Re: [Qemu-devel] [PATCH] qemu-img: add FUSE-based image access

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/25/2010 05:27 PM, Jan Kiszka wrote:
> Anthony Liguori wrote:
>    
>> On 03/25/2010 04:46 PM, Jan Kiszka wrote:
>>      
>>> Anthony Liguori wrote:
>>>
>>>        
>>>> On 03/25/2010 12:52 PM, Jan Kiszka wrote:
>>>>
>>>>          
>>>>> This adds the "map" subcommand to qemu-img. It is able to expose the
>>>>> raw
>>>>> content of a disk image via a FUSE filesystem. Both the whole disk can
>>>>> be accessed, e.g. to run partitioning tools against it, as well as
>>>>> individual partitions. This allows to create new filesystems in the
>>>>> image or loop-back mount exiting ones. Using the great mountlo tool
>>>>> from the FUSE collection [1][2], the latter can even be done by
>>>>> non-root
>>>>> users (the former anyway).
>>>>>
>>>>> There are some dependency to fulfill to gain all features: Partition
>>>>> scanning is done via recent libblkid (I used version 2.17.1). If this
>>>>> library is not available, only the disk file is provide. Fortunately,
>>>>> mountlo can do partition scanning as well ("-p n") to work around this.
>>>>>
>>>>> Moreover, libfuse>= 2.8 and a host kernel>= 2.6.29 is required for
>>>>> seamless disk access via fdisk. Otherwise, the BLKGETSIZE64 IOCTL
>>>>> cannot
>>>>> be provided, and the number of cylinders has to set explicitly (e.g.
>>>>> via
>>>>> "-C n").
>>>>>
>>>>> This work was inspired by Ashley Saulsbury's qemu-diskp [3].
>>>>>
>>>>> [1]
>>>>> http://sourceforge.net/apps/mediawiki/fuse/index.php?title=FileSystems#Mountlo
>>>>>
>>>>>
>>>>> [2] http://sourceforge.net/projects/fuse/files/mountlo/
>>>>> [3] http://www.saulsbury.org/software/virtualization.html
>>>>>
>>>>> Signed-off-by: Jan Kiszka<jan.kiszka@web.de>
>>>>>
>>>>>
>>>>>            
>>>> This has been proposed quite a few times.
>>>>
>>>> In fact, I wrote something like this prior to implementing qemu-nbd.
>>>>
>>>> The problem with fuse is that as default configured, you can't actually
>>>> enter into a fuse filesystem as root and since you need to be root to
>>>> loopback mount it, it pretty nasty from a usability perspective.
>>>>
>>>>          
>>> You don't, see mountlo.
>>>
>>>        
>> That definitely changes things.  I assume it just uses libe2fs et al to
>> display filesystem contents?
>>      
> Nope. It's a bit like libguestfs as it uses Linux to access the
> filesystems, but that Linux runs in UML mode, thus does not require any
> qemu/kvm underneath. It simply maps the FUSE requests on corresponding
> VFS services in the UML kernel.
>
>    
>> Does it preserve ownership?
>>      
> Yep.
>
>    
>> You still can't do things as root I take it which is problematic.
>>      
> At least my default config does not prevent running qemu-img map as root
> and then performing a classic "mount -o loop" on the partitions it
> provides. Or what do you mean?
>    

You need user_allow_other set in /etc/fuse.conf which isn't set by default.

Regards,

Anthony Liguori