Reply To: Security

Very good! Yes, you can see the Novel bit as implementing a writeback cache, and deferred table update (described as “lazy” in the talk) works as you suggest.

As for the missed question (with my hearing I miss quite a few), the “next call” is for explicit calls, not for interrupts, traps, or faults. The pending grant(s) are state. We once had a way to push them in the PLB and fix them later, but there were issues if they got evicted before the call, so it’s just spiller-food now.

A region descriptor cannot have both execute and portal permission, but you could create two overlapping descriptors. Which you got would be search happenstance. If you wound up looking at a portal block as code then you would not transit and would be due for an invalidInstruction fault Real Soon Now. If you wound up looking at code as a portal, and by accident you happened to pass the security check by satisfying the ids that the bits in the id fields implied, then you would transit to the turf implied by the bits in the turf field, and then try to jump to the address implied by the bits in the target field. That address would have to have execute permission, and be in fact the address of an EBB entry (or you are up for invalidInstruction again) and probably must be the address of the entry of a function with no arguments or you are up for invalidOperand because the belt contents wouldn’t match what the code expects.

So, if the OS portal-bless service screws up and does overlap two descriptors, and the bitsies are just exactly right, then you can call a function in a random service. That’s why portal-bless is in the kernel.

As for distinguishing portal from non-portal calls, the basic reason is uniformity. We wanted a single pointer representation, one you could pass on to code that did not know whether it’s a portal or not. Consider a numeric integration package, which takes a data vector and a pointer to the function to integrate. The integrator should work the same whether the function pointer is to an application function, or a portal pointer to something in a math service.