How does the grub efi loader find the correct grub.cfg and boot directory

biosbootgrub

As I understand it, the uefi bios loads grub.efi from the efi partition. This now loads a bunch of grub modules and the configuration file from the /boot directory.

But how does grub find the correct device in which the boot partition resides? In grub legacy, you'd usually install grub to a specific partition, using

# grub-install --root-directory=/dev/sda

So it's quite clear where it will look for the boot directory. How does grub-efi figure out the correct device?

Best Answer

After using ghex to examine my "BOOTX64.EFI" file in the efi partition I found this line.

search.fs_uuid a43d1f11-6ebe-477d-8be3-321a33bc37f9 root hd2,gpt4 
set prefix=($root)'/boot/grub'

This shows that the information for the location of grub (the boot partition) has been embedded by grub2-install into the BOOTX64.EFI file generated for the system.

Grub2 is then executed by the EFI loader and you get the grub interface to choose the operating system (kernel) to boot into or the default kernel is loaded.

Just to expand a bit: ".EFI" files are kind of like applications that the UEFI subsystem can execute. Intended to be used to both boot the system and provide a convenient place to run execution protection and anti-virus type programs.

They put the extensible in "Unified Extensible Firmware Interface"

Related Solutions

Centos – grub error : “File not found” for separate /boot/efi partition

Read Rod's books on this but basically centos6 isn't the most fit distro for UEFI.

EFI Boot – How to Recreate EFI Boot Partition

I don't know why you're using grub in the first place. UEFI acts as a boot loader and it allows to select different operating systems or individual kernels from a boot menu. Although there are some exceptions, it usually is not required to chain a second boot loader, grub in this case.

You mention, you installed elementary OS instead of Fedora, which means you only need to load one operating system. Here I present a way to do it without using grub. The kernel needs to be compiled with EFI_STUB, if that's the case you can check with

grep EFI_STUB /boot/config-<version>

Copy the kernel and initramfs to the ESP (EFI system partition)

cp /boot/vmlinuz-<version> /boot/efi/EFI/elementary/vmlinuz-<version>.efi
cp /boot/initrd.img-<version> /boot/efi/EFI/elementary/initrd.img-<version>

Register kernel as boot option in UEFI

echo "root=UUID=<disk_uuid> ro quiet rootfstype=ext4 add_efi_memmap initrd=\\EFI\\elementary\\initrd.img-<version>" |
  iconv -f ascii -t ucs2 |
  efibootmgr \
    --create --gpt \
    --disk /dev/<disk> --part <partition_number> \
    --label "Elementary OS" \
    --loader "\\EFI\\elementary\\vmlinuz-<version>.efi" \
    --write-signature --append-binary-args -

The --disk argument takes the device name of the disk, e.g. --disk /dev/sda, the --part argument takes the partition number of the ESP, e.g. 4. You can find the ESP partition number with the following command:

gdisk -l /dev/sda | awk '$6=="EF00" {print $1}'

Ensure that you repeat the steps after each kernel update

Either you this manually (just repeat the steps above) or you write a small script which does the job. To fully automatise it, the script could be hooked into the kernel post-install procedure, into the initramfs post-update procedure and into the kernel postrm procedure (to remove the UEFI boot entry). Actually, I don't know why this isn't done by default in the distributions, it's just a few lines of code.