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

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

[.. ink ..]

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tested with cryptsetup version 1.6.0

http://pastebin.com/G1jtyCZN

link above has source code for the "tt" program below

[root@mtz ~]# cryptsetup create rrr /home/ink/rrr.img
Enter passphrase:
[root@mtz ~]#
[root@mtz ~]# /home/ink/tt /dev/mapper/rrr
UUID=PLAIN-rrr
[root@mtz ~]# /home/ink/tt /dev/mapper/zuluCrypt-500-NAAN-truecrypt-2273
crypt_get_uuid() failed: No such file or directory


The source code and the test show crypt_get_uuid does not work as expected
with plain volumes but does with truecrypt volumes and luks volumes. The
expected behavior is to return NULL on volumes that do not support UUID.

with plain volumes,instead of "NULL",crypt_get_uuid returns "PLAIN-XYZ"
where "XYZ" is the last component of the mapper path.

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

[Milan Broz]

On 01/18/2013 07:29 PM, .. ink .. wrote:

> The source code and the test show crypt_get_uuid does not work as
> expected with plain volumes but does with truecrypt volumes and luks
> volumes. The expected behavior is to return NULL on volumes that do
> not support UUID.
> 
> with plain volumes,instead of "NULL",crypt_get_uuid returns
> "PLAIN-XYZ" where "XYZ" is the last component of the mapper path.

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).

Well, I'll check what I can do here now and not break existing logic.

Thanks,
Milan

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

[.. ink ..]

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> 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?

[ink@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

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

[Milan Broz]

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@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

 CRYPT-TCRYPT-zuluCrypt-500-NAAN-truecrypt-2273
   |      |   ---------------------------------
   |      |          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:
 CRYPT-LUKS1-f57ccd0520f943b9973aed73c6b67f07-zuluCrypt-500-NAAN-sdc6-781
   |      |        on-disk UUID                  activated device name
   |       - CRYPT device context type (LUKS1/PLAIN/etc)
    - device managed by cryptsetup

Milan