If we use CopyFileExW in copy2(), then also copystat() and copymode() should be able to copy the same metadata/security-bits/etc as CopyFileExW. I don't know which Windows APIs should be used though.

I sort of agree with Steven that CopyFileExW could be used everywhere (meaning copyfile()), but that basically means breaking backward compatibility re. what is promised in the doc, and am not sure how to deal with that. For that reason it's probably it's better to leave copyfile() alone. 

On macOS it seems we can use fcopyfile(3) syscall (which is already exposed) and its flag argument (COPYFILE_METADATA, COPYFILE_DATA. COPYFILE_XATTR, etc,) to implement both copy2() and copystat() / copymode().