Get the sender PID from gDBus

[Simon McVittie]

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