Cannot mount LVM volumes after manually decrypting LUKS partition

I’ve got a full drive image 2022-06-11_fedora.iso that I’m trying to manipulate its partitions on another computer. (e.g. resize LVM logical volumes, move ESP, and expand /boot partition).

I cannot seem to mount the LVM volumes after decrypting the LUKS partition.
How can I mount the LVM volumes after decrypting the LUKS partition that they sit on in order to manipulate the volumes with the LVM command suite?

fdisk of image contents

3 partitions:

  1. 2022-06-11_fedora.iso1 is /boot
  2. 2022-06-11_fedora.iso2 is ESP
  3. 2022-06-11_fedora.iso3 is LVM on LUKS
$ fdisk -l 2022-06-11_fedora.iso 
Disk 2022-06-11_fedora.iso: 238.47 GiB, 256060514304 bytes, 500118192 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 

Device                      Start       End   Sectors   Size Type
2022-06-11_fedora.iso1   2048    514047    512000   250M Linux filesystem
2022-06-11_fedora.iso2 514048    808959    294912   144M EFI System
2022-06-11_fedora.iso3 808960 500117503 499308544 238.1G Linux filesystem

Decrypting third partition

$ sudo cryptsetup plainOpen --offset=808960 2022-06-11_fedora.iso cryptdisk
Enter passphrase for 2022-06-11_fedora.iso: 

$ lsblk
loop0             7:0    0 238.5G  0 loop  
└─cryptdisk     253:2    0 238.1G  0 crypt 
sda               8:0    0   1.8T  0 disk  
└─sda1            8:1    0   1.8T  0 part  
nvme0n1         259:0    0   1.8T  0 disk  
├─nvme0n1p1     259:1    0   487M  0 part  /boot/efi
├─nvme0n1p2     259:2    0   3.8G  0 part  /recovery
└─nvme0n1p3     259:3    0   1.8T  0 part  
  └─cryptdata   253:0    0   1.8T  0 crypt 
    └─data-root 253:1    0   1.8T  0 lvm   /

$ sudo fdisk -l /dev/mapper/cryptdisk 
Disk /dev/mapper/cryptdisk: 238.09 GiB, 255646326784 bytes, 499309232 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Trying to mount /dev/mapper/cryptdisk

$ sudo mount /dev/mapper/cryptdisk /mnt/crypt
mount: /mnt/crypt: wrong fs type, bad option, bad superblock on /dev/mapper/cryptdisk, missing codepage or helper program, or other error.

How can I mount the LVM volumes after decrypting the LUKS partition?

Asked By: EarthIsHome


First of all, this

sudo cryptsetup plainOpen --offset=808960 2022-06-11_fedora.iso cryptdisk

is wrong. If the third partition is really a LUKS partition you didn’t open it correctly, the plain mode in cryptsetup uses hash of the provided passphrase to decrypt the data which won’t work for LUKS.

Second: The /dev/mapper/cryptdisk device is the LVM physical volume (or it would be if it was decrypted correctly) it is not mountable. You need to mount the logical volume(s) on it.

To do this correctly:

  • Use losetup to create the loop device: sudo losetup -f 2022-06-11_fedora.iso --partscan
  • You should now see all three partitions in the lsblk output.
  • Open the third partition with sudo cryptsetup luksOpen /dev/loop0p3 cryptdisk.
  • If you don’t see the logical volumes in the lsblk output, they were not automatically activated, use vgscan to scan for the volume group and then use sudo vgchange -ay <vg name> to activate it.
  • Mount the logical volume(s) in the VG. The path will be /dev/mapper/<vgname>-<lvname> (same as for your system data-root LV).

Note: VG name is a unique identifier in LVM so if the VG on the image is also called data, you won’t be able to activate it and you need to rename it first by identifying the correct VG by its UUID using vgrename <uuid> <new name>. (You will notice this, LVM will complain about having two VGs in the system with the same name.)

Answered By: Vojtech Trefny
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