Devtmpfs and permissions

The permissions of /dev/null and /dev/zero are not policy: you can't sanely change them. /dev/null has its permissions specified by POSIX (as do /dev/tty and /dev/console, which are similarly magical), and lots of critical things including the dynamic linker rely on /dev/zero in particular (thanks to mmap() of /dev/zero being one way to get anonymous memory).