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
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
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:
SCM_RIGHTScontrol message containing one end of the fresh socket pair.
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:
content/browser/sandbox_ipc_linux.h defines HandleLocalTime which is implemented in
content/browser/sandbox_ipc_linux.h defines HandleMakeSharedMemorySegment which is implemented in