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doc: add introductory docs for portable services

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Lennart Poettering 2018-04-17 18:43:42 +02:00
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# Portable Services Introduction
This systemd version includes a preview of the "portable service"
concept. "Portable Services" are supposed to be an incremental improvement over
traditional system services, making two specific facets of container management
available to system services more readily. Specifically:
1. The bundling of applications, i.e. packing up multiple services, their
binaries and all their dependencies in a single image, and running them
directly from it.
2. Stricter default security policies, i.e. sand-boxing of applications.
The primary tool for interfacing with "portable services" is the new
"portablectl" program. It's currently shipped in /usr/lib/systemd/portablectl
(i.e. not in the `$PATH`), since it's not yet considered part of the officially
supported systemd interfaces — it's a preview still after all.
Portable services don't bring anything inherently new to the table. All they do
is put together known concepts in a slightly nicer way to cover a specific set
of use-cases in a nicer way.
# So, what *is* a "Portable Service"?
A portable service is ultimately just an OS tree, either inside of a directory
tree, or inside a raw disk image containing a Linux file system. This tree is
called the "image". It can be "attached" or "detached" from the system. When
"attached" specific systemd units from the image are made available on the host
system, then behaving pretty much exactly like locally installed system
services. When "detached" these units are removed again from the host, leaving
no artifacts around (except maybe messages they might have logged).
The OS tree/image can be created with any tool of your choice. For example, you
can use `dnf --installroot=` if you like, or `debootstrap`, the image format is
entirely generic, and doesn't have to carry any specific metadata beyond what
distribution images carry anyway. Or to say this differently: the image format
doesn't define any new metadata as unit files and OS tree directories or disk
images are already sufficient, and pretty universally available these days. One
particularly nice tool for creating suitable images is
[mkosi](https://github.com/systemd/mkosi), but many other existing tools will
do too.
If you so will, "Portable Services" are a nicer way to manage chroot()
environments, with better security, tooling and behavior.
# Where's the difference to a "Container"?
"Container" is a very vague term, after all it is used for
systemd-nspawn/LXC-type OS containers, for Docker/rkt-like micro service
containers, and even certain 'lightweight' VM runtimes.
The "portable service" concept ultimately will not provide a fully isolated
environment to the payload, like containers mostly intend to. Instead they are
from the beginning more alike regular system services, can be controlled with
the same tools, are exposed the same way in all infrastructure and so on. Their
main difference is that the use a different root directory than the rest of the
system. Hence, the intention is not to run code in a different, isolated world
from the host — like most containers would do it —, but to run it in the same
world, but with stricter access controls on what the service can see and do.
As one point of differentiation: as programs run as "portable services" are
pretty much regular system services, they won't run as PID 1 (like Docker would
do it), but as normal process. A corollary of that is that they aren't supposed
to manage anything in their own environment (such as the network) as the
execution environment is mostly shared with the rest of the system.
The primary focus use-case of "portable services" is to extend the host system
with encapsulated extensions, but provide almost full integration with the rest
of the system, though possibly restricted by effective security knobs. This
focus includes system extensions otherwise sometimes called "super-privileged
containers".
Note that portable services are only available for system services, not for
user services. i.e. the functionality cannot be used for the stuff
bubblewrap/flatpak is focusing on.
# Mode of Operation
If you have portable service image, maybe in a raw disk image called
`foobar_0.7.23.raw`, then attaching the services to the host is as easy as:
```
# /usr/lib/systemd/portablectl attach foobar_0.7.23.raw
```
This command does the following:
1. It dissects the image, checks and validates the `/etc/os-release` data of
the image, and looks for all included unit files.
2. It copies out all unit files with a suffix of `.service`, `.socket`,
`.target`, `.timer` and `.path`. whose name begins with the image's name
(with the .raw removed), truncated at the first underscore (if there is
one). This prefix name generated from the image name must be followed by a
".", "-" or "@" character in the unit name. Or in other words, given the
image name of `foobar_0.7.23.raw` all unit files matching
`foobar-*.{service|socket|target|timer|path}`,
`foobar@.{service|socket|target|timer|path}` as well as
`foobar.*.{service|socket|target|timer|path}` and
`foobar.{service|socket|target|timer|path}` are copied out. These unit files
are placed in `/etc/systemd/system/` like regular unit files. Within the
images the unit files are looked for at the usual locations, i.e. in
`/usr/lib/systemd/system/` and `/etc/systemd/system/` and so on, relative to
the image's root.
3. For each such unit file a drop-in file is created. Let's say
`foobar-waldo.service` was one of the unit files copied to
`/etc/systemd/system/`, then a drop-in file
`/etc/systemd/system/foobar-waldo.service.d/20-portable.conf` is created,
containing a few lines of additional configuration:
```
[Service]
RootImage=/path/to/foobar.raw
Environment=PORTABLE=foobar
LogExtraFields=PORTABLE=foobar
```
4. For each such unit a "profile" drop-in is linked in. This "profile" drop-in
generally contains security options that lock down the service. By default
the `default` profile is used, which provides a medium level of
security. There's also `trusted` which runs the service at the highest
privileges, i.e. host's root and everything. The `strict' profile comes with
the toughest security restrictions. Finally, `nonetwork` is like `default`
but without network access. Users may define their own profiles too (or
modify the existing ones)
And that's already it.
Note that the images need to stay around (and the same location) as long as the
portable service is attached. If an image is moved, the `RootImage=` line
written to the unit drop-in would point to an non-existing place, and break the
logic.
The `portablectl detach` command executes the reverse operation: it looks for
the drop-ins and the unit files associated with the image, and removes them
again.
Note that `portable attach` won't enable or start any of the units it copies
out. This still has to take place in a second, separate step. (That said We
might add options to do this automatically later on.).
# Requirements on Images
Note that portable services don't introduce any new image format, but most OS
images should just work the way they are. Specifically, the following
requirements are made for an image that can be attached/detached with
`portablectl`.
1. It must contain a binary (and its dependencies) that shall be invoked,
including all its dependencies. If binary code, the code needs to be
compiled for an architecture compatible with the host.
2. The image must either be a plain sub-directory (or btrfs subvolume)
containing the binaries and its dependencies in a classic Linux OS tree, or
must be a raw disk image either containing only one, naked file system, or
an image with a partition table understood by the Linux kernel with only a
single partition defined, or alternatively, a GPT partition table with a set
of properly marked partitions following the [Discoverable Partitions
Specification](https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/).
3. The image must at least contain one matching unit file, with the right name
prefix and suffix (see above). The unit file is searched in the usual paths,
i.e. primarily /etc/systemd/system/ and /usr/lib/systemd/system/ within the
image. (The implementation will check a couple of other paths too, but it's
recommended to use these two paths.)
4. The image must contain an os-release file, either in /etc/os-release or
/usr/lib/os-release. The file should follow the standard format.
Note that generally images created by tools such as `debootstrap`, `dnf
--installroot=` or `mkosi` qualify for all of the above in one way or
another. If you wonder what the most minimal image would be that complies with
the requirements above, it could consist of this:
```
/usr/bin/minimald # a statically compiled binary
/usr/lib/systemd/minimal-test.service # the unit file for the service, with ExecStart=/usr/bin/minimald
/usr/lib/os-release # an os-release file explaining what this is
```
And that's it.
Note that qualifying images do not have to contain an init system of their
own. If they do, it's fine, it will be ignored by the portable service logic,
but they generally don't have to, and it might make sense to avoid any, to keep
images minimal.
Note that as no new image format or metadata is defined, it's very
straight-forward to define images than can be made use of it a number of
different ways. For example, by using `mkosi -b` you can trivially build a
single, unified image that:
1. Can be attached as portable service, to run any container services natively
on the host.
2. Can be run as OS container, using `systemd-nspawn`, by booting the image
with `systemd-nspawn -i -b`.
3. Can be booted directly as VM image, using a generic VM executor such as
`virtualbox`/`qemu`/`kvm`
4. Can be booted directly on bare-metal systems.
Of course, to facilitate 2, 3 and 4 you need to include an init system in the
image. To facility 3 and 4 you also need to include a boot loader in the
image. As mentioned `mkosi -b` takes care of all of that for you, but any other
image generator should work too.
# Execution Environment
Note that the code in portable service images is run exactly like regular
services. Hence there's no new execution environment to consider. Oh, unlike
Docker would do it, as these are regular system services they aren't run as PID
1 either, but with regular PID values.
# Access to host resources
If services shipped with this mechanism shall be able to access host resources
(such as files or AF_UNIX sockets for IPC), use the normal `BindPaths=` and
`BindReadOnlyPaths=` settings in unit files to mount them in. In fact the
`default` profile mentioned above makes use of this to ensure
`/etc/resolv.conf`, the D-Bus system bus socket or write access to the logging
subsystem are available to the service.
# Instantiation
Sometimes it makes sense to instantiate the same set of services multiple
times. The portable service concept does not introduce a new logic for this. It
is recommended to use the regular unit templating of systemd for this, i.e. to
include template units such as `foobar@.service`, so that instantiation is as
simple as:
```
# /usr/lib/systemd/portablectl attach foobar_0.7.23.raw
# systemctl enable --now foobar@instancea.service
# systemctl enable --now foobar@instanceb.service
…
```
The benefit of this approach is that templating works exactly the same for
units shipped with the OS itself as for attached portable services.
# Immutable images with local data
It's a good idea to keep portable service images read-only during normal
operation. In fact all but the `trusted` profile will default to this kind of
behaviour, by setting the `ProtectSystem=strict` option. In this case writable
service data may be placed on the host file system. Use `StateDirectory=` in
the unit files to enable such behaviour and add a local data directory to the
services copied onto the host.