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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/