[dm-crypt] UUID question

Marc Ballarin Ballarin.Marc at gmx.de
Tue Dec 18 09:12:01 CET 2012


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



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