[dm-crypt] UUID question

Arno Wagner arno at wagner.name
Tue Dec 18 09:57:44 CET 2012


On Tue, Dec 18, 2012 at 09:12:01AM +0100, Marc Ballarin wrote:
> Am 18.12.2012 01:36, schrieb Arno Wagner:
> >On Mon, Dec 17, 2012 at 04:10:50PM -0800, David Li wrote:
> >>Hi, I wonder if the dm-crypt partition UUID (shown in blkid -p <dev>) can
> >>be used to uniquely associate it with the set of keys the partition will
> >>need. Are there any cases that the UUID would change during the partition's
> >>lifetime?
> >
> >The UUID is actually a filesystem attribute, not a partition
> >attribute...
> 
> This depends on the partition format in use. For example GPT, and
> maybe others, provide an additional UUID for partititons (actually
> GPT even supports Labels):
> 
> $ sudo blkid -p /dev/sda1
> /dev/sda1: LABEL="data_usb"
> UUID="9b70c4bf-6b40-4be3-9cb7-030db682ad35" VERSION="1.0"
> TYPE="ext4" USAGE="filesystem" PART_ENTRY_SCHEME="gpt"
> PART_ENTRY_UUID="3d18a590-d329-4a82-be02-c3588098d625"
> PART_ENTRY_TYPE="0fc63daf-8483-4772-8e79-3d69d8477de4"
> PART_ENTRY_NUMBER="1" PART_ENTRY_OFFSET="2048"
> PART_ENTRY_SIZE="3907027087" PART_ENTRY_DISK="8:0"
> 
> Whereas dos/mbr does not:
> 
> $sudo blkid -p /dev/sda1
> /dev/sda1: UUID="b786a3a4-26e7-4765-aed1-9bc472522c06" VERSION="1.0"
> TYPE="ext4" USAGE="filesystem" PART_ENTRY_SCHEME="dos"
> PART_ENTRY_TYPE="0x83" PART_ENTRY_FLAGS="0x80" PART_ENTRY_NUMBER="1"
> PART_ENTRY_OFFSET="2048" PART_ENTRY_SIZE="63997952"
> PART_ENTRY_DISK="8:0"
> 
> While the GPT UUID should never change, it might happen if some
> bogus resizing tool is used.
> 
> So, if a LUKS-UUID is available I would always prefer it and only
> fall back to partition UUIDs when not using LUKS.
> 
> Regards,
> Marc

Interesting. Seems (again), my view of things is too simplistic ;-)

Anyways, the gist is that change of UUIDs is not something that 
should ordionarily happen during the lifetime of whatever
carries it.

Arno
-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
One of the painful things about our time is that those who feel certainty
are stupid, and those with any imagination and understanding are filled
with doubt and indecision. -- Bertrand Russell


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