This is a toy Linux kernel module that prints 🫀 to the console (aka dmesg), so that you can know if the kernel logging is working. Maybe you want to check whether your logging pipeline works end to end, maybe you just want to learn how these things work, maybe you just want emojis there. It's your funeral.

The module is implemented in both C and Rust. I started with Rust, but then I had to re-implement it in C because the Rust version kept crashing.

You'll need linux-headers installed to build either one.

To build the module:

make -C c

To insert the module:

sudo insmod c/dmesg_heartbeat.ko

You should see these pop up in dmesg:

ivan@vm:~$ sudo dmesg -T | tail -n3
[Fri May 12 04:42:29 2023] 🫀
[Fri May 12 04:42:39 2023] 🫀
[Fri May 12 04:42:49 2023] 🫀

To remove the module:

sudo rmmod dmesg_heartbeat

The C version is very simple and it keeps the timer in a static variable at a fixed address, so very little can go wrong (and nothing should).

On Ubuntu Lunar Lobster things might work a little easier:

• https://discourse.ubuntu.com/t/ubuntu-kernel-is-getting-rusty-in-lunar/34977

I'm using Linux v6.5-rc1 on Debian and I needed a few patches for that:

• 8fac97511408 rust: enable allocator_api to allow Box usage
• a053ba6b56c8 rust: include in deb package for linux-headers

If you are running on aarch64, you'll also need the following (in order):

• dd50b8163346 arm64: rust: Enable Rust support for AArch64
• 02d425fa78f6 arm64: rust: Enable PAC support for Rust
• 0c078d5cca69 arm64: rust: add missing BINDGEN_TARGET_arm64 in rust/Makefile

You will need to use a specific version of Rust, the same one the kernel was built with. For Linux v6.5-rc1 that's v1.68.2. Same goes for bindgen v0.56.0. To make this happen with already installed Rust via rustup:

rustup toolchain add 1.68.2-aarch64-unknown-linux-gnu
rustup default 1.68.2-aarch64-unknown-linux-gnu

Adjust the command above if you have a different host architecture.

cargo install --locked --version 0.56.0 bindgen

You'll also need rust-src component:

rustup component add rust-src

If you try building the module:

make -C rust

There will be an error, because for some reason it tries to create a file in the system header directory. You'll just have to let it:

sudo chown $(id -u) /usr/src/linux-headers-$(uname -r)/

It should build successfully after that.

To insert the module:

sudo insmod c/dmesg_heartbeat.ko

You should see these pop up in dmesg:

ivan@vm:~/projects/printk_heartbeat$ sudo dmesg -T | tail -n3
[Sat May 13 02:39:09 2023] dmesg_heartbeat: 🫀
[Sat May 13 02:39:14 2023] dmesg_heartbeat: 🫀
[Sat May 13 02:39:19 2023] dmesg_heartbeat: 🫀

There's also the name of the module present, which Rust macro adds for some reason when calling pr_info!() macro. In C pr_info doesn't do this.

To remove the module:

sudo rmmod dmesg_heartbeat

The Rust version has a wrapper for a Boxed bindings::timer_list. It is critical that the timer is stored within a Box, because otherwise it can and will be moved around in memory by Rust and you will get some nasty kernel complaints and crashes due to mangled memory.

Ideally there should be a Pin in addition to Box, but Box works okay:

• https://doc.rust-lang.org/std/pin/index.html