This is why in UNIX-like systems, file/device permissions have to be checked using the authorisation that was in place at open() time and not at write() / read(

This is why in UNIX-like systems, file/device permissions have to be checked using the authorisation that was in place at open() time and not at write() / read() time; this allows passed or inherited file descriptors to function like capabilities in this sense. For example, when you run a setuid binary, it inherits file descriptors (stdin, stdout and stderr) that were opened by the caller - to prevent a certain class of confused deputy attacks, writes and reads on these file descriptors should not use the elevated privileges of the binary but the original privileges of the opener.