This rejection is indeed problematic.  If mmap.mmap receives an explicit size, checking that size against stat will break on devices or special files.  It's perfectly reasonable that mmap.mmap's automatic logic when passed length=0 (to map the entire file) won't work if stat says the file has length 0, but the mmap syscall works fine on special files with length 0 (or devices), and mmap.mmap should work in that case as well.