[dm-crypt] crypt_get_uuid does not work as expected with plain volumes

Milan Broz gmazyland at gmail.com
Mon Jan 21 15:19:07 CET 2013

On 01/21/2013 02:40 PM, .. ink .. wrote:
>     Hm. That function is confusing.
>     There are two UUIDs in fact
>      - on disk (applies only to LUKS)
>      - for activated device (aka DM-UUID)
>     DM-UUID is constructed from format type, on-disk UUID (if present) and
>     activated device name (so you are able to activate e.g. snapshot of LUKS
>     even with the same on-disk UUID).
>     crypt_get_uuid() was formerly designed to return DM-UUID but apparently I
>     mixed up something here (PLAIN prefix should not be visible).
> so crypt_get_uuid() was supposed to return something like one of the below entries?

In some old testing API yes, but not now :)

> [ink at mtz ~]$ ls /dev/disk/by-id/dm-uuid
> dm-uuid-CRYPT-LUKS1-f57ccd0520f943b9973aed73c6b67f07-zuluCrypt-500-NAAN-sdc6-781
> dm-uuid-CRYPT-PLAIN-rrr
> dm-uuid-CRYPT-PLAIN-zuluCrypt-500-NAAN-image-01.img-2216
> dm-uuid-CRYPT-TCRYPT-zuluCrypt-500-NAAN-truecrypt-2273

These are is udev links created from DM_UUID (it is adding dm-uuid prefix)
(and created by generic DM udev rules, you will similar links for LVM, multipath etc).

Anyway, I'll switch crypt_get_uuid() to return only on-disk UUID
IOW it will work for LUKS  and VERITY where UUID is persistent.
(I did not found users which rely on this old and confusing behaviour for PLAIN/loopAES devices.)

The rest (DM_UUID handling) is internal for libcryptsetup and will remain hidden.

But just for reference (you should not rely on this, internal for libcryptsetup),
DM-UUID is generated like this

   |      |   ---------------------------------
   |      |          activated device name
   |       - CRYPT device context type (LUKS1/PLAIN/ etc)
    - device managed by cryptsetup (similar to LVM,DMRAID,MPATH prefixes)

For device with on-disk UUID:
   |      |        on-disk UUID                  activated device name
   |       - CRYPT device context type (LUKS1/PLAIN/etc)
    - device managed by cryptsetup


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