In a message to tech@ , Theo de Raadt ( deraadt@ ) provided some insights into ongoing work on pledge(2) :

One is execpromises. This will become the 2nd argument of pledge(). This allows one to set the pledge for the new image after pledge "exec"-allowed execve(). A warning though: utilizing this in software isn't as easy as you might think! The fork+exec + startup sequences needed to be studied quite carefully to ensure the newly-executed child doesn't ask for more than the parent's execpromises. In my experiments such a circumstance is exceedingly common, so the problem is eased by introducing a new pledge feature which allows pledge violations to return ENOSYS or such rather than killing the process. This feature also needs to be used with great caution (especially in privileged programs) because programs which fail to observe errors may continue operating forward very incorrectly; you've lost the ability to catch it failing, and provide care by fixing the problem. The other is pledgepaths. The semantics are still being tuned a bit. Before the first call to pledge() in a process, one can pledgepath() directories. Then later after pledge(), file access operations only work if the traversal of the path crosses one of those pre-declared directories (but better make sure you don't move a directory, because the kernel remembers and reasons about the vnode of the directory rather than the path). Something similar is being worked on for files, but we are still adjusting that, as well as a flag parameter for the pledgepath() call which may constrain the operations done on such files. As such, pledgepath() will become a filesystem containment mechanism unlike chroot() because paths will still be based upon true /. Patience.