A small exploration of GRUB and initramfs

In the menuentry code, there are the following points worth noting:

1. insmod gzio

   Due to the loading of the gzio module, support for GZIP compression and decompression functions is provided.

   My first reaction when I saw this was that I felt a bit disconnected. Why does this seemingly insignificant decompression function need to be available before the kernel is started? Then I remembered that when configuring the kernel, there is an option like this:

   

   In the configuration options, we selected support for initramfs and checked Support initial ramdisk/ramfs compressed using gzip , which means that the size of the initramfs is compressed by gzip during compilation to save space.

   Therefore, the initramfs we hold is in a compressed state before the kernel starts. Therefore, we naturally need to install the gzio module before the kernel starts to support subsequent decompression of initramfs.

2. insmod ext2

   This code shows that GRUB’s temporary file system is of ext2 type. This code is actually the necessary dependency package for installing GRUB to create temporary files, so that the GRUB program can create its temporary file system and obtain it from /boot/initrd.img initramfs image.

3. linux /boot/vmlinuz-6.4.0-rc3 + root=UUID=XXX ro text

   The startup parameters are specified, that is, the root file system is mounted on the block device corresponding to root=UUID=XXX in a read-only (ro) manner, and the default is < Start the kernel in code>text mode (that is, non-graphical Shell interface).

   The startup parameters here can be personalized in the grub file described in the next section.

Generation and modification of grub.cfg

In actual application, it is often necessary to make some modifications to the startup parameters. The following describes two methods of modifying grub.cfg.

/etc/default/grub

As you can see, grub.cfg actually has a relatively fixed format (that is, it consists of a series of menuentries with similar content). Therefore, we actually generate grub.cfg through grub.d (this is actually covered in the 6.S081 experiment), and /etc /default/grub is the configuration file generated by the GRUB program and grub.cfg. The following introduces the main configuration options of this file.

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
# info -f grub -n 'Simple configuration'

# The duration of the GRUB interface when booting, set here to 30s
GRUB_TIMEOUT=30
GRUB_CMDLINE_LINUX=""

# Do not use graphical interface
#GRUB_TERMINAL=console
# Size of graphical interface
#GRUB_GFXMODE=640x480
# Do not use UUID
#GRUB_DISABLE_LINUX_UUID=true

# Hide recovery mode
#GRUB_DISABLE_RECOVERY="true"

Focus on these parameters:

1. GRUB_CMDLINE_LINUX

   Indicates what parameters need to be appended to the linux line in each menuentry in the final generated grub.cfg.

   For example, if set to:

   # Indicates that initramfs needs to wait 120s before mounting the real root file system, which is used to prevent mounting failure caused by the disk not being ready.
   GRUB_CMDLINE_LINUX="rootdelay=120"
   

   Then, the final startup parameters in menuentry are:

   linux /boot/vmlinuz-6.4.0-rc3 + root=UUID=XXX ro rootdelay=120 text
   

   Some other common options:

   # Identify block devices directly by path instead of using UUID. This is an old option. It is recommended to use UUID as much as possible.
   GRUB_CMDLINE_LINUX="root=/dev/sda3"
   # Indicate the specific path of the init process (the first process after startup). Specified here as `/bin/sh`
   GRUB_CMDLINE_LINUX="init=/bin/sh"
   

2. GRUB_DEFAULT

   Reference can be used to specify kernel options on reboot. For example, GRUB_DEFAULT="1> 0" means selecting the 2nd column (Advanced for Ubuntu) of the first menu interface and the 1st kernel of the second menu.

After modifying the grub file, we need to execute sudo update-grub to regenerate the grub.cfg file for next startup.

Modify directly in the GRUB interface

We can select the required kernel in the GRUB interface and press the e key:

Then you can modify the startup parameters and ^X exit.

It is worth noting that this modification is only effective for this launch. If long-term modification is required, it is recommended to modify it through the first method.

initramfs

The GRUB program will start initramfs through initrd.img to mount the real root file system.

initrd.img is an image file of the initial RAM disk in a Linux system. It is a compressed file system image that is usually loaded into memory during the boot process and provides a temporary root file system to provide necessary functionality and module.

We can use the unmkinitramfs /boot/initrd.img-6.4.0-rc3 + /tmp/initrd/ command to decompress initrd and explore what is inside.

├── bin -> usr/bin
├── conf
├── etc
├── init
├── lib -> usr/lib
├── lib32 -> usr/lib32
├── lib64 -> usr/lib64
├── libx32 -> usr/libx32
├── run
├── sbin -> usr/sbin
├── scripts
├── usr
└── var
init

As you can see, this is actually a small file system, namely initramfs. It has its own built-in Shell (BusyBox):

There are a few shell commands (in the bin and sbin directories), and the code logic used to mount the real root file system (stored in the scripts directory). [I guess] Under normal circumstances, the system will execute the script code under scripts to mount the real file system. When an exception occurs during the mount, the system will hand over control to the Shell BusyBox built into the initramfs, and it is up to the user to figure out what went wrong.

We can next trace the file system mounting process in the script directory of initramfs.

The main function for mounting a real file system is local_mount_root:

# Only show the main process code
local_mount_root()
{<!-- -->
# Preprocessing, obtaining parameters, etc. (i.e. root=UUID configured in grub.cfg above)
local_top
if [ -z "${ROOT}" ]; then
panic "No root device specified. Boot arguments must include a root= parameter."
fi
\t
# Get the corresponding block device based on UUID
local_device_setup "${ROOT}" "root file system"
ROOT="${DEV}"

# Preprocessing before mounting
local_premount
\t
# mount
mount ${roflag} ${FSTYPE: + -t "${FSTYPE}"} ${ROOTFLAGS} "${ROOT}" "${rootmnt?}"
}

Since researching this is an error driver, I only looked at local_device_setup:

# $1=device ID to mount device ID
# $2=optionname (for root and etc) What is to be mounted? This should be the root file system
# $3=panic if device is missing (true or false, default: true)
# Sets $DEV to the resolved device node $DEV is the finally obtained block device
local_device_setup()
{<!-- -->
local dev_id="$1"
local name="$2"
local may_panic="${3:-true}"
local real_dev
local time_elapsed
local count

# Get the device waiting time of the rootdelay parameter of grub.cfg. Without this parameter, the default is 30 seconds
local slumber=30
if [ "${ROOTDELAY:-0}" -gt $slumber ]; then
slumber=$ROOTDELAY
fi

# Wait for device
case "$dev_id" in
UUID=*|LABEL=*|PARTUUID=*|/dev/*)
FSTYPE=$( wait-for-root "$dev_id" "$slumber" )
;;
*)
wait_for_udev 10
;;
esac

# The wait is over. If the condition is true, it means that the corresponding device still cannot be obtained, which can only mean that the device is dead.
# So we have to tell the user the problem, let the user solve it by themselves, and enter the BusyBox Shell
# We've given up, but we'll let the user fix matters if they can
while ! real_dev=$(resolve_device "${dev_id}") ||
! get_fstype "${real_dev}" >/dev/null; do
if ! $may_panic; then
echo "Gave up waiting for ${name}"
return 1
fi
echo "Gave up waiting for ${name} device. Common problems:"
echo "-Boot args (cat /proc/cmdline)"
echo "-Check rootdelay= (did the system wait long enough?)"
if [ "${name}" = root ]; then
echo "-Check root= (did the system wait for the right device?)"
fi
echo "-Missing modules (cat /proc/modules; ls /dev)"
panic "ALERT! ${dev_id} does not exist. Dropping to a shell!"
done

DEV="${real_dev}"
}

As you can see, if it enters an error state here, the final effect will be like this 2333: