Get the sender PID from gDBus

[Amira S]

Thanks for the detailed explanation!

Calling g_dbus_method_invocation_get_sender() and then
GetConnectionUnixProcessID work and I can get the peer's PID now.


On Mon, May 23, 2022 at 2:15 PM Simon McVittie <smcv at collabora.com> wrote:

> On Sun, 22 May 2022 at 11:40:35 +0300, Amira S wrote:
> > Any idea if/how it is possible to get the peer's PID connecting to a
> SYSTEM
> > bus?
>
> g_dbus_method_invocation_get_sender() will tell you the peer's
> unique connection name, and then you can look that up with
> GetConnectionUnixProcessID or GetConnectionCredentials to get the process
> ID (for example you might use g_dbus_connection_call() to call one of
> those methods).
>
> polkit <https://gitlab.freedesktop.org/polkit/polkit/> is an example of a
> system service that does this.
>
> > According to the documentation, it is possible to obtain a GCredentials
> from
> > the connection by calling [1]g_dbus_connection_get_peer_credentials,
> (and from
> > there I can get a unix ucred that has the pid), but I always get a NULL
> > credentials.
> > The documentation for this function says:
> > "Gets the credentials of the authenticated peer. This will always
> return [2]
> > NULL unless connection acted as a server
>
> You are not the server, so the documentation correctly says that it will
> return NULL for you.
>
> Normal use of D-Bus on the system or session bus is in a "star" topology,
> where the message bus (dbus-daemon or dbus-broker) is the middle of the
> star, and all clients and services are at the points of the star:
>
>                 app 2         }
>                   |           } (AF_UNIX socket clients)
>           app 1   |   app 3   }
>               \   |    /
>                \  |   /
>               message bus (AF_UNIX socket server)
>                /   |  \
>               /    |   \
>         service A  |  service C }
>                    |            } (also AF_UNIX socket clients)
>                service B        }
>
> We often talk about services as though they were something special,
> but there is not really any fundamental difference between services and
> other applications: they're all clients of the message bus. The role of
> the message bus is often called a "broker" in other IPC protocols.
>
> Depending who you ask, a service is either a D-Bus peer that owns a
> well-known name like "com.microsoft.mscrypt", or a D-Bus peer that was
> started automatically by the message bus to provide the implementation of
> a well-known name, or a D-Bus peer that provides methods to be called by
> other peers. In practice, most peers that are referred to as "services"
> do all of these things.
>
> > I also tried getting the PID by calling programmatically the following
> command:
> >
> > dbus-send --system --print-reply --dest=org.freedesktop.DBus
>  /org/freedesktop/
> > DBus org.freedesktop.DBus.GetConnectionUnixProcessID
> > ‘string:com.microsoft.mscrypt’
> >
> > But I always receive the server's PID, and not the client's.
>
> No, you receive the process ID of the service that owns the name
> "com.microsoft.mscrypt". Don't confuse the service (a high-level concept)
> with the AF_UNIX server (a lower-level concept).
>
> If you are using GDBus, you should call D-Bus methods with something like
> g_dbus_connection_call() or g_dbus_connection_call_sync(),
> g_dbus_proxy_call() or client code generated by gdbus-codegen, instead of
> running dbus-send as a subprocess (which does the same thing, but less
> efficiently).
>
>     smcv
>
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