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Merge tag 'v6.5/vfs.mount' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs
Pull vfs mount updates from Christian Brauner:
"This contains the work to extend move_mount() to allow adding a mount
beneath the topmost mount of a mount stack.
There are two LWN articles about this. One covers the original patch
series in [1]. The other in [2] summarizes the session and roughly the
discussion between Al and me at LSFMM. The second article also goes
into some good questions from attendees.
Since all details are found in the relevant commit with a technical
dive into semantics and locking at the end I'm only adding the
motivation and core functionality for this from commit message and
leave out the invasive details. The code is also heavily commented and
annotated as well which was explicitly requested.
TL;DR:
> mount -t ext4 /dev/sda /mnt
|
└─/mnt /dev/sda ext4
> mount --beneath -t xfs /dev/sdb /mnt
|
└─/mnt /dev/sdb xfs
└─/mnt /dev/sda ext4
> umount /mnt
|
└─/mnt /dev/sdb xfs
The longer motivation is that various distributions are adding or are
in the process of adding support for system extensions and in the
future configuration extensions through various tools. A more detailed
explanation on system and configuration extensions can be found on the
manpage which is listed below at [3].
System extension images may – dynamically at runtime — extend the
/usr/ and /opt/ directory hierarchies with additional files. This is
particularly useful on immutable system images where a /usr/ and/or
/opt/ hierarchy residing on a read-only file system shall be extended
temporarily at runtime without making any persistent modifications.
When one or more system extension images are activated, their /usr/
and /opt/ hierarchies are combined via overlayfs with the same
hierarchies of the host OS, and the host /usr/ and /opt/ overmounted
with it ("merging"). When they are deactivated, the mount point is
disassembled — again revealing the unmodified original host version of
the hierarchy ("unmerging"). Merging thus makes the extension's
resources suddenly appear below the /usr/ and /opt/ hierarchies as if
they were included in the base OS image itself. Unmerging makes them
disappear again, leaving in place only the files that were shipped
with the base OS image itself.
System configuration images are similar but operate on directories
containing system or service configuration.
On nearly all modern distributions mount propagation plays a crucial
role and the rootfs of the OS is a shared mount in a peer group
(usually with peer group id 1):
TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID
/ / ext4 shared:1 29 1
On such systems all services and containers run in a separate mount
namespace and are pivot_root()ed into their rootfs. A separate mount
namespace is almost always used as it is the minimal isolation
mechanism services have. But usually they are even much more isolated
up to the point where they almost become indistinguishable from
containers.
Mount propagation again plays a crucial role here. The rootfs of all
these services is a slave mount to the peer group of the host rootfs.
This is done so the service will receive mount propagation events from
the host when certain files or directories are updated.
In addition, the rootfs of each service, container, and sandbox is
also a shared mount in its separate peer group:
TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID
/ / ext4 shared:24 master:1 71 47
For people not too familiar with mount propagation, the master:1 means
that this is a slave mount to peer group 1. Which as one can see is
the host rootfs as indicated by shared:1 above. The shared:24
indicates that the service rootfs is a shared mount in a separate peer
group with peer group id 24.
A service may run other services. Such nested services will also have
a rootfs mount that is a slave to the peer group of the outer service
rootfs mount.
For containers things are just slighly different. A container's rootfs
isn't a slave to the service's or host rootfs' peer group. The rootfs
mount of a container is simply a shared mount in its own peer group:
TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID
/home/ubuntu/debian-tree / ext4 shared:99 61 60
So whereas services are isolated OS components a container is treated
like a separate world and mount propagation into it is restricted to a
single well known mount that is a slave to the peer group of the
shared mount /run on the host:
TARGET SOURCE FSTYPE PROPAGATION MNT_ID PARENT_ID
/propagate/debian-tree /run/host/incoming tmpfs master:5 71 68
Here, the master:5 indicates that this mount is a slave to the peer
group with peer group id 5. This allows to propagate mounts into the
container and served as a workaround for not being able to insert
mounts into mount namespaces directly. But the new mount api does
support inserting mounts directly. For the interested reader the
blogpost in [4] might be worth reading where I explain the old and the
new approach to inserting mounts into mount namespaces.
Containers of course, can themselves be run as services. They often
run full systems themselves which means they again run services and
containers with the exact same propagation settings explained above.
The whole system is designed so that it can be easily updated,
including all services in various fine-grained ways without having to
enter every single service's mount namespace which would be
prohibitively expensive. The mount propagation layout has been
carefully chosen so it is possible to propagate updates for system
extensions and configurations from the host into all services.
The simplest model to update the whole system is to mount on top of
/usr, /opt, or /etc on the host. The new mount on /usr, /opt, or /etc
will then propagate into every service. This works cleanly the first
time. However, when the system is updated multiple times it becomes
necessary to unmount the first update on /opt, /usr, /etc and then
propagate the new update. But this means, there's an interval where
the old base system is accessible. This has to be avoided to protect
against downgrade attacks.
The vfs already exposes a mechanism to userspace whereby mounts can be
mounted beneath an existing mount. Such mounts are internally referred
to as "tucked". The patch series exposes the ability to mount beneath
a top mount through the new MOVE_MOUNT_BENEATH flag for the
move_mount() system call. This allows userspace to seamlessly upgrade
mounts. After this series the only thing that will have changed is
that mounting beneath an existing mount can be done explicitly instead
of just implicitly.
The crux is that the proposed mechanism already exists and that it is
so powerful as to cover cases where mounts are supposed to be updated
with new versions. Crucially, it offers an important flexibility.
Namely that updates to a system may either be forced or can be delayed
and the umount of the top mount be left to a service if it is a
cooperative one"
Link: https://lwn.net/Articles/927491 [1]
Link: https://lwn.net/Articles/934094 [2]
Link: https://man7.org/linux/man-pages/man8/systemd-sysext.8.html [3]
Link: https://brauner.io/2023/02/28/mounting-into-mount-namespaces.html [4]
Link: https://github.com/flatcar/sysext-bakery
Link: https://fedoraproject.org/wiki/Changes/Unified_Kernel_Support_Phase_1
Link: https://fedoraproject.org/wiki/Changes/Unified_Kernel_Support_Phase_2
Link: https://github.com/systemd/systemd/pull/26013
* tag 'v6.5/vfs.mount' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
fs: allow to mount beneath top mount
fs: use a for loop when locking a mount
fs: properly document __lookup_mnt()
fs: add path_mounted()
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| char_dev.c | ||
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| dax.c | ||
| dcache.c | ||
| direct-io.c | ||
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| file_table.c | ||
| file.c | ||
| filesystems.c | ||
| fs_context.c | ||
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| fs_pin.c | ||
| fs_struct.c | ||
| fs_types.c | ||
| fs-writeback.c | ||
| fsopen.c | ||
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| internal.h | ||
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| Kconfig | ||
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| kernel_read_file.c | ||
| libfs.c | ||
| locks.c | ||
| Makefile | ||
| mbcache.c | ||
| mnt_idmapping.c | ||
| mount.h | ||
| mpage.c | ||
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| no-block.c | ||
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| open.c | ||
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