Who-T: udev-hid-bpf: quickstart tooling to fix your HID devices with eBPF
For the last few months, Benjamin Tissoires and I have been working on and polishing a little tool called udev-hid-bpf [1]. This is the scaffolding required quickly and easily write, test and eventually fix your HID input devices (mouse, keyboard, etc.) via a BPF program instead of a full-blown custom kernel driver or a semi-full-blown kernel patch. To understand how it works, you need to know two things: HID and BPF [2].
HID is the Human Interface Device standard and the most common way input devices communicate with the host (HID over USB, HID over Bluetooth, etc.). It has two core components: the "report descriptor" and "reports", both of which are byte arrays. The report descriptor is a fixed burnt-in-ROM byte array that (in rather convoluted terms) tells us what we'll find in the reports. Things like "bits 16 through to 24 is the delta x coordinate" or "bit 5 is the binary button state for button 3 in degrees celcius". The reports themselves are sent at (usually) regular intervals and contain the data in the described format, as the devices perceives reality. If you're interested in more details, see Understanding HID report descriptors.
BPF or more correctly eBPF is a Linux kernel technology to write programs in a subset of C, compile it and load it into the kernel. The magic thing here is that the kernel will verify it, so once loaded, the program is "safe". And because it's safe it can be run in kernel space which means it's fast. eBPF was originally written for network packet filters but as of kernel v6.3 and thanks to Benjamin, we have BPF in the HID subsystem. HID actually lends itself really well to BPF because, well, we have a byte array and to fix our devices we need to do complicated things like "toggle that bit to zero" or "swap those two values".
