[dm-crypt] renaming of device

Arno Wagner arno at wagner.name
Thu Jan 14 22:42:57 CET 2010

Hi Sven,

On Thu, Jan 14, 2010 at 07:18:50PM +0100, Sven Eschenberg wrote:
> Hi Arno,
> Agreed.
> Concerning the actual problem: This should not go into cryptsetup, it's  
> not the right place. 

Well, it is actually already in there, just at the moment it 
gives you the first it finds. I think it is acceptable to have
it in there, as cryptsetup already is a frontend. Writing a 
frontend for a frontend is a bit redundant.

> If we talk convetions, then the convention is:  
> There is only ONE inode representing the device under /dev. If your  
> hotplug manager (and it's configuration) creates multiple such entries,  
> it is an indication for misconfiguration (imho). I.E.: Each device  
> should have one physical identification (the inode carrying major:minor,  
> together with the pathname) and as many logical names (via softlinks) as  
> wanted. This guarantees that the right pathname can be found (not only  
> for dmcrypt btw) and the device is yet useable via it's symbolic names  
> (i.e. UUIDs, volume labels etc.).
> If the preference of the system administrator is on UUID names, then  
> those should be created as device files, and the 'traditional names' as  
> softlinks. If yet there is a need to know all device names that link to  
> a certain major:minor, then this can and should be done in a simple  
> shell script, where it belongs in my opinion.
> Just my 2 cents

I agree to that. It came when I added udev to my system, which
I do not like, and it creating devices under /dev/scsi/ is the
default under Debian. Clearly this has not been thought through.
As such broken configs can happen and it seems easily, it seems
they need to be expected. 


> Regards
> -Sven
> Arno Wagner schrieb:
>> Well, I think listing all in /dev/ would be an improvement to
>> the current behaviour. Of course people can put their device
>> spceials somewhere else, but that is a pretty big break of
>> convention that IMO does not need to be supported.
>> Arno
>> On Wed, Jan 13, 2010 at 05:18:27PM +0100, Sven Eschenberg wrote:
>>> Hi Arno,
>>> Indeed cryyptsetup traverses /dev stating each file for major:minor 
>>> and  stops on the first match - at least a trace suggests this. The 
>>> question  is though, if this is an appropriated approach. One thing 
>>> that annoys me   when doing this: There's no guarantee that device 
>>> inodes are in /dev,  nor links to it. In theory it cann be any place 
>>> in the filesystem. there  can be as many links as you want and as 
>>> many inodes for the same device,  as you want.
>>> Regards
>>> -Sven
>>> Arno Wagner schrieb:
>>>> Having had a bit of time to think about it, it seems that
>>>> it would indeed be necessary to store the device name.
>>>> A possible alternative is to traverse all of /dev/ and
>>>> report all devices that match major/minor. This may be
>>>> the solution causing the least effort, as cryptsetup
>>>> is doing something like it already, just that it currently
>>>> stops after the first match.
>>>> In fact, I think I prefer for it to report all devices
>>>> that match. 
>>>> Arno
>>>> On Wed, Jan 13, 2010 at 11:20:36AM +0100, Sven Eschenberg wrote:
>>>>> Hi Luca,
>>>>> I doubt this is possible. The only possible thing would be: Modify DM,
>>>>> when a device name is passed in, to keep the passed in name alongside
>>>>> the actual major:minor and provide an IOCTL, to access the information.
>>>>> Regards
>>>>> -Sven
>>>>> On Wed, 2010-01-13 at 08:01 +0100, Luca Berra wrote:
>>>>>> On Sun, Jan 10, 2010 at 09:33:13PM +0100, Milan Broz wrote:
>>>>>>> The algorithm is very simple (and was probably written before
>>>>>>> udev was used so these special links in /dev did not exist).
>>>>>>> So it need to add some preferred names and not print the first entry.
>>>>>>> Please can you add an issue to project pages to not forget about this?
>>>>>>> Probably good idea to fix it in next minor release.
>>>>>> it could be useful to have the functionality in device-mapper itself,
>>>>>> instead of duplicating in dm-crypt, lvm, dmraid, whatever ?
>>>>>> Regards,
>>>>>> L.
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Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

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