I always assumed that doing open(/proc/self/fd/NUM, flags) was equivalent to dup(NUM), but apparently this is not the case! For example, if you dup a file descriptor, then set the new fd to non-blocking, this also affects the original file descriptor (because non-blocking state is a property of the file description, and the two file descriptors both point to the same file description). However, if you open /proc/self/fd/NUM, then you seem to get a new independent file description, and can set the non-blocking state of your old and new fds independently. You can even use this to get two file descriptions referring to the same anonymous pipe, which is otherwise impossible (example). On the other hand, while you can dup a socket fd, open("/proc/self/fd/NUM", flags) fails if NUM refers to a socket.
Now I'd like to be able to see how this works for other types of special file, and answer questions like "what permission checking is done when re-opening a file this way?", so I was trying to find the code in Linux that actually implements this path, but when I started reading fs/proc/fd.c I quickly got lost in a maze of twisty operations structs, all different.
So my question is: can anyone explain the code path followed by doing open("/proc/self/fd/NUM", flags)? For concreteness let's say that NUM refers to a pipe and we're talking about the latest kernel release.