kernel patching with dkms

How do you maintain a patched version of an in-tree Linux kernel module?

The Problem

For my Dockerized router project, I had to find a way to broadcast a 5 GHz Wi-Fi signal from my desktop hardware – a function that’s almost exclusively the domain of embedded wireless routers and access points.

It turns out that the Wi-Fi module that came with my Intel NUC is notoriously non-cooperative. The iwlwifi kernel driver for Intel Wi-Fi modules simply refuses to do such a thing.

After some Googling, I learned that Qualcomm / Atheros Wi-Fi chipsets are more amenable to operating in AP mode at 5 GHz. So I had to go find a suitable Wi-Fi module that would work.

A quick trip to the local laptop repair shop later, I had a Lenovo-branded M.2 Wi-Fi module with an Atheros chipset ready to swap with the stock Intel hardware.

For anyone wondering, here’s the exact hardware spec:

$ lspci -kvnn
02:00.0 Network controller [0280]: Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter [168c:003e] (rev 20)
	Subsystem: Lenovo QCA6174 802.11ac Wireless Network Adapter [17aa:3044]
	Flags: bus master, fast devsel, latency 0, IRQ 131
	Memory at 91000000 (64-bit, non-prefetchable) [size=2M]
	Capabilities: <access denied>
	Kernel driver in use: ath10k_pci
	Kernel modules: ath10k_pci

Wireless chipsets supported by the ath family of kernel modules are able to broadcast on 5 GHz, but there’s a catch: the stock driver code compiled for and delivered with most Linux distributions almost certainly will not allow you to do it.

By default, the kernel driver looks at two things: the wireless regulatory domain etched in to the EEPROM of the hardware, and the software wireless regulatory database. The combined restrictions from both are what determine the channels on which a radio can initiate a broadcast, a function required to run in access point mode. A more detailed overview can be found here.

The OpenWrt project, an embedded Linux distribution designed to run on wireless routers and networking gear, maintains a set of patches[4] for the ath kernel module which allow for a custom regulatory domain to be set by the user. Which leads us to…

The Solution

Running an OpenWrt filesystem on a stock Linux kernel is possible, but we need a way to incorporate those tweaks if we want to run our wireless hardware in 5GHz AP mode. That’s what ath_user_regd is for.

If you’ve ever installed VirtualBox or an external kernel driver for some hardware, you probably came across DKMS, the dynamic kernel module support package. DKMS is a collection of clever shell scripts available for most Linux distros that can automatically recompile the source code for a kernel module whenever a new kernel is installed. VirtualBox uses DKMS hooks to do exactly that to keep your virtual machines running after a kernel upgrade. I use it for ath_user_regd to maintain a patched version of the ath kernel module with the OpenWrt patches applied.

Instructions for use are in the repo link.

But please, before using it, read the disclaimer. I’m not responsible if you fry your Wi-Fi hardware, break laws, or do other not-smart things.

Credit to Renaud Cerrato’s DIY router article for the original idea.

References

  1. https://badgateway.qc.to/contain-your-router
  2. https://wireless.wiki.kernel.org/en/developers/regulatory/wireless-regdb
  3. https://wireless.wiki.kernel.org/en/users/drivers/ath#regulatory
  4. https://github.com/openwrt/openwrt/blob/master/package/kernel/mac80211/patches/ath/402-ath_regd_optional.patch and https://github.com/openwrt/openwrt/blob/master/package/kernel/mac80211/patches/ath/406-ath_relax_default_regd.patch
  5. https://github.com/dell/dkms
  6. https://github.com/oofnikj/ath-user-regd
  7. https://medium.com/@renaudcerrato/how-to-build-your-own-wireless-router-from-scratch-part-3-d54eecce157f

Leave a Reply

© 2020 bad gateway | Theme: Storto by CrestaProject WordPress Themes.