Mike Fleetwood authored Apr 16, 2015 and Curtis Gedak committed Jan 29, 2016

Load basic details of active Device-mapper encryption mappings from the
kernel.  Use dmsetup active targets.

    # cryptsetup luksFormat /dev/sdb5
    # cryptsetup luksFormat /dev/sdb6
    # cryptsetup luksOpen /dev/sdb6 sdb6_crypt
    # ls -l /dev/mapper/sdb6_crypt /dev/dm-0
    lrwxrwxrwx. 1 root root 7 Nov 15 09:03 /dev/mapper/sdb6_crypt -> ../dm-0
    brw-rw----. 1 root disk 253, 0 Nov 15 09:03 /dev/dm-0
    # ls -l /dev/sdb6
    brw-rw----. 1 root disk 8, 22 Nov 15 09:02 /dev/sdb6
    # dmsetup table --target crypt
    sdb6_crypt: 0 1044480 crypt aes-cbc-essiv:sha256 0000000000000000000000000000000000000000000000000000000000000000 0 8:22 4096

So far just load the mapping name and underlying block device reference
(path or major, minor pair).

Note that all supported kernels appear to report the underlying block
device as major, minor pair in the dmsetup output.  Underlying block
device paths are added to the cache when found during a search to avoid
stat(2) call on subsequent searches for the same path.

Prints debugging to show results, like this:

    # ./gpartedbin
    ======================
    libparted : 2.4
    ======================
    DEBUG: /dev/sdb5: LUKS closed
    DEBUG: /dev/sdb6: LUKS open mapping /dev/mapper/sdb6_crypt

Bug 760080 - Implement read-only LUKS support