On Tue, Mar 29, 2022 at 12:35:07PM +0200, Christian Brauner wrote:
> From: "Christian Brauner (Microsoft)" <brauner@xxxxxxxxxx>
>
> Hey,
>
> This adds support for mounting overlay on top of idmapped layers.
>
> I have to start by saying a massive thank you to Amir! He did not just
> answer my constant overlay questions but also provided quite a few
> patches himself in this series in addition to reviews, comments and a
> lot of suggestions. Thank you!
>
> There have been a lot of requests to unblock this. For just a few select
> examples see [3], [4], and [5]. I've worked closely with various
> communities among them containerd, Kubernetes, Podman, LXD, runC, crun,
> and systemd (For the curious please see the various pull-request and
> issues below.) a lot of them already support idmapped mounts since they
> are enabled for btrfs, ext4, and xfs (and f2fs and fat fwiw). In
> additon, a few colleagues at Microsoft and from Red Hat work on a
> Kubernetes Enhancement Proposals (KEP) that also relies on overlayfs
> supporting idmapped layers, see [12].
>
> Overlayfs on top of idmapped layers will be used in various ways:
>
> * Container managers use overlayfs to efficiently share layers between
> containers. However, this only works for privileged containers.
> Layers cannot be shared if both privileged and unprivileged containers
> are used. Layers can also not be shared if non-overlapping idmappings
> are used for unprivileged containers. Layers cannot be shared because
> of the conflicting ownership requirements between the containers.
Hi Christian,
Thank you for this work. This is awesome.
Wanted to test it. I was wondering how to test it. Some simple
instructions will help.
I am assuming I will need to be priviliged to be able to setup idmapped
mounts. And then overlay can be mounted on top of these layers (either
as a priviliged operation outside container or unprivileged overlayfs
inside container). Sorry, just trying to wrap my head around which
operations will require me to be priviliged (root in init_user_ns)
and which operations I can do unprivileged (as normal user).
Thanks
Vivek
>
> Both the KEP proposal (see [13]) and LXD (see [14]) use
> non-overlapping idmappings to increase isolation.
>
> All of these cases can be supported if overlayfs can be mounted on
> idmapped layers. The container runtime will create idmapped mounts for
> the layers supposed to be shared. Each container will be given
> idmapped mounts with the correct idmapping. Either by attaching a
> custom or its own user namespace depending on the use-case. Then an
> overlay mount can be mounted on top of the idmapped layers. The
> underlying idmapped mounts can then be unmounted and the mount table
> will be in a clean state.
> This approach has been tested an verified by Giuseppe and others.
>
> * Because of the inability to share layers preparing a layer causes a
> big runtime overhead as ownership needs to be recursively changed.
> This becomes increasingly costly with the bigger the layers are.
> Especially for a large rootfs it becomes prohibitively expensive.
>
> This recursive ownership change also causes a lot storage overhead.
> Without metacopy it can quickly become unmanagable. With metacopy it
> still wastes a lot of space and inodes.
>
> For both the KEP and container manager being able to use idmapped
> layer with overlayfs on top of it solves all these problems.
>
> * Container managers such as LXD do run full system containers. Such
> systems are managed like virtual machines and users run application
> containers inside. Since LXD uses an idmapped mounts for the rootfs of
> the container with the container's user namespace attached to the
> mount container runtimes cannot user overlayfs inside.
>
> Once overlayfs can be mounted on top of idmapped layers it will be
> possible for container runtimes to use overlayfs inside LXD
> containers.
>
> * The systemd-homed daemon and toolsuite is a new component of systemd
> to manage home areas in a portable way. systemd-homed makes use of
> idmapped mounts for the home areas. If the kernel and used file system
> support it the user's home area will be mounted as an idmapped mount
> when they log into their system.
>
> All files are internally owned by the "nobody" user (i.e. the user
> typically used for indicating "this ownership is not mapped"), and
> dynamically mapped to the {g,u}id used locally on the system via
> idmapped mounts. This makes migrating home areas between different
> systems trivial because recursively chown()ing file system trees is no
> longer necessary. This also means that it is impossible to store files
> as {g,u}id 0 on disk.
>
> Once overlayfs can be mounted on top of idmapped layers it will be
> possible for container runtimes to work better together with
> systemd-homed.
>
> * systemd provides various sandboxing features (see [11]) for services.
> These serve as hardening measures. In this context, idmapped mounts
> can be used to prevent services from creating files on disk as {g,u}id
> 0, making specific files inaccessible, or to prevent access to whole
> directories. Since such services may also make use of overlayfs for
> e.g. the ExtensionImages= option supporting overlayfs on top of
> idmapped layers would be another huge hardening win.
>
> * systemd provides the ability to use system extension images [15] for
> /usr and /opt (with a look to /etc in the future). Such system
> extension images contain files and directories similar in fashion to
> a regular OS system tree. When one or more system extension images are
> activated, their /usr/ and /opt/ hierarchies are combined via
> overlay with the same hierarchies of the host OS, and the host /usr/
> and /opt/ overmounted with it ("merging"). These images are read-only.
>
> This feature is available to unprivileged container and sandboxed
> services as well. Idmapped layers are used here to avoid runtime and
> storage overhead from recursively changing ownership and ultimately an
> overlay mount is supposed to be created on top of it.
>
> Giuseppe provided testing for runC/crun/Podman and he put up a pull
> request to support overlayfs on top of idmapped layers at [16]. That
> already covers a lot of users. Other tools have put up pull requests as
> well and they are linked below.
>
> The patchset has been extensively tested for about 2 weeks with
> xfstests. The tests and results are explained in the following
> paragraphs.
>
> In order to test overlayfs with idmapped mounts a simple patch to
> xfstests has been added which is part of this series. The patchset
> simply allows for each test in the xfstests suite to be run on top of
> idmapped mounts. That is in addition to the generic idmapped mount tests
> that have existed and are already run for a long time.
>
> Since idmapped mounts can be created on top of btrfs, ext4, and xfs and
> these are the most relevant filesystems for users they were taken into
> the test matrix.
>
> Amir ensured that the test matrix also includes metacopy. So all tests
> are run once with metacopy=on and once with metacopy=off.
>
> Additionally, the unionmount tesuite that Amir maintains was run as part
> of xfstests. This brings testing for multi layer overlay and a few
> rarely used overlayfs use-cases.
>
> And last, xfstests were run with and without idmapped mounts.
>
> Since the patch series is based on Linux 5.17 the 5.17 kernel was chosen
> as a baseline kernel. The baseline kernel is pure 5.17 upstream without
> any of the patches in this series. The baseline kernel was used to run
> all xfstests with the same parameters minus the idmapped mount part of
> the test matrix. This ensured that regressions would be immediately
> noticeable.
>
> So the full test matrix is:
>
> ext4 x metacopy=off x -idmapped mounts
> ext4 x metacopy=on x -idmapped mounts
> ext4 x metacopy=off x +idmapped mounts
> ext4 x metacopy=on x +idmapped mounts
>
> xfs x metacopy=off x -idmapped mounts
> xfs x metacopy=on x -idmapped mounts
> xfs x metacopy=off x +idmapped mounts
> xfs x metacopy=on x +idmapped mounts
>
> btrfs x metacopy=off x -idmapped mounts
> btrfs x metacopy=on x -idmapped mounts
> btrfs x metacopy=off x +idmapped mounts
> btrfs x metacopy=on x +idmapped mounts
>
> The test runs were started with:
>
> sudo ./check -overlay
>
> so they encompass all xfstests apart from those that needed to be
> excluded because they triggered known issues (One such example is
> generic/530 which causes an xfs corruption that is currently under
> discussion for a fix upstream.).
>
> A single testrun with over 750 tests takes a bonkers long time given
> that I run them in a beefy VM on top of an ssd and not on bare metal.
>
> The successes and failures are identical for the whole test matrix with
> and without idmapped mounts. The tests and failures are also identical
> compared to the baseline kernel. All in all this should provide a good
> amount of convidence. The full test output can be found in the following
> repo: https://gitlab.com/brauner/fs.idmapped.overlay.xfstests.output
>
> In order to create an idmapped mount the mount_setattr() system call
> (see [17]) can be used together with the MOUNT_ATTR_IDMAP flag to attach
> a userns fd to a detached mount created with open_tree()'s
> OPEN_TREE_CLONE flag. The only effect is that the idmapping of the
> userns becomes the idmapping of the relevant. The links below should
> serve to illustrate how widely they are already used.
>
> For people who would like to test I've created a tag fs.idmapped.overlay.v1
> which can be fetched:
>
> git fetch git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git fs.idmapped.overlay.v1
>
> the same goes for xfstests:
>
> git fetch git://git.kernel.org/pub/scm/linux/kernel/git/brauner/xfstests-dev.git fs.idmapped.overlay.v1
>
> Thanks!
> Christian
>
> [1]: OCI runtime-spec extension for idmapped mounts
> https://github.com/opencontainers/runtime-spec/pull/1143
>
> [2]: Support for idmapped mounts for shared volumes
> https://github.com/opencontainers/runc/pull/3429
>
> [3]: containerd support for idmapped mounts with focus on overlayfs
> https://github.com/containerd/containerd/pull/5890
>
> [4]: runC support for idmapped mounts with focus on overlayfs
> https://github.com/opencontainers/runc/issues/2821
>
> [5]: Podman support for idmapped mounts with focus on overlayfs
> https://github.com/containers/podman/issues/10374
>
> [6]: systemd-nspawn support for idmapped mounts
> https://github.com/systemd/systemd/pull/19438
>
> [7]: systemd-homed support for idmapped mounts
> https://github.com/systemd/systemd/pull/21136
>
> [8]: LXD support for idmapped mounts
> https://github.com/lxc/lxd/pull/8778
>
> [9]: LXC support for idmapped mounts
> https://github.com/lxc/lxc/pull/3709
>
> [10]: crun support for idmapped mounts
> https://github.com/containers/crun/pull/874
>
> [11]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Sandboxing
>
> [12]: https://github.com/kubernetes/enhancements/pull/3065
>
> [13]: https://github.com/kubernetes/enhancements/pull/3065/files#diff-a9ca9fbce1538447b92f03125ca2b8474e2d875071f1172d2afa0b1e8cadeabaR118
>
> [14]: https://github.com/lxc/lxd/blob/master/doc/instances.md
> (The relevant entry can be found by looking for the
> "security.idmap.isolated" key.)
>
> [15]: https://www.freedesktop.org/software/systemd/man/systemd-sysext.html
>
> [16]: https://github.com/containers/storage/pull/1180
>
> [17]: https://man7.org/linux/man-pages/man2/mount_setattr.2.html
>
> Amir Goldstein (3):
> ovl: use wrappers to all vfs_*xattr() calls
> ovl: pass layer mnt to ovl_open_realfile()
> ovl: store lower path in ovl_inode
>
> Christian Brauner (15):
> fs: add two trivial lookup helpers
> exportfs: support idmapped mounts
> ovl: pass ofs to creation operations
> ovl: handle idmappings in creation operations
> ovl: pass ofs to setattr operations
> ovl: use ovl_do_notify_change() wrapper
> ovl: use ovl_lookup_upper() wrapper
> ovl: use ovl_path_getxattr() wrapper
> ovl: handle idmappings for layer fileattrs
> ovl: handle idmappings for layer lookup
> ovl: use ovl_copy_{real,upper}attr() wrappers
> ovl: handle idmappings in ovl_permission()
> ovl: handle idmappings in layer open helpers
> ovl: handle idmappings in ovl_xattr_{g,s}et()
> ovl: support idmapped layers
>
> fs/exportfs/expfs.c | 5 +-
> fs/namei.c | 52 +++++++--
> fs/overlayfs/copy_up.c | 97 +++++++++-------
> fs/overlayfs/dir.c | 133 +++++++++++-----------
> fs/overlayfs/export.c | 3 +-
> fs/overlayfs/file.c | 44 ++++----
> fs/overlayfs/inode.c | 63 +++++++----
> fs/overlayfs/namei.c | 49 ++++++---
> fs/overlayfs/overlayfs.h | 231 ++++++++++++++++++++++++++++-----------
> fs/overlayfs/ovl_entry.h | 7 +-
> fs/overlayfs/readdir.c | 48 ++++----
> fs/overlayfs/super.c | 57 +++++-----
> fs/overlayfs/util.c | 142 +++++++++++++++++++-----
> include/linux/namei.h | 2 +
> 14 files changed, 607 insertions(+), 326 deletions(-)
>
>
> base-commit: f443e374ae131c168a065ea1748feac6b2e76613
> --
> 2.32.0
>
