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Linux Sandbox IPC

The Sandbox IPC system is separate from the 'main' IPC system. The sandbox IPC is a lower level system which deals with cases where we need to route requests from the bottom of the call stack up into the browser.

The motivating example used to be Skia, which uses fontconfig to load fonts. Howvever, the OOP IPC for FontConfig was moved to using Font Service and the components/services/font/public/cpp/font_loader.h interface.

These days, only the out-of-process localtime implementation as well as an OOP call for making a shared memory segment are using the Sandbox IPC file-descriptor based system. See sandbox/linux/services/libc_interceptor.cc.

Thus we define a small IPC system which doesn't depend on anything but base and which can make synchronous requests to the browser process.

The zygote starts with a UNIX DGRAM socket installed in a well known file descriptor slot (currently 4). Requests can be written to this socket which are then processed on a special "sandbox IPC" process. Requests have a magic int at the beginning giving the type of the request.

All renderers share the same socket, so replies are delivered via a reply channel which is passed as part of the request. So the flow looks like:

  1. The renderer creates a UNIX DGRAM socketpair.
  2. The renderer writes a request to file descriptor 4 with an SCM_RIGHTS control message containing one end of the fresh socket pair.
  3. The renderer blocks reading from the other end of the fresh socketpair.
  4. A special "sandbox IPC" process receives the request, processes it and writes the reply to the end of the socketpair contained in the request.
  5. The renderer wakes up and continues.

The browser side of the processing occurs in chrome/browser/renderer_host/render_sandbox_host_linux.cc. The renderer ends could occur anywhere, but the browser side has to know about all the possible requests so that should be a good starting point.

Here is a (possibly incomplete) list of endpoints in the renderer:

localtime

content/browser/sandbox_ipc_linux.h defines HandleLocalTime which is implemented in sandbox/linux/services/libc_interceptor.cc.

Creating a shared memory segment

content/browser/sandbox_ipc_linux.h defines HandleMakeSharedMemorySegment which is implemented in content/browser/sandbox_ipc_linux.cc.