[EN] How I fixed a non-booting Grub + LUKS + BtrFS system

[PhillyBass]

This is not a request for help.
I am sharing what I did to fix my system for others to use as a reference.

This is a story about how I broke my Grub2 EFI Cryptodisk bootloader and then how I fixed it
Note: I don't know the proper name for the little Grub that unlocks LUKS partitions before loading the bigger Grub

How did I notice the problem?
I turned on my computer and was presented with the usual Grub2 EFI Cryptodisk unlock request

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Welcome to GRUB!
Attempting to decrypt master key...
Enter passphrase for hd2,gpt2 (a3.....
Slot 0 opened
But then there was an error I had never seen before

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error: file `/@snapshots/122/snapshot/boot/grub/x86_64-efi/normal.mod' not found.
I realize now that I deleted snapshot 122 and never updated Grub.  The process below is an abridged version of what actually transpired.  These are my notes that I typed into a second computer as I performed the fixing process.

# Useful articles I found on the Internet to educate myself on how Grub works
https://nixventure.blogspot.com/2015/08/grub-rescue-btrfs.html
https://superuser.com/questions/1588626/grub-bootloader-with-luks-encryption-stuck
https://www.linuxfoundation.org/blog/blog/classic-sysadmin-how-to-rescue-a-non-booting-grub-2-on-linux


# We are starting in the EFI limited functionality Grub bootloader

# Use the set command to see settings

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grub rescue> set
# Some minimal settings are provided

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cmdpath=(hd2,gpt1)/EFI/siduction
prefix=(cryptouuid/####a_bunch_of_hex_codes/@snapshots/122/snapshot/boot.....
# Set the root to crypto0 because I already provided the LUKS password

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grub rescue> set root=(crypto0)
# Set the prefix to where the grub bootloader is located

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grub rescue> set prefix=(crypto0)/@/boot/grub/
# Load the normal module
# In normal mode, commands, filesystem modules, and cryptography modules
# are automatically loaded, and the full GRUB script parser is available.

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grub rescue> insmod normal
# Call normal to load the full-service Grub

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grub rescue> normal
# Now we are in the normal highly-functioning Grub bootloader

# Type the letter "C" to get to the Grub command line

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c
# Show settings

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grub> set# a lot of settings are displayed here

# Prepare Grub by loading necessary modules
# I don't know if I need every one of these
# Loading more than needed will not hurt the process
# Loading fewer than needed will hurt the process

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grub> insmod cryptodisk
grub> insmod luks
grub> insmod lvm
grub> insmod btrfs
grub> insmod part_msdos
# Instruct Grub to automatically mount all LUKS partitions

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grub> cryptomount -a# Provide my LUKS password here

# Set the root device

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grub> set root=(crypto0)
# List the kernel and initrd files available

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grub> ls /@/boot# Decide which kernel to use

# Specify the kernel and initrd
# Do not fail to pass the root and rootflags to the kernel
# Notice the ro means to mount filesystems as read-only

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grub> linux /@/boot/vmlinuz-6.3.8-2-siduction-amd64 root=/dev/dm-0 ro rootflags=subvol=@
grub> initrd /@/boot/initrd.img-6.3.8-2-siduction-amd64
# Boot

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grub> boot
# If the process failed, then you will likely be dropped into the initramfs
# This is how I discovered the proper root device name to pass to the kernel

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(initramfs) fdisk -l
/dev/dm-0   # This is the root filesystem
/dev/dm-1   # This is the swap partition
# The process worked, I am presented with the usual login screen

# Install and update Grub

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$ sudo grub-install
$ sudo update-grub
# Reboot to see if the system has been fixed

[edlin]

Thank you for your detailed report. I hope no one needs it, but if they do, it's a verified guide. Thank you.

edlin

[dibl]

+1