Kernel modules

“Tending kernel modules”

A kernel module is a self-contained driver library file.

modules keep the kernel lighter, as opposed to compiling them into the kernel
load or unload them dynamically without needing to reboot
module files use .ko extension

Types of kernel modules:

Device driver: Communicates with hardware device
Filesystem driver: Required for fs I/O
Network driver: Implement network protocols
System calls: Additional functions for adding/modifying system services
Executable loader: Allows additional executable formats to load

Module and config files

Stored in /lib/modules/ directory:

# view kernel versions
ls -F /lib/modules 
6.5.0-18-generic/  6.5.0-26-generic/

# config files generated during install or by sys admin
ls -hogl /etc/modprobe.d/
total 40K
-rw-r--r-- 1 2.5K Feb 22  2021 alsa-base.conf
-rw-r--r-- 1  154 Aug 20  2023 amd64-microcode-blacklist.conf
-rw-r--r-- 1  325 Aug 17  2021 blacklist-ath_pci.conf
-rw-r--r-- 1 1.5K Aug 17  2021 blacklist.conf
-rw-r--r-- 1  210 Aug 17  2021 blacklist-firewire.conf
-rw-r--r-- 1  677 Aug 17  2021 blacklist-framebuffer.conf
-rw-r--r-- 1  156 Feb 22  2021 blacklist-modem.conf
lrwxrwxrwx 1   41 Mar 21 20:12 blacklist-oss.conf -> /lib/linux-sound-base/noOSS.modprobe.conf
-rw-r--r-- 1  583 Aug 17  2021 blacklist-rare-network.conf
-rw-r--r-- 1  154 Nov 14 20:03 intel-microcode-blacklist.conf
-rw-r--r-- 1  347 Aug 17  2021 iwlwifi.conf

# config files for 3rd party packages
ls -hogl /etc/modules-load.d/
total 4.0K
-rw-r--r-- 1 119 May 15  2023 cups-filters.conf
lrwxrwxrwx 1  10 Mar 21 20:12 modules.conf -> ../modules

# same as /lib/modprobe.d/
ls -hogl /usr/lib/modprobe.d/
total 20K
-rw-r--r-- 1  655 Aug 17  2021 aliases.conf
-rw-r--r-- 1 1.6K Feb  7 04:12 blacklist_linux-hwe-6.5_6.5.0-18-generic.conf
-rw-r--r-- 1 1.6K Mar 12 04:50 blacklist_linux-hwe-6.5_6.5.0-26-generic.conf
-rw-r--r-- 1  390 Sep 26  2023 fbdev-blacklist.conf
-rw-r--r-- 1  773 Mar 11  2022 systemd.conf

# shows current kernel ring buffer
sudo dmesg | grep -i driver
[501249.099477] usbcore: registered new interface driver usb-storage
[501249.101243] usbcore: registered new interface driver uas

# status of modules currently within linux kernel
# Used by is number of processes or modules currently using that Module
lsmod
Module                  Size  Used by
dm_crypt               53248  1
uas                    28672  0
usb_storage            77824  2 uas
tcp_diag               16384  0
inet_diag              24576  1 tcp_diag
sctp                  393216  10
...

# get detailed info about specific module
sudo modinfo bridge
filename:       /lib/modules/5.15.0-91-generic/kernel/net/bridge/bridge.ko
alias:          rtnl-link-bridge
version:        2.3
license:        GPL
srcversion:     65DA8B280548959EDB1E02D
depends:        stp,llc
retpoline:      Y
intree:         Y
name:           bridge
vermagic:       5.15.0-91-generic SMP mod_unload modversions 
...

Installing kernel modules

Also called inserting or loading a module.

insmod

Insert a single module:

Very basic, so you have to provide an absolute directory reference to the module file.
Doesn’t load module dependencies

modprobe

Insert module by module name:

Inserts unloaded dependencies too

# install dm_mirror and dependencies with verbose flag
# calls insmod to do work
sudo modprobe -v dm_mirror
insmod /lib/modules/6.5.0-26-generic/kernel/drivers/md/dm-log.ko 
insmod /lib/modules/6.5.0-26-generic/kernel/drivers/md/dm-region-hash.ko 
insmod /lib/modules/6.5.0-26-generic/kernel/drivers/md/dm-mirror.ko 

# get status of dm_mirror
lsmod | grep -i dm_mirror
dm_mirror              24576  0
dm_region_hash         24576  1 dm_mirror
dm_log                 20480  2 dm_region_hash,dm_mirror

# get infor about dm_mirror
sudo modinfo dm_mirror
filename:       /lib/modules/6.5.0-26-generic/kernel/drivers/md/dm-mirror.ko
license:        GPL
author:         Joe Thornber
description:    device-mapper mirror target
srcversion:     E16D5E7307354AFA415433B
depends:        dm-region-hash,dm-log
retpoline:      Y
intree:         Y
name:           dm_mirror
vermagic:       6.5.0-26-generic SMP preempt mod_unload modversions 
...

# module dependencies are listed in /lib/modules/modules.deb
cat /lib/modules/6.5.0-26-generic/modules.dep | head
kernel/arch/x86/events/amd/amd-uncore.ko:
kernel/arch/x86/events/intel/intel-cstate.ko:
kernel/arch/x86/events/rapl.ko:
kernel/arch/x86/kernel/cpu/mce/mce-inject.ko:

# view module and dependencies
# dependencies are listed after the colon (:)
grep -i mirror /lib/modules/6.5.0-26-generic/modules.dep
kernel/drivers/md/dm-mirror.ko: kernel/drivers/md/dm-region-hash.ko kernel/drivers/md/dm-log.ko

# update modules.dep file
sudo depmod -v

Removing kernel modules

# rm module but not its dependencies
sudo rmmod -v <module-name>

# remove module and its dependencies
# -r recursive
# -v verbose
sudo modprobe -rv dm_mirror
rmmod dm_mirror
rmmod dm_region_hash
rmmod dm_log