The kdbuswreck [LWN.net]

The kdbuswreck

Posted Apr 25, 2015 8:08 UTC (Sat) by cortana (subscriber, #24596)
In reply to: The kdbuswreck by kentonv
Parent article: The kdbuswreck

I believe you've misunderstood the D-Bus model. You are confusing addresses with interfaces. This is understandable since in simple interfaces the same string is used to identify both!

In order to communicate with a peer, you need to talk to an address. For instance, "org.freedesktop.ColorManager". This is a well-known name, that the client would use to identify *who* it wants to talk to. The dbus-daemon has a set of rules that determine which processes are allowed to take ownership of a well-known name. This ensures that you are really talking to colord, and not an imposter. These are currently defined by files in /etc/dbus-1/systemd.d; I don't know what happens to them in the brave new kdbus world.

Once you've connected to an address, you can obtain a list of objects exported by the peer. Simple services will just export one object, such as "/org/freedesktop/ColorManager"; by convention these are similar to the address above, but with a slash instead of a period used to separate components.

More complex interfaces will export additional objects, for instance /org/freedesktop/ColorManager/devices/printer1, /org/freedesktop/ColorManager/devices/profiles/icc_{hexstring} and so on. In colord's case these are used to represent different devices and colour profiles on the system.

Having selected an object to talk to, for instance, /org/freedesktop/ColorManager, you now choose an *interface*. Even in the case of the simplest service, that just exports a single object, that object will export multiple interfaces. In our example we see 'org.freedesktop.DBus.Introspectable', 'org.freedesktop.DBus.Peer' and 'org.freedesktop.DBus.Properties' and finally 'org.freedesktop.ColorManager'.

Now, *these* are the interfaces that you mistook for addresses earlier. All the methods & properties supported by an object are accessed via one of these interfaces. So in order to find a list of the colour sensing devices attached to my computer, I would call the GetSensors() method of the 'org.freedesktop.ColorManager' interface of the /org/freedesktop/ColorManager object exported by the 'org.freedesktop.ColorManager' peer. Whereas if I wanted to see which interfaces an object implements, along with their associated methods and properties, I would call the Introspect() method of the 'org.freedesktop.DBus.Introspectable' interface of the same object. Those three 'org.freedesktop.DBus.*' interfaces, by the way, are supported by every object on the bus.

So, if you wanted to communicate with a special-purpose peer for testing, you would tell your code to connect to a different *address*, e.g. ":1.340" (these are non-well-known addresses that are assigned when a client connects to the dbus-daemon), but the same object and interface.

Hope that makes sense; if not then please install the dbus introspection tool "d-feet" and click around for a couple of minutes, things should become obvious then. For command-line introspection I usually use the 'qdbus' program, but it's much quicker to navigate the peers on a bus with d-feet, so start there.

Personally I think the D-Bus model is a little over-complex, and the fact that simple services use the same name for each of their address, object path _and_ interface makes things appear a more obscure than they really are, but we are where we are and D-Bus has gained a lot of adoption over the last 10 years, having replaced the IPC mechanisms previously used by both GNOME and KDE.

That said, I really do wish there was a dbus(7) man page that explained the above in simple terms that would be useful for busy sysadmins and users curious about the internals of their system. :)

to post comments

The kdbuswreck

Posted Apr 25, 2015 12:27 UTC (Sat) by lsl (subscriber, #86508) [Link] (12 responses)

> Having selected an object to talk to, for instance, /org/freedesktop/ColorManager, you now choose an *interface*

That seems backwards to me. Why would I even care what object I talk to? I just want *some* object that implements the interface I need.

So when calling methods on the 'org.freedesktop.ColorManager' interface those get dispatched to an implementation that makes sense accorrding to local system configuration, say colord, KolorManager or whatever the user set up for this.

Is it possible to sanely use dbus this way? I mean, I can certainly enumerate the bus und search for something that implements the wanted interface but that doesn't seem reasonable.

So let's take a step back here. How would one implement the concept "I want this functionality but I don't care who provides it" in dbus? Are interfaces a red herring here and I better look at well-known names? What those resolve to is up to system configuration, right? So is this the point where it is commonly decided what program will handle my requests regarding color management? Whatever owns the name?

The kdbuswreck

Posted Apr 25, 2015 13:34 UTC (Sat) by mchapman (subscriber, #66589) [Link] (5 responses)

> How would one implement the concept "I want this functionality but I don't care who provides it" in dbus?

I would say that is *exactly* what D-Bus provides now. There is nothing in D-Bus's policy configuration that locks a service to a particular binary. A service name can be claimed by any process that matches that service's policy (for system services this is typically just a check that the connection was authenticated as root). Of course, only one D-Bus connection can own a service name at any particular time.

Service activation is a bit different, as D-Bus (or systemd, if it's doing the activation) needs to know which binary to launch when the service is requested. I suppose you could use something like alternatives to cater for multiple implementations of that service.

The kdbuswreck

Posted Apr 25, 2015 15:17 UTC (Sat) by mathstuf (subscriber, #69389) [Link]

> Service activation is a bit different, as D-Bus (or systemd, if it's doing the activation) needs to know which binary to launch when the service is requested. I suppose you could use something like alternatives to cater for multiple implementations of that service.

There should already be examples for this at the session level with the different NetworkManager UIs, kwallet vs. gnome-keyring, etc.

The kdbuswreck

Posted Apr 27, 2015 12:16 UTC (Mon) by javispedro (guest, #83660) [Link] (3 responses)

> I would say that is *exactly* what D-Bus provides now. There is nothing in D-Bus's policy configuration that locks a service to a particular binary. A service name can be claimed by any process that matches that service's policy (for system services this is typically just a check that the connection was authenticated as root). Of course, only one D-Bus connection can own a service name at any particular time.

You actually _cannot_ do what is being asked with current D-Bus, and it is one of my major gripes with it (which is why I prefer anything else over it). The "using the service name as what every other IPC would call interface name" idea works well until you realize you cannot register more than one service with the same name. Thus, you start seeing ugly tricks such as what MPRIS does, which require greping over the service names, etc.

The kdbuswreck

Posted Apr 27, 2015 12:54 UTC (Mon) by mchapman (subscriber, #66589) [Link] (2 responses)

> The "using the service name as what every other IPC would call interface name" idea works well until you realize you cannot register more than one service with the same name. Thus, you start seeing ugly tricks such as what MPRIS does, which require greping over the service names, etc.

I don't think it's necessarily an ugly trick. The difference between getting a list of service names with some prefix and in getting a list of services providing some object (assuming this were even possible with D-Bus) is mostly superficial.

If efficiency is the problem, getting a list of service names for things like MPRIS could be optimized by extending the protocol slightly, e.g. by having org.freedesktop.DBus.ListNames take a prefix as an argument.

But allowing a service name to be owned by at most one connection is essential for lsl's use case. You can't sanely dispatch "to an implementation that makes sense accorrding to local system configuration" if more than one such implementation is on the bus at the same time.

The kdbuswreck

Posted Apr 27, 2015 13:28 UTC (Mon) by MrWim (subscriber, #47432) [Link]

I don't think it's necessarily an ugly trick. The difference between getting a list of service names with some prefix and in getting a list of services providing some object (assuming this were even possible with D-Bus) is mostly superficial.

If efficiency is the problem, getting a list of service names for things like MPRIS could be optimized by extending the protocol slightly, e.g. by having org.freedesktop.DBus.ListNames take a prefix as an argument.

Indeed, this is the purpose of the arg0namespace match rule. You register for NameOwnerChanged events with some prefix, call ListNames (or ListActivatableNames) filtering on the prefix and then you can efficiently and asynchronously keep your local list of remote names up-to-date.

The kdbuswreck

Posted Apr 28, 2015 8:10 UTC (Tue) by javispedro (guest, #83660) [Link]

It introduces a bunch of additional problems. For example, what if you want the same process to expose more than one instance of the service? You will be hit by the fact that despite having multiple service names you still have one object namespace only... and no way to setup different policies, etc.

The MPRIS trick obviously works, but it puts the design of DBus upside down.

The kdbuswreck

Posted Apr 27, 2015 6:45 UTC (Mon) by krake (guest, #55996) [Link]

> That seems backwards to me. Why would I even care what object I talk to? I just want *some* object that implements the interface I need.

It will depend on the type of service, i.e. if there is some object related context.

For example, a service which provides functionality on a set of real world objects will expose these objects again as a set of D-Bus objects.
It makes it easier for programmers on both sides (service and clients) if there is a one-to-one mapping, e.g. NetworkManager exposing each network device as a separate object.

For a service that provides only one interface on one object, the convention seems to be to use the same name parts for the well-known connection name, the object path and the interface name (with respective separator characters).

The kdbuswreck

Posted Apr 27, 2015 7:30 UTC (Mon) by cortana (subscriber, #24596) [Link] (2 responses)

> That seems backwards to me. Why would I even care what object I talk to? I just want *some* object that implements the interface I need.

If you're saving passwords then you want to be sure that the 'org.freedesktop.secrets' address has not been taken by a password-stealing program.

The kdbuswreck

Posted Apr 27, 2015 9:06 UTC (Mon) by mchapman (subscriber, #66589) [Link] (1 responses)

> If you're saving passwords then you want to be sure that the 'org.freedesktop.secrets' address has not been taken by a password-stealing program.

That seems like a completely orthogonal problem to me.

I'm going to reiterate what I said in my other post: D-Bus *already provides* the ability for a client to talk to "any object that implements a particular interface": simply replace the word "object" with "service" and "interface" with "object".

The kdbuswreck

Posted Apr 27, 2015 9:16 UTC (Mon) by mchapman (subscriber, #66589) [Link]

> I'm going to reiterate what I said in my other post: D-Bus *already provides* the ability for a client to talk to "any object that implements a particular interface": simply replace the word "object" with "service" and "interface" with "object".

Meh, I screwed that comment up. I should have said: simply replace the word "object" with "connection" and "interface" with "service".

That is, a D-Bus client does not care what connection provides a particular service; it relies on bus policy for that to be authorized appropriately.

That being said, I have the feeling there is very little stopping some malicious piece of software from killing off gnome-keyring-daemon, say, and grabbing the org.freedesktop.secrets bus name before GNOME has a chance to restart the daemon.

The kdbuswreck

Posted Apr 27, 2015 15:17 UTC (Mon) by hp (guest, #5220) [Link] (1 responses)

> That seems backwards to me. Why would I even care what object I talk to? > I just want *some* object that implements the interface I need.

An interface is implemented by N objects, so for example an interface might be implemented by each open document in a word processor. I would say you do not want "some document that implements the Document interface" when you call `org.whatever.Document.Delete()`, you want the specific document you plan to delete :-)

*Services* are generally pluggable - i.e. the entire word processor application, could implement a set of objects (each with a set of interfaces) conforming to some sort of standard, potentially, and then you could interop with whichever word processor owns a certain `org.whatever.WordProcessor` service, or something.

Well-known name: like a DNS entry, a way to find an entire *program* to talk to (service locator)

Object path: equivalent to a pointer ... a specific instance of an object in the "object-oriented programming" sense of object

Interface: means same thing as in Java (set of methods on an object instance)

The fact that some programs have only one object instance with only one interface, in no way means that these are redundant.

Yes you can write a program in Java that only contains `class MyProgram` and `static MyProgram theInstanceOfMyProgram = new MyProgram()`.

This does not mean that Java should _only_ provide support for singleton objects!

The kdbuswreck

Posted Apr 27, 2015 18:59 UTC (Mon) by lsl (subscriber, #86508) [Link]

> An interface is implemented by N objects, so for example an interface might be implemented by each open document in a word processor. I would say you do not want "some document that implements the Document interface" when you call `org.whatever.Document.Delete()`, you want the specific document you plan to delete :-)

Ah ok, thanks. Didn't thought about it that way. For most of the stuff on my local system bus it wouldn't make a difference: it doesn't matter who tells me the hostname or who is going to set the timezone. But then there's logind (which I missed the last time), where it in fact matters whose session is going to be terminated.

The kdbuswreck

Posted Apr 25, 2015 19:55 UTC (Sat) by kentonv (subscriber, #92073) [Link] (7 responses)

Sorry, I don't think I've expressed my point very clearly.

When I say "singleton" what I essentially mean is "an object addressed by a global well-known name or path". The problems that I have with singletons are not fixed by saying "ok, you can have a list of objects with different names" -- all those objects are still singletons.

For example, the path "/org/freedesktop/ColorManager/devices/printer1" refers to the *same* device regardless of who is calling. The problem with this is that it means the calling code decides which printer to connect to. That's bad because:

1. It's probably the user, not the app, that knows best which printer to connect to. So now the app needs to implement a picker dialog. Many apps will skip this and just hard-code the first object. (In practice you don't usually see this problem for printers, but you *do* see it for, say, audio output devices. Every app that plays audio should be asking me which device to use, but, sadly, they do not. I must choose a system-wide default device, and I cannot easily have different apps playing to different speakers. Yes, some systems support advanced configuration of audio sources and destinations within the audio control panel, but my point is that we should have this kind of configurability for all resources.)

2. The app necessarily has the ability to enumerate the devices and connect to all of them. For security reasons, it would be better if the app *only* had the ability to connect to the device that I, as the user, chose for it to access. Traditionally desktop systems have made the unfortunate assumption that I trust all my apps to wield all the power of my user account, but I'd really prefer that each of my apps runs in a sandbox with only the power it needs to do its job.

What I want is for apps to make requests like "I need something that implements org.freedesktop.AudioOutput" (or whatever interface), and then the system displays a dialog to the *user* asking which device or service to use. The app only ever receives access to the device the user chooses, and the app can be used in a broader range of use cases without burdening the app developer with implementing the requisite configurability.

The kdbuswreck

Posted Apr 25, 2015 20:38 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link] (6 responses)

For example, the path "/org/freedesktop/ColorManager/devices/printer1" refers to the *same* file regardless of who is opening it. The problem with this is that it means the calling code decides which file to open. That's bad because:

1. It's probably the user, not the app, that knows best which file to open. So now the app needs to implement a picker dialog. Many apps will skip this and just hard-code the first file. (In practice you don't usually see this problem for files, but you *do* see it for, say, audio output devices. Every app that plays audio should be asking me which device to use, but, sadly, they do not. I must choose a system-wide default device, and I cannot easily have different apps playing to different speakers. Yes, some systems support advanced configuration of audio sources and destinations within the audio control panel, but my point is that we should have this kind of configurability for all resources.)

2. The app necessarily has the ability to enumerate the files and open all of them. For security reasons, it would be better if the app *only* had the ability to open the file that I, as the user, chose for it to access. Traditionally desktop systems have made the unfortunate assumption that I trust all my apps to wield all the power of my user account, but I'd really prefer that each of my apps runs in a sandbox with only the power it needs to do its job.
...

> What I want is for apps to make requests like "I need something that implements org.freedesktop.AudioOutput" (or whatever interface), and then the system displays a dialog to the *user* asking which device or service to use.
Try to watch DBUS with a sniffer. Now imagine that you have to MANUALLY select each and every endpoint.

The kdbuswreck

Posted Apr 25, 2015 21:03 UTC (Sat) by kentonv (subscriber, #92073) [Link] (5 responses)

> Try to watch DBUS with a sniffer. Now imagine that you have to MANUALLY select each and every endpoint.

Yes obviously what I'm describing can't be dropped on top of the existing set of dbus endpoints and just work. Lots of stuff would need to be redesigned and organized differently. It is possible to design such an environment and have it work well (CapDesk did it, and Sandstorm.io is doing it), but I don't honestly expect today's dbus-using desktop environments to entirely switch anytime soon. Still, it's useful for people to understand the ideal in order to guide improvements to what we have today.

The kdbuswreck

Posted Apr 25, 2015 22:42 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link] (4 responses)

I still have no idea how your design will be any different from the current one. You still need to have well-known starting points for lookups.

The kdbuswreck

Posted Apr 26, 2015 20:17 UTC (Sun) by luto (subscriber, #39314) [Link] (1 responses)

Not really. Yes, something needs to create a starting point, but that something could just be whatever creates the resource in the first place.

For example, gdm or logind could start my shell with access to an object implementing the "find a printer" interface. Programs that inherit access to that object would use it.

Sandboxed programs, on the other hand, might get access to a different "find a printer" interface that behaves differently.

dbus can do this right now. On my Fedora 21 system, my shell and everything it starts has access to a standard implementation of a lot of these things. It looks like:

DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-qB3T8DFwej,guid=1453a3565ca58487e6a024fe5538ad89

Too bad that doesn't seem to apply to the system bus.

The kdbuswreck

Posted Apr 27, 2015 4:12 UTC (Mon) by Cyberax (✭ supporter ✭, #52523) [Link]

There's nothing really special about the system bus. It's possible to override it for each application.

But as I understand, it was designed to be a globally visible namespace with access being controlled by PolKit.

The kdbuswreck

Posted Apr 26, 2015 20:19 UTC (Sun) by kentonv (subscriber, #92073) [Link]

The "starting point" would be a file descriptor inherited from the parent process. Today we have "standard input", "standard output", and "standard error"; now imagine adding "standard desktop" which is a socket that implements some protocol to talk to the desktop session.

This file descriptor would support a bunch of standard functionality that all apps need (like, opening windows, or raising notifications, etc., but NOT things where the user might want to choose the resource used or deny it for security reasons, like printers or audio devices or connected OAuth accounts). It would also have a way to say "I need an object (file descriptor) implementing protocol X", and that's when the user is prompted to choose which object to use. Once the user chooses something, the app can save a long-term token representing that choice and re-request the same object later using that token.

Yes, this is "standard desktop" FD is similar to the dbus session bus, except that I can decide which programs that I run are allowed to access it, and I can mock it out, audit usage, sandbox, etc., as described previously, and I can make choices about individual resources accessed by any app.

The kdbuswreck

Posted Apr 27, 2015 12:13 UTC (Mon) by javispedro (guest, #83660) [Link]

Yes, but you can make symlinks ;)