Hey everyone, This is v3 of loopfs. The branch is available in three locations: https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=loopfs https://gitlab.com/brauner/linux/-/commits/loopfs https://github.com/brauner/linux/tree/loopfs I've added a few more people to the Cc that want to make use of this and I've added the missing ucount part that David pointed out and expanded a little more on how this is used so this is used. I also removed the "max" mount option which people argued wasn't very useful, bringing down the loc count quite a bit actually. This implements loopfs, a loop device filesystem. It takes inspiration from the binderfs filesystem I implemented about two years ago and with which we had overall good experiences so far. Parts of it are also based on [3] but it's mostly a new, imho cleaner approach. Loopfs allows to create private loop devices instances to applications for various use-cases. It covers the use-case that was expressed on-list and in-person to get programmatic access to private loop devices for image building in sandboxes. An illustration for this is provided in [4]. Also loopfs is intended to provide loop devices to privileged and unprivileged containers which has been a frequent request from various major tools (Chromium, Kubernetes, LXD, Moby/Docker, systemd). I'm providing a non-exhaustive list of issues and requests (cf. [5]) around this feature mainly to illustrate that I'm not making the use-cases up. Currently none of this can be done safely since handing a loop device from the host into a container means that the container can see anything that the host is doing with that loop device and what other containers are doing with that device too. And (bind-)mounting devtmpfs inside of containers is not secure at all so also not an option (though sometimes done out of despair apparently). The workloads people run in containers are supposed to be indiscernible from workloads run on the host and the tools inside of the container are supposed to not be required to be aware that they are running inside a container apart from containerization tools themselves. This is especially true when running older distros in containers that did exist before containers were as ubiquitous as they are today. With loopfs user can call mount -o loop and in a correctly setup container things work the same way they would on the host. The filesystem representation allows us to do this in a very simple way. At container setup, a container manager can mount a private instance of loopfs somehwere, e.g. at /dev/loopfs and then bind-mount or symlink /dev/loopfs/loop-control to /dev/loop-control, pre allocate and symlink the number of standard devices into their standard location and have a service file or rules in place that symlink additionally allocated loop devices through losetup into place as well. With the new syscall interception logic this is also possible for unprivileged containers. In these cases when a user calls mount -o loop <image> <mountpoint> it will be possible to completely setup the loop device in the container. The final mount syscall is handled through syscall interception which we already implemented and released in earlier kernels (see [1] and [2]) and is actively used in production workloads. The mount is often rewritten to a fuse binary to provide safe access for unprivileged containers. Loopfs also allows the creation of hidden/detached dynamic loop devices and associated mounts which also was a often issued request. With the old mount api this can be achieved by creating a temporary loopfs and stashing a file descriptor to the mount point and the loop-control device and immediately unmounting the loopfs instance. With the new mount api a detached mount can be created directly (i.e. a mount not visible anywhere in the filesystem). New loop devices can then be allocated and configured. They can be mounted through /proc/self/<fd>/<nr> with the old mount api or by using the fd directly with the new mount api. Combined with a mount namespace this allows for fully auto-cleaned up loop devices on program crash. This ties back to various use-cases and is illustrated in [4]. The filesystem representation requires the standard boilerplate filesystem code we know from other tiny filesystems. And all of the loopfs code is hidden under a config option that defaults to false. This specifically means, that none of the code even exists when users do not have any use-case for loopfs. In addition, the loopfs code does not alter how loop devices behave at all, i.e. there are no changes to any existing workloads and I've taken care to ifdef all loopfs specific things out. Each loopfs mount is a separate instance. As such loop devices created in one instance are independent of loop devices created in another instance. This specifically entails that loop devices are only visible in the loopfs instance they belong to. The number of loop devices available in loopfs instances are hierarchically limited through /proc/sys/user/max_loop_devices via the ucount infrastructure (Thanks to David Rheinsberg for pointing out that missing piece.). An administrator could e.g. set echo 3 > /proc/sys/user/max_loop_devices at which point any loopfs instance mounted by uid x can only create 3 loop devices no matter how many loopfs instances they mount. This limit applies hierarchically to all user namespaces. Thanks! Christian [1]: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace") [2]: fb3c5386b382 ("seccomp: add SECCOMP_USER_NOTIF_FLAG_CONTINUE") [3]: https://lore.kernel.org/lkml/1401227936-15698-1-git-send-email-seth.forshee@xxxxxxxxxxxxx [4]: https://gist.github.com/brauner/dcaf15e6977cc1bfadfb3965f126c02f [5]: https://github.com/kubernetes-sigs/kind/issues/1333 https://github.com/kubernetes-sigs/kind/issues/1248 https://lists.freedesktop.org/archives/systemd-devel/2017-August/039453.html https://chromium.googlesource.com/chromiumos/docs/+/master/containers_and_vms.md#loop-mount https://gitlab.com/gitlab-com/support-forum/issues/3732 https://github.com/moby/moby/issues/27886 https://twitter.com/_AkihiroSuda_/status/1249664478267854848 https://serverfault.com/questions/701384/loop-device-in-a-linux-container https://discuss.linuxcontainers.org/t/providing-access-to-loop-and-other-devices-in-containers/1352 https://discuss.concourse-ci.org/t/exposing-dev-loop-devices-in-privileged-mode/813 Christian Brauner (7): kobject_uevent: remove unneeded netlink_ns check loopfs: implement loopfs loop: use ns_capable for some loop operations kernfs: handle multiple namespace tags loop: preserve sysfs backwards compatibility loopfs: start attaching correct namespace during loop_add() loopfs: only show devices in their correct instance Documentation/filesystems/sysfs-tagging.txt | 1 - MAINTAINERS | 5 + block/genhd.c | 85 ++++ block/partitions/core.c | 1 + drivers/base/devtmpfs.c | 4 +- drivers/block/Kconfig | 4 + drivers/block/Makefile | 1 + drivers/block/loop.c | 205 +++++++-- drivers/block/loop.h | 7 + drivers/block/loopfs/Makefile | 3 + drivers/block/loopfs/loopfs.c | 439 ++++++++++++++++++++ drivers/block/loopfs/loopfs.h | 38 ++ fs/kernfs/dir.c | 38 +- fs/kernfs/kernfs-internal.h | 33 +- fs/kernfs/mount.c | 11 +- fs/sysfs/mount.c | 14 +- include/linux/device.h | 3 + include/linux/genhd.h | 3 + include/linux/kernfs.h | 44 +- include/linux/kobject_ns.h | 7 +- include/linux/sysfs.h | 8 +- include/linux/user_namespace.h | 3 + include/uapi/linux/magic.h | 1 + kernel/ucount.c | 3 + lib/kobject.c | 17 +- lib/kobject_uevent.c | 2 +- net/core/net-sysfs.c | 6 - 27 files changed, 891 insertions(+), 95 deletions(-) create mode 100644 drivers/block/loopfs/Makefile create mode 100644 drivers/block/loopfs/loopfs.c create mode 100644 drivers/block/loopfs/loopfs.h base-commit: ae83d0b416db002fe95601e7f97f64b59514d936 -- 2.26.2
