linux-kernel - Re: [RFC PATCH 0/2] Loop device psuedo filesystem

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Date:	Mon, 15 Sep 2014 19:20:52 -0400
From:	Shea Levy <shea@...alevy.com>
To:	linux-kernel@...r.kernel.org, lxc-devel@...ts.linuxcontainers.org
Subject: Re: [RFC PATCH 0/2] Loop device psuedo filesystem

Hi Seth,

I applied your patches to 3.15-rc7, and had the same problem, the build
fails with:

> make[3]: *** No rule to make target `fs/loopfs/loopfs.c', needed by `fs/loopfs/loopfs.o'.  Stop.

I've attached the config I used, I generated it via make allnoconfig and
then enabled MODULES, BLK, BLK_DEV, and BLK_DEV_LOOP=m in make nconfig.

The build succeeds if I set BLK_DEV_LOOP=y, so I'll go ahead with my
testing using that.

~Shea

On Mon, Sep 15, 2014 at 03:55:32PM -0500, Seth Forshee wrote:
> On Mon, Sep 15, 2014 at 04:38:44PM -0400, Shea Levy wrote:
> > Hi,
> > 
> > I wanted to test these patches (to support creating and filling a disk
> > image containing a btrfs filesystem and several subvolumes as an
> > unprivileged user), but the build fails due to what looks like a missing
> > loopfs.c in fs/loopfs (or alternatively an erroneous line in
> > fs/loopfs/Makefile). I built based off of 3.17-rc5.
> 
> There's no loopfs.c, loopfs.o gets built from inode.o which is in turn
> built from inode.c. I'm pretty sure the patches built when I posted
> them, which seems to be 3.15-rc7 based on the branch I've got here.
> 
> Seth
> 
> > 
> > ~Shea
> > 
> > 
> > On Tue, May 27, 2014 at 11:58:54PM +0200, Seth Forshee wrote:
> > > I'm posting these patches in response to the ongoing discussion of loop
> > > devices in containers at [1].
> > > 
> > > The patches implement a psuedo filesystem for loop devices, which will
> > > allow use of loop devices in containters using standard utilities. Under
> > > normal use a loopfs mount will initially contain a single device node
> > > for loop-control which can be used to request and release loop devices.
> > > Any devices allocated via this node will automatically appear in that
> > > loopfs mount (and in devtmpfs) but not in any other loopfs mounts.
> > > CAP_SYS_ADMIN in the userns of the process which performed the mount is
> > > allowed to perform privileged loop ioctls on these devices.
> > > 
> > > Alternately loopfs can be mounted with the hostmount option, intended
> > > for mounting /dev/loop in the host. This is the default mount for any
> > > devices not created via loop-control in a loopfs mount (e.g. devices
> > > created during driver init, devices created via /dev/loop-control, etc).
> > > This is only available to system-wide CAP_SYS_ADMIN.
> > > 
> > > I still have some testing to do on these patches, but they work at
> > > minimum for simple use cases. It's possible to use an unmodified losetup
> > > if it's new enough to know about loop-control, with a couple of caveats:
> > > 
> > >  * /dev/loop-control must be symlinked to /dev/loop/loop-control
> > >  * In some cases losetup attempts to use /dev/loopN when the device node
> > >    is at /dev/loop/N. For example, 'losetup -f disk.img' fails.
> > > 
> > > Device nodes for loop partitions are not created in loopfs. These
> > > devices are created by the generic block layer, and the loop driver has
> > > no way of knowing when they are created, so some kind of hook into the
> > > driver will be needed to support this.
> > > 
> > > Thanks,
> > > Seth
> > > 
> > > [1] http://article.gmane.org/gmane.linux.kernel/1703988
> > > 
> > > Seth Forshee (2):
> > >   loop: Add loop filesystem
> > >   loop: Permit priveleged operations within user namespaces
> > > 
> > >  drivers/block/loop.c       | 137 +++++++++++++----
> > >  drivers/block/loop.h       |   2 +
> > >  fs/Makefile                |   1 +
> > >  fs/loopfs/Makefile         |   6 +
> > >  fs/loopfs/inode.c          | 360 +++++++++++++++++++++++++++++++++++++++++++++
> > >  include/linux/loopfs.h     |  53 +++++++
> > >  include/uapi/linux/magic.h |   1 +
> > >  7 files changed, 535 insertions(+), 25 deletions(-)
> > >  create mode 100644 fs/loopfs/Makefile
> > >  create mode 100644 fs/loopfs/inode.c
> > >  create mode 100644 include/linux/loopfs.h
> > > 
> > > --
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> > > 
> > > 

View attachment "config" of type "text/plain" (22187 bytes)