"arbitrary code I've installed" How do you do that if there is no facility to load and run code? That's what he's getting at, the binary installed has fixed fu

"arbitrary code I've installed" How do you do that if there is no facility to load and run code? That's what he's getting at, the binary installed has fixed functionality and has no facilities to do anything but what it's programmed to do. It may have no file system nor almost any other feature that an OS has. It likely supports network ports, but they are unlikely to allow arbitrary code execution becuase again, they will have a fixed function. The only way to do what you're talking about is replacing or modifying the binary, which means you have physical access or there is a serious breakdown in security which nothing could withstand.