64 -m 8G -M virt -cpu host -accel hvf -smp 8 -bios QEMU_EFI.fd -drive if=none,file=debian-12-nocloud-arm64-20250316-2053.qcow2,id=hd0 -device virtio-blk-device,drive=hd0 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 -nographic -serial telnet:localhost:4321,server,wait

For AArch64 on Arm Linux:

qemu-system-aarch64 -m 8G -M virt -cpu host -accel kvm -smp 8 -bios QEMU_EFI.fd -drive if=none,file=debian-12-nocloud-arm64-20250316-2053.qcow2,id=hd0 -device virtio-blk-device,drive=hd0 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 -nographic -serial telnet:localhost:4321,server,wait

AArch64 otherwise:

qemu-system-aarch64 -m 8G -M virt -cpu cortex-a53 -smp 8 -bios QEMU_EFI.fd -drive if=none,file=debian-12-nocloud-arm64-20250316-2053.qcow2,id=hd0 -device virtio-blk-device,drive=hd0 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 -nographic -serial telnet:localhost:4321,server,wait

For x86-64 on x86-64 Windows:

qemu-system-x86_64 -m 8G -M q35 -accel whpx -smp 8 -hda debian-12-nocloud-amd64-20250316-2053.qcow2 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 -nographic -serial telnet:localhost:4321,server,wait

For x86-64 on x86-64 Linux:

qemu-system-x86_64 -m 8G -M q35 -accel kvm -smp 8 -hda debian-12-nocloud-amd64-20250316-2053.qcow2 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 -nographic -serial telnet:localhost:4321,server,wait

x86-64 otherwise:

qemu-system-x86_64 -m 8G -M q35 -smp 8 -hda debian-12-nocloud-amd64-20250316-2053.qcow2 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp:127.0.0.1:5555-:22 -nographic -serial telnet:localhost:4321,server,wait

In all cases I gave the machine 8 GiB of RAM and 8 CPU cores. You may want to tweak that to suit your needs.

Connect to your virtual machine over Telnet (you can use PuTTY, or your favourite telnet client) on localhost:4321 You should end up at a login prompt. The user is root and there is no password.

Task 4 - Resize the partition

The Debian image we downloaded has quite a small root partition, and we're going to need a lot more space to build the Linux kernel. We already made the virtual disk (the QCOW file) larger, so now let's go in and resize it the partition to use the extra space.

Inside your VM, run:

apt update
apt install fdisk
cfdisk /dev/*da

Once in cfdisk, use the arrow keys to select [ Sort ], then select the root filesystem (the bottom one) and pick [ Resize ], then [ Write ], and [ Quit ]. You'll need to type yes in response to [ Write ].

Now run reboot and connect to the VM again. Running df -h should show that / is about 35 GiB in size - Debian's start-up scripts automatically resized the root filesystem to fill our newly enlarged the partition.

Task 5 - Pre-requisites

If you have an SSH key, you can install it now and start an SSH server.

apt install openssh-server
nano .ssh/authorized_keys # paste your key into this file

If you do that, you can SSH into the VM using localhost:5555 right away. You might prefer that to whatever telnet client you were using. Or, keep using telnet - either is fine.

Now install some more tools:

apt install build-essential libssl-dev python3 flex bison bc libncurses-dev gawk openssl libssl-dev libelf-dev libudev-dev libpci-dev libiberty-dev autoconf llvm clang lld git 
curl https://sh.rustup.rs | bash
source $HOME/.cargo/env
cargo install --locked bindgen-cli
rustup component add rust-src

Task 6 - Build a kernel

Let's grab Linux 6.14 and build it.

curl -O https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.14.tar.xz
tar xvf linux-6.14.tar.xz
cd linux-6.14
make LLVM=1 rustavailable
make LLVM=1 defconfig 
make LLVM=1 menuconfig  # General setup / [*] Rust support

In the menuconfig interface you will need to enter General Support and scroll down to Rust support. Press Y to enable Rust support. Then use the arrow keys to select < Exit > and < Exit > again, then < Yes > to save.

Now we can build and install our kernel.

make LLVM=1 -j8
make LLVM=1 modules_install
make LLVM=1 install
reboot

This takes around 12 minutes or so on a fast laptop (using Hypervisor acceleration). It'll be much longer if you're on the 'opposite' architecture and are having to fully emulate the guest processor.

Task 7 - Build a kernel module

OK! Now we have Linux 6.14 with Rust support enabled. Let's build an out-of-tree kernel module.

git clone https://github.com/Rust-for-Linux/rust-out-of-tree-module
cd rust-out-of-tree-module
git checkout 15de8569df46e16f4940b52c91ee8f6bfbe5ab22
make KDIR=../linux-6.14 LLVM=1

The kernel module has been compiled as rust_out_of_tree.ko. Let's load it.

insmod ./rust_out_of_tree.ko

As it loaded, you should see a message from the kernel - if you're on SSH, check the telnet window. If you don't have telnet connected, you can review the kernel logs with dmesg.

root@localhost:~/rust-out-of-tree-module# insmod rust_out_of_tree.ko
[   13.933513] rust_out_of_tree: loading out-of-tree module taints kernel.
[   13.938155] rust_out_of_tree: Rust out-of-tree sample (init)
root@localhost:~/rust-out-of-tree-module# dmesg
...
[   13.933513] rust_out_of_tree: loading out-of-tree module taints kernel.
[   13.938155] rust_out_of_tree: Rust out-of-tree sample (init)
root@localhost:~/rust-out-of-tree-module#

Now let's unload it.

rmmod rust_out_of_tree

Again, you should see some output:

root@localhost:~/rust-out-of-tree-module# rmmod rust_out_of_tree
[   72.677287] rust_out_of_tree: My numbers are [72, 108, 200]
[   72.677835] rust_out_of_tree: Rust out-of-tree sample (exit)
root@localhost:~/rust-out-of-tree-module# dmesg
...
[   13.933513] rust_out_of_tree: loading out-of-tree module taints kernel.
[   13.938155] rust_out_of_tree: Rust out-of-tree sample (init)
[   72.677287] rust_out_of_tree: My numbers are [72, 108, 200]
[   72.677835] rust_out_of_tree: Rust out-of-tree sample (exit)
root@localhost:~/rust-out-of-tree-module#

Task 8 - Create a device

As of Linux 6.14, Rust for Linux has Rust APIs for:

Block Devices
Miscellaneous Devices
Network Devices

A Miscellaneous Device has an entry like /dev/foobar and we can open it, close it, and send it ioctl requests.

Looking at the documentation for the MiscDevice::register method we can see that we get an opaque object that implements PinInit rather than a concrete type. So, we're going to need to make a bunch of changes, step by step.

First, let's remove that example Vec and hold MiscDeviceRegistration instead. We mark the struct with #[pin_data(PinnedDrop)] to promise that we're not going to be moving things around in memory whilst the module is loaded, and mark the _miscdev field with #[pin]:

#[pin_data(PinnedDrop)]
struct RustOutOfTree {
    #[pin]
    _miscdev: kernel::miscdevice::MiscDeviceRegistration<RustOutOfTreeDevice>,
}

Now instead of implementing kernel::Module, let's implement kernel::InPlaceModule:

impl kernel::InPlaceModule for RustOutOfTree {
    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
        pr_info!("Rust out-of-tree sample (init)\n");

        let options = kernel::miscdevice::MiscDeviceOptions {
            name: kernel::c_str!("rust-misc-device"),
        };

        try_pin_init!(Self {
            _miscdev <- kernel::miscdevice::MiscDeviceRegistration::register(options),
        })
    }
}

Instead of a plain Result we're returning something that implements PinInit. The try_pin_init! macro will do this for us. The name in our kernel::miscdevice::MiscDeviceOptions sets the name of our device in /dev. Pick something else if you like!

We need to adjust our Drop impl, to deal with our newly pinned data structure.

#[pinned_drop]
impl PinnedDrop for RustOutOfTree {
    fn drop(self: Pin<&mut Self>) {
        pr_info!("Rust out-of-tree sample (exit)\n");
    }
}

(We've also removed the bit that prints the Vec we removed).

Finally, let's make the RustOutOfTreeDevice we referenced earlier in our RustOutOfTree structure. It's as basic as we can get away with.

struct RustOutOfTreeDevice {}

#[vtable]
impl kernel::miscdevice::MiscDevice for RustOutOfTreeDevice {
    type Ptr = Pin<KBox<Self>>;

    fn open(
        _file: &kernel::fs::File,
        _misc: &kernel::miscdevice::MiscDeviceRegistration<Self>,
    ) -> Result<Pin<KBox<Self>>> {
        return Err(ENOTTY);
    }
}

Our full file looks like this

// SPDX-License-Identifier: GPL-2.0

//! Rust out-of-tree sample

use kernel::prelude::*;

module! {
    type: RustOutOfTree,
    name: "rust_out_of_tree",
    author: "Rust for Linux Contributors",
    description: "Rust out-of-tree sample",
    license: "GPL",
}

#[pin_data(PinnedDrop)]
struct RustOutOfTree {
    #[pin]
    _miscdev: kernel::miscdevice::MiscDeviceRegistration<RustOutOfTreeDevice>,
}

impl kernel::InPlaceModule for RustOutOfTree {
    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
        pr_info!("Rust out-of-tree sample (init)\n");

        let options = kernel::miscdevice::MiscDeviceOptions {
            name: kernel::c_str!("rust-misc-device"),
        };

        try_pin_init!(Self {
            _miscdev <- kernel::miscdevice::MiscDeviceRegistration::register(options),
        })
    }
}

#[pinned_drop]
impl PinnedDrop for RustOutOfTree {
    fn drop(self: Pin<&mut Self>) {
        pr_info!("Rust out-of-tree sample (exit)\n");
    }
}

struct RustOutOfTreeDevice {}

#[vtable]
impl kernel::miscdevice::MiscDevice for RustOutOfTreeDevice {
    type Ptr = Pin<KBox<Self>>;

    fn open(
        _file: &kernel::fs::File,
        _misc: &kernel::miscdevice::MiscDeviceRegistration<Self>,
    ) -> Result<Pin<KBox<Self>>> {
        return Err(ENOTTY);
    }
}

Let's load it and see if we get a device:

$ make KDIR=../linux-6.14 LLVM=1
$ insmod ./rust_out_of_tree.ko
[ 2337.507487] rust_out_of_tree: Rust out-of-tree sample (init)
$ ls /dev/rust*
crw------- 1 root root 10, 124 Apr  2 16:29 /dev/rust-misc-device
$ rmmod rust_out_of_tree
[ 2345.938810] rust_out_of_tree: Rust out-of-tree sample (exit)

Nice, we got a device!

Task 9 - Implement `open`

Let's implement the open function for our RustOutOfTreeDevice.

Our RustOutOfTreeDevice will need to hold onto a reference to our open Device:

#[pin_data]
struct RustOutOfTreeDevice {
    dev: kernel::types::ARef<kernel::device::Device>,
}

Yes, more pinning was required. The clue was the return type of the open function in the MiscDevice trait: Result<Pin<KBox<Self>>>.

Let's re-write that open function to actually open our device.

#[vtable]
impl kernel::miscdevice::MiscDevice for RustOutOfTreeDevice {
    type Ptr = Pin<KBox<Self>>;

    fn open(
        file: &kernel::fs::File,
        misc: &kernel::miscdevice::MiscDeviceRegistration<Self>,
    ) -> Result<Pin<KBox<Self>>> {
        let dev = kernel::types::ARef::from(misc.device());

        dev_info!(
            dev,
            "Opening Rust Misc Device Sample (uid = {})\n",
            file.cred().euid().into_uid_in_current_ns()
        );

        KBox::try_pin_init(
            try_pin_init! {
                RustOutOfTreeDevice {
                    dev: dev,
                }
            },
            GFP_KERNEL,
        )
    }
}

Our full file looks like this:

// SPDX-License-Identifier: GPL-2.0

//! Rust out-of-tree sample

use kernel::prelude::*;

module! {
    type: RustOutOfTree,
    name: "rust_out_of_tree",
    author: "Rust for Linux Contributors",
    description: "Rust out-of-tree sample",
    license: "GPL",
}

#[pin_data(PinnedDrop)]
struct RustOutOfTree {
    #[pin]
    _miscdev: kernel::miscdevice::MiscDeviceRegistration<RustOutOfTreeDevice>,
}

impl kernel::InPlaceModule for RustOutOfTree {
    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
        pr_info!("Rust out-of-tree sample (init)\n");

        let options = kernel::miscdevice::MiscDeviceOptions {
            name: kernel::c_str!("rust-misc-device"),
        };

        try_pin_init!(Self {
            _miscdev <- kernel::miscdevice::MiscDeviceRegistration::register(options),
        })
    }
}

#[pinned_drop]
impl PinnedDrop for RustOutOfTree {
    fn drop(self: Pin<&mut Self>) {
        pr_info!("Rust out-of-tree sample (exit)\n");
    }
}

#[pin_data]
struct RustOutOfTreeDevice {
    dev: kernel::types::ARef<kernel::device::Device>,
}

#[vtable]
impl kernel::miscdevice::MiscDevice for RustOutOfTreeDevice {
    type Ptr = Pin<KBox<Self>>;

    fn open(
        file: &kernel::fs::File,
        misc: &kernel::miscdevice::MiscDeviceRegistration<Self>,
    ) -> Result<Pin<KBox<Self>>> {
        let dev = kernel::types::ARef::from(misc.device());

        dev_info!(
            dev,
            "Opening Rust Misc Device Sample (uid = {})\n",
            file.cred().euid().into_uid_in_current_ns()
        );

        KBox::try_pin_init(
            try_pin_init! {
                RustOutOfTreeDevice {
                    dev: dev,
                }
            },
            GFP_KERNEL,
        )
    }
}

Now we should be able to see a log when we try and open our device.

$ make KDIR=../linux-6.14 LLVM=1
$ insmod ./rust_out_of_tree.ko
[ 3918.696311] rust_out_of_tree: Rust out-of-tree sample (init)
$ cat /dev/rust-misc-device
cat: /dev/rust-misc-device: Invalid argument
[ 3990.836103] misc rust-misc-device: Opening Rust Misc Device Sample (uid = 0)

Great, we can see that the device has been opened by cat. As of Linux 6.14 there's no support for Read operations - only ioctl operations - so cat gets an error from the Kernel. That's expected.

Task 10 - implement `ioctl`

You've got the hang of this now, so as a bonus exercise, why not implement ioctl. See https://rust.docs.kernel.org/kernel/miscdevice/trait.MiscDevice.html for details.

You'll need some IOCTL numbers to use. Try creating a "Hello" ioctl, with no argument (no data read and no data written):

const RUST_MISC_DEV_HELLO: u32 = _IO('|' as u32, 0x80);

I chose | as the ioctl type for Miscellaneous Devices, because that's what is in the example code. If you need help with this step, that's a great place to look.

To send an ioctl to your device, you can use this Rust program. You'll need to put it in a package (cargo new --bin openfile) and add the nix crate (cargo add nix).

use std::os::fd::AsRawFd;

const HELLO: u8 = 0x80;
nix::ioctl_none!(hello_ioctl, '|', HELLO);

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = std::fs::File::open("/dev/rust-misc-device")?;
    let fd = f.as_raw_fd();
    let result = unsafe { hello_ioctl(fd) };
    println!("ioctl returned {:?}", result);
    Ok(())
}

Or you could try and write the equivalent in Rust (you'll probably need the nix crate and the libc crate).

Here's an example `ioctl` method if you need one

fn ioctl(
    me: Pin<&RustOutOfTreeDevice>,
    _file: &kernel::fs::File,
    cmd: u32,
    arg: usize,
) -> Result<isize> {
    dev_info!(me.dev, "IOCTLing Rust Out Of Tree Device\n");

    let size = kernel::ioctl::_IOC_SIZE(cmd);

    match cmd {
        RUST_MISC_DEV_HELLO => {
            dev_info!(me.dev, "-> hello received (size {}, arg {})\n", size, arg);
            Ok(100)
        }
        _ => {
            dev_err!(me.dev, "-> IOCTL not recognised: {}\n", cmd);
            Err(ENOTTY)
        }
    }
}

Here's what your output might look like if we run that example:

$ make KDIR=../linux-6.14 LLVM=1
$ insmod ./rust_out_of_tree.ko
[12147.696311] rust_out_of_tree: Rust out-of-tree sample (init)
$ cd ../openfile
$ cargo run
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/openfile`
ioctl returned Ok(100)
$ dmesg
[12150.540427] misc rust-misc-device: Opening Rust Misc Device Sample (uid = 0)
[12150.541318] misc rust-misc-device: IOCTLing Rust Misc Device Sample
[12150.541606] misc rust-misc-device: -> hello received (size 0, arg 0)

Task 11 - Keep going!

OK! If you still want more, try implementing 'read' and 'write' ioctls, so you can communicate with your driver. Or look at the Kernel mailing list for the patches that will let you do ordinary read and write syscalls on your device, rather than just ioctls. Happy kernel hacking in Rust!

Rust Exercises