Consideration is currently going into Linux to add sendfile file-to-file semantics, which would be basically equivalent to a copyfile system call. The major problem is that, if you were to copy a 2GiB file, for example, it would freeze the system as the kernel took up all available processing time doing the copy. So, copyfile system calls would tend to expose a local DoS attack, because they happen in a place where execution can't normally be interrupted to allow for multitasking.