Perhaps the most reliable way to know exactly what all a permission allows, is to write an analyzer tool that analyzes android platform sources and/or bytecode

Perhaps the most reliable way to know exactly what all a permission allows, is to write an analyzer tool that analyzes android platform sources and/or bytecode looking for calls to permission APIs, creates a list of such calling methods, and finally maps these callers to what data they affect. Not sure if the underlying native binaries too use these permissions, but if they do, they too can probably be analyzed. I wonder if such a tool already exists.