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D-Bus Mojo Connection Service

Overview

D-Bus Mojo Connection Service in Chrome is a D-Bus service that helps to bootstrap CrOS services' Mojo connection.

Bootstrap a new CrOS service

D-Bus Mojo Connection Service lives in //chrome/browser/ash/dbus/mojo_connection_service_provider.h.

Follow the example of CrOS Sensors' changelist.

Steps to Add a usage for a CrOS process with a new D-Bus method:

  1. Add a method name in both CrOS platform and Chrome. (Recommend: platform2/system_api)
  2. Add the busconfig policy in MojoConnectionService.conf.
  3. Upon a D-Bus request coming from the CrOS service, pass one endpoint of the generated Mojo pipe to the component in Chrome that needs a Mojo channel to the CrOS service. Ex: RegisterServer in CrOS Sensors' usage.
  4. Respond to the D-Bus request with the other endpoint of the generated Mojo pipe. (Recommend: use the helper function SendResponse)

The Mojo pipe can also be generated in the CrOS process, and pass the endpoint of it as the D-Bus argument to the service provider, instead of allowing Chrome to generate the pipe.

Steps to Add a usage for a CrOS process with an existing D-Bus method:

  1. Add the busconfig policy in MojoConnectionService.conf.

And that’s it. The method name and the logic in the service provider can be reused.

Security

UID filtering should be used to ensure only the needed processes are calling the specific D-Bus methods, as processes/applications calling D-Bus APIs are trusted (written and reviewed by Chromium/CrOS teams) and should have a well-known UID to be filtered.

UID filtering: Define access permission for each UID in MojoConnectionService.conf. Only the processes run under the specific UIDs can send respective D-Bus requests to the service provider.

Arguments/tokens in D-Bus methods are still available if needed, which should be enough for multi-login situations and handling failures. The arguments can also be used to determine if Chromium should accept the request, and which Mojo interface should be used to establish the Mojo channel.