Fedora runtimes and sandboxed desktop applications

Alexander Larsson alexl at redhat.com
Wed May 27 09:50:15 UTC 2015

I'm working on an application sandboxing system called xdg-app, and
the surrounding echo-system. I won't go into details, but there are
some links to technical details at the end of the mail[1]. The basic
idea is that we have two sources (both ostree trees), one called the
runtime (/usr), and one called the app (/app), which get combined in a
container-like environment at launch time by xdg-app, with configurable
degrees of sandboxing. There is also some interaction with the host
system so that installed desktop files show up in the shell and dbus
session services from apps work.

xdg-app has two major goals, the first is to allow applications to run
in a sandboxed environment with minimal access to the host. The second
is to allow third parties to create and distribute applications that
run on all distributions (and the sandboxing helps making this less
risky). To facilitate and test this I've also created an upstream
gnome runtime[2] that gnome applications can chose to use. I'm also
working on various Portal APIs[3] that allow sandboxed applications to
safely interact with the system.

But, third party applications are a long way off in the future, and so
are sandboxed applications (which need not only a fleshed out set of
sandbox APIs, but also modifications to individual apps). In the
meantime it is still interesting to run bundled applications. For
instance in the context of an Atomic-based Fedora Workstation (with
readonly /usr), or if you want to run older or newer versions of
applications than your distribution is shipping.

It would make a lot of sense to use the existing packages in Fedora
for this. And in order to kickstart discussions about this I've
created an initial sample of a possible Fedora runtime using
rpm-ostree and the Fedora 22 repositories:


Building this produces a repo with the main runtime
(org.fedoraproject.Platform) and a sdk runtime (org.fedoraproject.Sdk)
which has some additional tools for app creation and debugging. It
also builds .Var subruntimes that contain /var with the rpm db
for use when building apps. The current package set is mostly based on
what is in the gnome one, so don't consider this official in any way.

These runtimes are made to work with xdg-app 0.3.0 which are now in
updates-testing for F21 and F22. You also need rpm-ostree from git
master for the container support to build the fedra runtime.

In order to test the runtime I've also created a firefox application
bundle. Unfortunately it requires some hacks in order to be relocated
to /app, and to work as a bundle. However, it works pretty well,
running somewhat contained with only access to the Downloads xdg
directory and with ~/.mozilla being redirected to the per-app
persistant data directory (~/.var/app/org.fedoraproject.firefox). Note
however, that I would not consider this properly sandboxed, as it uses
X11 for rendering, which is not very secure.

The scripts to build the firefox bundle is here:


I've also put up a xdg-app repository so that you can easily test this:
(Run these commands as non-root, due to the --user args)

  xdg-app add-remote --user --no-gpg-verify fedora http://people.redhat.com/~alexl/repo
  xdg-app install-runtime --user fedora org.fedoraproject.Platform 22
  xdg-app install-app --user fedora org.fedoraproject.firefox
  xdg-app run org.fedoraproject.firefox

Once installed the desktop file will be exported to the session, so
you should also be able to launch it from the shell. In theory this
should work on any distro, but I've only tried it on F21.

The question is, would the Fedora project be interested in supplying
and maintaining a Fedora runtime like this, which allows us (and
others) to create applications using the fedora base. And if we do,
exactly what should be considered part of the basic platform.

Defining a platform requires a lot of care. It is a balance between
wanting as much as possible in it to share more maintainance and
updates and make it easier to write apps and on the other hand trying
to keep the basic platfrom small, maintained and of high quality. The
ABI requirements for a platform is a bit different than a regular
distro. The goal is to allow apps built against a particular platform
to run for years to come, so it should only do security and very
important bugfix updates, and *never* break ABI. On the other hand,
its easy to break ABI in a later version of the runtime, because they
are parallel installed. This is somewhat different than our current
updates which revs libraries during the distro lifetime as long as it
also updates all users of it.

An added complexity is that anything in the runtime that relies on
some kind of IPC to the host (say the pulseaudio client libraries, or
something that uses a dbus interface on the host) requires the host
side of that IPC to remain available and backwards compatible in
future versions of the distro, so we should err on exposing a minimal
set of IPC.

There is also some changes that may be required in the distro to keep
unnecessary dependencies out of the runtime. For instance anything
related to hardware or system initialization is completely
unnecessary. Luckily much of this is being fixed by similar needs from
docker already.

One thing that is different from docker however, is that we generally
want translations/locales in the runtime. But these are pretty large,
so we want to handle them in a smart way. xdg-app has a feature that
allows splitting a runtime into a base and a set of optional
extensions that are mounted in the sandbox only when they are
installed. I use this in the gnome runtime to split out each
locale. Unfortunately Fedora uses a single large locale-archive file
for its locales, so its hard to do that. This needs some research to
figure out how to do properly.

There are also some custom configuration that a runtime needs. For
instance, the build script for the fedora runtime above drops in a new
fontconfig file that adds a /app/share/fonts fontdir for app-bundled
fonts and configuration to include the host fonts (which xdg-app
exposes in /run/host/fonts). This would be nicer if this was just a
fontconfig subpackage we could pull in, and I'm sure there are more
similar configuration options that would be needed.

Creating applications from fedora packages is a bit more
complicated. The primary reason is that they need to be relocated to
/app. In the hacky script above I just extract the rpm contents to
/app and then rewrite some rpaths and shell scripts. A less hacky
approach is to rebuild the rpms with a different rpm %_prefix macro,
or to use relocatable rpms. It is not really significantly different
from creating rpms to use in software collections though, so we have
some experience with this.

Packaging an application also requires some changes other than the
relocatability itself. First of all, all "exported" files from an
application (such as icons, desktop files, dbus service files, or
other files that are read from outside the app itself) must have the
application id (which is a dbus/java-style reverse dns name) as a
filename prefix. This is both for avoiding conflicts, and for security
reasons, see [4]. However, it wouldn't necessarily be a problem if we
just did such changes on the regular fedora packages too, as a regular
fedora packaging policy.

One issue is that as third party packagers of applications we should
not use the "official" app ids for our builds, as they would conflict
with upstream doing their own release. So, our package of
org.gnome.gedit should probably be called org.fedoraproject.gedit,
which may require some minor tweaks to the build (for instance, this
affects the dbus name the app uses for dbus activated desktop files,
as well as the name of the desktop file itself).

There are clearly a lot of details here, but I'll stop here and throw
this out as the start of a discussion. I'll gladly reply to any
questions people may have.

[1] More information:
   My devconf talk: https://www.youtube.com/watch?v=t-2a_XYJPEY
   gnome wiki: https://wiki.gnome.org/Projects/SandboxedApps
   xdg-app code: https://github.com/alexlarsson/xdg-app

[2] Announcement of gnome runtime:

[3] https://wiki.gnome.org/Projects/SandboxedApps/DocumentPortal


 Alexander Larsson                                            Red Hat, Inc 
       alexl at redhat.com            alexander.larsson at gmail.com 
He's a war-weary Republican vampire hunter on a search for his missing 
sister. She's a ditzy winged queen of the dead with the power to see 
death. They fight crime! 

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