> I would prefer extending poll to support things other than file descriptors, instead of converting everything a file descriptor to be able to use poll.

Why? The ability to block on these descriptors as a one off rather than wrapping into a poll makes them extremely useful and avoids the race issues that exist with signal handlers and other non-blocking mechanisms.

signalfd, timerfd, eventfd, userfaultfd, pidfd are all great applications of this strategy.