Linux – How to reinstall grub to the EFI partition after initially installing it in Linux root. Do I have to start over

grub2linux-mintuefi

My machine was running Mint 15 KDE 64-bit (GPT partitioning, EFI bootloader without 'secure boot'), and I wanted to try Mint 16 before committing to it. So I installed it into another partition and told it to put GRUB into /dev/sda6 instead of /dev/sda.

I then ran the Ubuntu boot-repair tool on my Mint 15 setup, which detected and added Mint 16 to 15's GRUB menu. So far, so good. I can boot 16 by selecting 15's GRUB from my EFI boot menu (I'm using rEFInd for that), and having that boot 16 for me. But now I want to make 16 directly bootable from rEFInd – so it won't rely on keeping 15 around.

Is boot-repair still the tool for the job?

Mint 16 seems to know how to handle EFI booting directly, so I would expect it to have its own tools for managing the boot process. There's a KDE GRUB2 bootloader tool, but that just seems to manage the grub menu entries in my partition. It does have an option on its 'Advanced' tab to 'Install/Recover Bootloader', but that tool just lists out my partitions – and doesn't show my EFI partition. I guess, because I didn't set up EFI booting at install time, I also don't have /boot/efi mounted.

Any advice on how to proceed?

Best Answer

You're advised to read EFI bootloader principles -- this should help you understand that installing a bootloader into MBR or a partition's boot sector is not going to do anything when booting in EFI mode. You basically want to establish or reuse an existing ESP (EFI System Partition) on a GPT partitioned disk and to store all of your bootloaders in its subdirectories.

Requirements:

Grub >= 2.00 (1.98 and 1.99 do not work)
Grub must be installed from a Linux kernel, that has support for EFI variables (CONFIG_EFI_VARS compiled in or as module efivars)
For creating the EFI boot entry you will need efibootmgr

Setup:

First mount your EFI partition to /boot

mount /dev/sdX1 /boot

If you look at the mount entry, you will see, that it is simply a FAT(32) partition. Under /boot you should find a directory efi.

As grub will call efibootmgr, you should load evivars, if it is not compiled into the kernel:

modprobe efivars

Now you can install grub:

# Replace x86_64 with i386 for 32 bit installations
grub2-install --target=x86_64-efi

Grub installs its files as usual to /boot/grub2. If everything worked correctly, you should now also have a folder /boot/efi/grub2 or /boot/efi/<name_of_your_distro>. With --bootloader-id=insert_name_here you can also specify the name for the folder yourself.

Grub calls efibootmgr automatically and creates a boot entry with that name in the EFI boot menu (in my case, that means it shows up as a bootable device in the EFI menu, not sure if this is the case on every EFI board)

Further setup does not differ from usual grub2 setup, grub2-mkconfig will add the appropriate modules for EFI to your grub.cfg.

Chainloading Windows:

As I asked for a dual boot with Windows, I will include the grub configuration for chainloading it:

Chainloading a Windows installation on EFI is slightly different from one on a MBR disk. You won't need the ntfs or part_mbr modules, instead fat and part_gpt are needed.

Also, setting root is not required, this information is stored by Windows' own boot manager. Instead specify the search command. The parameters needed for it can be determined by

grub-probe --target=hints_string /boot/efi/EFI/Microsoft/Boot/bootmgfw.efi

This will give you the parameters for search specifying the location of the EFI partition, it should look something like:

--hint-bios=hd0,gpt1 --hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1 1ce5-7f28

Instead of telling chainloader the number of sectors to read, you will need to set the path to Windows' EFI loader in the EFI partition. This is the same for all Windows EFI installations. The resulting entry should look like this:

menuentry "Microsoft Windows x86_64 UEFI-GPT" {
    insmod part_gpt
    insmod fat
    insmod search_fs_uuid
    insmod chain
    search --fs-uuid --no-floppy --set=root <insert ouput from grub-probe here>
    chainloader /efi/Microsoft/Boot/bootmgfw.efi
}

Sources: These cover some more cases, if you want to boot from EFI, they are worth reading:

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.