A readonly raspberry pi with /tmp
mounted in fstab as mode=1777
had mode 0755
after boot. This caused
several programs such as dhcpcd not to work correctly.
The same issue is also desribed in http://superuser.com/questions/1103101/how-can-i-mount-a-tmpfs-to-tmp-via-fstab-writable-to-anyone and http://unix.stackexchange.com/questions/289377/fstab-doenst-read-mode-value