Re: Capabilities, Anybody?

BGB <cr88192@gmail.com> wrote:

Presumably, in addition to the code, one needs some way for the code to
be able to access its own ".data" and ".bss" sections when called.

AIUI you derive a capability from PCC (the PC capability) that gives you
access to your local 'captable', which then holds pointers to your other
objects.  The captable can be read-only but the capabilities inside it can
be writable (ie pointers allow you to write to your globals etc).

Some options:
   PC-relative:
     Unclear if valid in this case.
   GOT:
     Table of pointers to things, loaded somehow.
     One example here being the ELF FDPIC ABI.
   Reloading a Global Pointer via a lookup table accessed via itself.
     This is what my ABI uses...
 
I couldn't seem to find any technical descriptions of the CHERI/Morello
ABI. I had made a guess that it might work similar to FDPIC, as this
could be implemented without needing to use raw addresses (and seemed
like a "best fit").

This is a description of the linkage model for CHERI MIPS; I'm not aware of
anything having changed significantly for RISC-V or Morello, although exact
usage of registers etc will be different.

https://www.cl.cam.ac.uk/research/security/ctsrd/pdfs/20190113-cheri-linkage.pdf

This also describes the OS-facing ABI on CheriBSD:
https://www.cl.cam.ac.uk/research/security/ctsrd/pdfs/201904-asplos-cheriabi.pdf

Theo