1. DBus, PolicyKit and YaST
Ing. Ladislav Slezák <lslezak@novell.com>
YaST Developer

2. DBus

3. © Novell, Inc. All rights reserved.3
DBus
• What is DBus?
– Local interprocess communication (IPC)
– Message based (sends messages, not streams)
– Messages are transferred in binary form
– Supports 1:1 and 1:N communication

4. © Novell, Inc. All rights reserved.4
DBus
• Why?
– Unified communication for desktop replacing DCOP (KDE)
and CORBA (GNOME)
– For communication between desktop applications or between
desktop applications and system daemons, for handling
system events...

5. © Novell, Inc. All rights reserved.5
DBus Concepts
• Object oriented view
– DBus services provides objects
– But OOP is not required, service can be written in C
• Data types
– Basic data types: integer, string, boolean...
– Containers: array, hash, structure, variant
> Containers can be recursive

6. © Novell, Inc. All rights reserved.6
DBus parts
• DBus daemon
– Runs a bus, processes and routes the messages
• DBus library (libdbus)
– Low-level C API
• Language bindings
– High-level API for Python, Ruby, Qt, Glib …
– Wrappers for low-level C calls
– Conversion between native and DBus data types
– May provide object proxy for mapping native objects
(GObject, QObject,...) and signals to DBus objects and signals

7. © Novell, Inc. All rights reserved.7
DBus Terminology
• Bus
– The place for transferring the messages
– Multiple buses can run in parallel
> System wide bus – single bus for system wide events and services, runs
under a system user (non-root)
> Session bus – one per user session, for desktop applications, created at
login, runs with user privileges
> Private bus – special separate bus can be started by applications

8. © Novell, Inc. All rights reserved.8
DBus Terminology
• Connection/Service name
– Unique ID (:1.42, assigned by DBus) or well-known
(org.freedesktop.PowerManagement, defined by service)
• Object path in object tree
– /org/freedesktop/PowerManagement
• Interface name
– Named group of methods and signals,
org.freedesktop.PowerManagement
• Method name
– Suspend

9. © Novell, Inc. All rights reserved.9
DBus Terminology
Service org.freedesktop.PowerManagement
Object /org/freedesktop/PowerManagement
Bus
Interface org.freedesktop.PowerManagement
Method Suspend()

10. © Novell, Inc. All rights reserved.10
DBus Message
• Message types
– Method call
– Method reply
– Error
– Signal (1:N publish/subscribe communication)
• Header
– Type, destination service, target object, method, sender,
serial number, data type signature...
• Data
– Arguments in binary form

11. © Novell, Inc. All rights reserved.11
Object Introspection
• Query available objects in a service
• Runtime object browsing
• Needed by language proxies and DBus browsers

12. © Novell, Inc. All rights reserved.12
Service Activation
• DBus service
– can be provided by a running application
– can start be started on demand
> Started service can continue running, it can exit immediately or exit after a
timeout

13. © Novell, Inc. All rights reserved.13
Access Control
• It's possible to allow or deny access to a specific
object, interface, method, signal...

14. © Novell, Inc. All rights reserved.14
Python Example
• Method call example
import dbus
bus = dbus.SessionBus()
obj = bus.get_object('org.freedesktop.PowerManagement',
'/org/freedesktop/PowerManagement')
iface = dbus.Interface(obj,'org.freedesktop.PowerManagement')
result = iface.CanHibernate()
print result

15. © Novell, Inc. All rights reserved.15
GUI Tools – qdbusviewer

16. © Novell, Inc. All rights reserved.16
GUI Tools – kdbus

17. © Novell, Inc. All rights reserved.17
Command line tools
• dbus-send
– Sends a message
• dbus-monitor
– Monitors activity on a bus
• qdbus
– DBus browsing, sending messages

18. © Novell, Inc. All rights reserved.18
DBus Documentation
• Introduction to DBus
– http://www.freedesktop.org/wiki/IntroductionToDBus
• DBus tutorial
– http://dbus.freedesktop.org/doc/dbus-tutorial.html
• DBus FAQ
– http://dbus.freedesktop.org/doc/dbus-faq.html
• Low-level C API
– http://dbus.freedesktop.org/doc/dbus/api/html/group__DBus.html
• C example
– http://dbus.freedesktop.org/doc/dbus/libdbus-tutorial.html

19. PolicyKit

20. © Novell, Inc. All rights reserved.20
PolicyKit
• Framework for central policy management of
privileged operations
– In contrast to /etc/sudoers, user group management...
• Extends DBus security model – more fine grained
privileges
– User can mount removable media, but cannot mount fixed
media
– User can shutdown the machine from a local session

21. © Novell, Inc. All rights reserved.21
PolicyKit Model
• Application is split to privileged and unprivileged part,
each part runs in a different process
– For security reasons the privileged part should be as small as
possible
– The privileged part is called “Mechanism”
• These parts communicate via IPC (DBus, pipes, ...)

22. © Novell, Inc. All rights reserved.22
PolicyKit Model
http://hal.freedesktop.org/docs/PolicyKit/diagram-bus-model.png

23. © Novell, Inc. All rights reserved.23
Mechanism
• Runs as a privileged user
• Parts of mechanism
– Decider part – checks
whether the requester is
allowed to call the action
part
– Action part – carries out the
privileged action
• The action is performed
only when the decider
part says “Yes”
Request
Mechanism
Allowed?
Do Action
Error

24. © Novell, Inc. All rights reserved.24
Input for Decision
• Subject
– Attributes of the requester – UID, application, local or remote
session, active session...
• Object
– Removable device, Package, Power management...
• Action
– What to do with the object
– Mount the device, install the package, reboot the system...
• PolicyKit encodes object and action to single Action ID
– e.g. org.freedesktop.hal.storage.mount-removable

25. © Novell, Inc. All rights reserved.25
Authorization Check
• Also the unprivileged part may check whether it can do
the privileged operation
– Disabled or locked options in UI

26. © Novell, Inc. All rights reserved.26
How to Obtain an Authorization
• User is allowed to do action
– By default (defined in configuration file)
– Under a special condition (e.g. active session)
– Administrator grants the authorization to the user
– Obtains the authorization via authentication

27. © Novell, Inc. All rights reserved.27
Authentication
• Authentication agent is a DBus service
(org.freedesktop.PolicyKit.AuthenticationAgent)

28. © Novell, Inc. All rights reserved.28
Workflow
• HAL example
• Mount request sent
http://hal.freedesktop.org/docs/PolicyKit/diagram-interaction.png
• Check the authorization

29. © Novell, Inc. All rights reserved.29
Workflow
• An exception is returned • The user authenticates

30. © Novell, Inc. All rights reserved.30
Workflow
• A token is written to DB • The second attept is OK

31. © Novell, Inc. All rights reserved.31
Command Line Tools
• polkit-auth
– Manage authorizations – display, obtain, revoke...
• polkit-action
– List actions, modify defaults
• polkit-policy-file-validate
– A validation tool for .policy files, should be called from
'make check'
• polkit-policy-file-validate
– Validates /etc/PolicyKit/PolicyKit.conf file

32. © Novell, Inc. All rights reserved.32
PolicyKit Documentation
• Design, API reference manual, tools...
– http://hal.freedesktop.org/docs/PolicyKit/index.html
– Package PolicyKit-doc

33. YaST DBus Service

34. © Novell, Inc. All rights reserved.34
YaST DBus Service
• Access the YaST functionality from other applications
• Needed for WebYaST
Why DBus?
• Auto start (no running daemon)
• Starts YaST as root user (no SUID flag)
Why PolicyKit?
• Admin tools available

35. © Novell, Inc. All rights reserved.35
YaST DBus Service
• Package yast2-dbus-server
• Service org.opensuse.YaST.modules
running on the system bus
• Object /org/opensuse/YaST/modules/<module>
• Interface org.opensuse.YaST.Values
• The service is started automatically
• Shuts down after 2 minutes

36. © Novell, Inc. All rights reserved.36
Solved Problems
• PolicyKit action ID
– Created from function name
– Checked before function call
• Stateful YaST modules
– Stateless wrappers (YaPI)
– Lock(), Unlock() methods
• YaST module autoloading
– No need to load all YaST modules
• YCP data types and special values
– Additional interface
• Bugfixes in Ruby DBus bindings

37. Questions?

38. Thank you!