[dm-crypt] renaming of device

Arno Wagner arno at wagner.name
Thu Jan 14 23:46:27 CET 2010


On Thu, Jan 14, 2010 at 11:16:29PM +0100, Sven Eschenberg wrote:
> Hi Arno,
>
> What I was trying to say, I think the current behavior usually should do  
> a good job, when there is only one device inode (as expected).

Ok. 

> Concerning udev - you could either modify the configuration for the scsi  
> subsystem, or you could copy it to you own rule file modify it and  
> override debian's defaults.
>
> This ways you are a little safer, when the distribution modifies some  
> rules (for some odd reason) and are sure that dmcrypt returns the name  
> you expect. In general this is true for most device names where you have  
> special needs.
>
> On my debian there are no rules for creating /dev/scsi/* names, at least  
> not for scsi disks. Might be you are stuck with an older udev or some  
> outdated configuration maybe?

I don't know and do not really have time to investigate now.
Come to think of it, it could also have some connection to a
recent kernel upgrade, I am running 2.6.32.2 from kernel.org.
I did not notice the issue before, so maybe the kernel now also
creates device files and there was something about a device
pseudo filesystem or the like in the patchnotes.

Anyways, I will just look up major and minor numbers manually 
until cryptsetup either reports all matching in /dev/ or has
a preferred device for a major/minor combination.


> Regards
>
> -Sven
>
>
> Arno Wagner schrieb:
>> 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. 
>>
>> Arno
>>
>>
>>
>>
>>> 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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>
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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
----
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