GRUB Bootloader

Chapter 2

GRUB 2 Implementation

On BIOS-Based Systems

• GRUB 2 on a BIOS-based system keeps all of its files in three different locations.

  • /boot/grub2/ (or /boot/grub/ in case of Ubuntu)

  • /etc/default/grub

  • /etc/grub.d/

/boot/grub2

• The location where GRUB 2 is installed and holds the bootloader' core files.

Contents of /boot/grub2

Device.map

• GRUB does not understand disk names like sda or vda since these disk naming conventions were created by the SCSI drivers of operating systems. That's why GRUB has it's own naming conventions.

Naming Conventions Of GRUB

• In GRUB, the hard disk starts at 0, and the partition numbers start at 1, whereas the OS naming conventions of disks and partitions start at 1.

• The Device.map file comes in handy here. Commands like grub2-install maps and converts the SCSI disk naming conventions to the GRUB disk naming conventions by reading the device.map file. Below is an example of this file:

# strace -o delete_it.txt  grub2-install  /dev/sda
      Installing for i386-pc platform.
      Installation finished. No error reported.
# cat delete_it.txt | grep -i 'device.map'
      openat(AT_FDCWD, "/boot/grub2/device.map", O_RDONLY) = 3
      read(3, "# this device map was generated "..., 4096) = 64
      openat(AT_FDCWD, "/boot/grub2/device.map", O_RDONLY) = 3
      read(3, "# this device map was generated "..., 4096) = 64

grub.cfg

• This is the main configuration file of GRUB. It's a huge script file that is generated by referring to some other script files.

The grub.cfg file

• Grub has it's own set of commands.

i386-pc

• This directory has all the GRUB-supported filesystem modules (drivers) in it. All the *.mod files are the modules. By using these modules, GRUB can load the kernel and initramfs files in memory.

• At the same time, you will find modules like http.mod and pxe.mod. This means GRUB 2’s part-3 can load the kernel and initramfs files from the http and pxe devices. In general, the *.mod files add features, not just devices. The features may include device support, filesystem support, or protocol support.

• There are also some other files present here:

• The core.img file is part-3 of GRUB 2. So, the Linux booting sequence becomes as follows:

-> Power on -> BIOS -> POST -> BIOS ->
-> part-1 of GRUB2 -> Part-2 of GRUB2 -> core3.img -> grub.cfg ->
-> if /boot is on an xfs filesystem -> /boot/grub2/i386-pc/xfs.mod ->
-> load vmlinuz & initramfs in main memory.

/etc/default/grub

• This file is used by GRUB to accept the cosmetic and kernel command-line changes from the user.

$ cat /etc/default/grub
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="resume=/dev/mapper/root_vg-swap rd.lvm.lv=root_vg/root rd.lvm.lv=root_vg/swap console=ttyS0,115200 console=tty0"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true

• We can change the default timeout of the GRUB welcome screen, the font, the submenus, and the default kernel command-line parameters like the root device name, the swap device name, etc.

/etc/grub.d

• GRUB 2 has a command called grub2-mkconfig . The name of command suggests that it will make the GRUB configuration file grub.cfg, which will be referred by part-3 of GRUB to show the welcome screen.

• The grub2-mkconfig file will first take the cosmetic and kernel command-line parameter inputs from /etc/default/grub and run the script files from the /etc/grub.d/ directory.

• The 00_header, 01_users, 08_fallback_counting, 10_reset_boot_success, and 12_menu_auto_hide script files do the housekeeping work.

10_linux

• Whenever a user executes the grub2-mkconfig command , it will run this script. The 10_linux file will find out what other Linux distributions you have installed on your system.

• It will literally go partition by partition and find all the other Linux versions that have been installed on your system. If there are any others, then it will make a menuentry of it in grub.cfg. Along with menuentry, it will add the respective kernel and initramfs entries.

20_linux_xen

• After grub2-mkconfig , this script file will find out whether your system has the XEN kernel installed.

• If it does, then it will add the appropriate entry for it in grub.cfg. Most of the Linux distributors ship XEN as a separate kernel package. XEN is mostly used by hypervisors.

20_ppc_terminfo

• If your system has PPC or a PowerPC architecture from IBM, then this script file will find the respective kernel for it and will add the appropriate entry into grub.cfg.

30_os_prober

• If you have any non-Linux-based OS installed on your HDD, then this script file will find that OS and will make the appropriate entry for it.

• In other words, if you have Windows installed on your system, it will automatically find that out and will make an appropriate entry for it in grub.cfg.

30_uefi-firmware

• This script will run successfully only if you have a UEFI system. The job of this script file is to add the appropriate entries of UEFI firmware in grub.cfg.

40_custom and 41_custom

• These are given to the user in case the user wants to add some custom entries to grub.cfg. For example, if grub2-mkconfig fails to add any of the installed OS as entries, then users can add a custom entry to these two custom files.

• You can make your own custom files, but you need to make sure each has a number assigned to it and has executable permission.

GRUB 2 on UEFI-Based System

• GRUB 2 stores it's files at:

  • /boot/grub2

  • /boot/efi/EFI/fedora/

  • /etc/default/grub

• The grub.cfg file that was shown earlier in /boot/grub2/ has been shifted inside ESP (/boot/efi/EFI/fedora/).

• Also, there is no i386-pc directory. This is because of the rich device and filesystem support provided by EFI. Inside ESP, you will find a couple of *.efi files, including our shim.efi and grubx64.efi binaries.

• The /etc/default/grub file, which is responsible for GRUB’s cosmetic changes and for kernel command-line parameters, is still at the same location.

• The device.map file is not available since the grub2-install command does not have significance on a UEFI system.