Comment by gpderetta

I don't think that Linux capabilities have much to do with the capabilities that the OP intends.

In a capabilities system, a program has permission to act on any object if it has a reference (aka a capability) to the object, there is no other access control. A program acquires a capability either by receiving it from is parent (or caller in the case of a function) or some other way like message passing. There is no other source of capabilities and they are unforgeable.

Unix file descriptors act in many ways as capabilities: they are inherited by processes from their parents and can be passed around via Unix sockets, and grant to the FD holder the same permissions to the referenced object as the creator of the file descriptor.

Of course as Unix has other ways from creating file descriptors other than inheritance and message passing is not truly a capabilities system.