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

[Anthony Liguori]

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<address@hidden>
> > > > >
> > > > >
> > > > >            
> > > > 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