GuessConfiguration

Overview

The make-initrd tries to guess what the root partition is made of and what features are used for the loaded system.

Most users don't have a good idea of how the distribution's installer created the root partition. It is completely unfair to force a user (even an advanced user) to delve into the nuances of booting the system and linux kernel modules.

The simplest /etc/initrd.mk config looks like this:

AUTODETECT = all

How It Works ?

Before actually generating the image, make-initrd parses the current configuration.

Quite a long time ago, a wonderful sysfs virtual file system appeared in the linux kernel. It is usually mounted in /sys. It is required for many purposes and is usually always mounted on the system. To guess the hardware configuration, make-initrd parses /sys and must have privileges to read any data from there.

Creating an image consists of two parts:

Creation of internal config based on the user config (/etc/initrd.mk).
Creating an image based on an internal temporary config.

After determining the list of devices of interest (paths in /sys), the autodetect process executes guess modules that complete the internal temporary configuration. These modules are located in the guess/* directory. Also, features can have callbacks to enable certain features.

Some of the guess modules require parameters. These options are also specified in /etc/initrd.mk.

You can read more about individual guess modules here.

Step-by-Step Example

To understand how /sys is used to find kernel modules, let's take an example and replay the steps of guess modules. Let's say, we need to find all kernel modules that requires a root mountpoint. Again, this is just an illustration of the sequence of steps.

First of all, we need to determine the device that is mounted as the root partition:

$ findmnt -no SOURCE /
/dev/nvme0n1p2

Now we need to define the path to this device in /sys:

$ ls -la /sys/class/block/nvme0n1p2
lrwxrwxrwx 1 root root 0 сен 12 11:44 /sys/class/block/nvme0n1p2 -> ../../devices/pci0000:00/0000:00:01.1/0000:01:00.0/nvme/nvme0/nvme0n1/nvme0n1p2

Some components can also be symlinks. Therefore, we get an absolute path.

$ readlink -e /sys/class/block/nvme0n1p2
/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/nvme/nvme0/nvme0n1/nvme0n1p2

Linux kernel modules are loaded using modalias. Therefore, in the /sys tree, we are primarily interested in the modalias file.

n=/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/nvme/nvme0/nvme0n1/nvme0n1p2

while [ "$n" != /sys/devices ]; do
	if [ -e "$n/modalias" ]; then
		cat "$n/modalias"
	fi
	n="${n%/*}"
done

For example output is:

pci:v000015B7d00005009sv000015B7sd00005009bc01sc08i02
pci:v00001022d00001483sv00001022sd00001234bc06sc04i00

Now we have enough information to search for modules:

$ modprobe -v -D -n pci:v000015B7d00005009sv000015B7sd00005009bc01sc08i02
insmod /lib/modules/5.14.0.162-centos-alt1.el9/kernel/block/t10-pi.ko
insmod /lib/modules/5.14.0.162-centos-alt1.el9/kernel/drivers/nvme/host/nvme-core.ko
insmod /lib/modules/5.14.0.162-centos-alt1.el9/kernel/drivers/nvme/host/nvme.ko

$ modprobe -v -D -n pci:v00001022d00001483sv00001022sd00001234bc06sc04i00
modprobe: FATAL: Module pci:v00001022d00001483sv00001022sd00001234bc06sc04i00 not found in directory /lib/modules/5.14.0

So, we need to add modules to the initramfs: nvme, nvme-core and t10-pi.

This is how make-initrd looks for kernel modules using /sys.