Hi sel4-devs, As I had mentioned in the last developer call, I am porting seL4 to morello architecture. To be exact, I plan to run a hybrid kernel with hybrid userspace. The hybrid mode essentially means that you define CHERI capability pointers explicitly, but if not stated, everything is just a standard 64bit pointer.
Based on browsing the code, I have jotted down the following list of items I need to change. 1,2 are just observations, and 3 is where I need some input.
1. Change the type of "registers" field in TCB in the kernel to store 128 bits instead of 64 bits per reg. This ties into the kernel_enter and restore_user_context code. It ensures that if a user app explicitly uses CHERI capabilities, it is not lost on a context switch.[1,2] - Done.
2. Change the syscall and IPC APIs to allow for passing CHERI caps. CHERIcaps are just 128 bits, so we should be able to pass them as arguments in IPC. The use case I have in mind is as follows - Imagine a mmap server. This server would return a VA to the caller, which in the CHERI world would be a CHERI capability. This mmap-server would allocate the memory and map it into the callee's virtual address space. Do you think this is a valid use case?
3. Update seL4_UserContext code to read and write newer regs from the user space. The seL4_UserContext struct will need to be updated to accommodate the new register DDC(default data capability reg). It will also need to be updated to accommodate 128 bit regs instead of 64 bits. This is where I ran into an issue - an assert statement in the build directory checks the size of the seL4_UserContext structure. Looks like it gets its info from the sel4arch.xml file[4]. I am not sure how to change XML processing to have it generate the correct size value for that struct.
I have not created ifdef guard for morello yet, as this is just a hacky pass to get it working. My diff so far is available at [3] if anyone wants to take a look. Any help and suggestions on the approach would be greatly appreciated.
Thanks, Sid
[1] https://gitlab.com/arm-research/security/icecap/seL4/-/blob/morello/include/... [2] https://gitlab.com/arm-research/security/icecap/seL4/-/blob/morello/include/... [3] https://gitlab.com/arm-research/security/icecap/seL4/-/compare/pre-morello-u... [4] https://gitlab.com/arm-research/security/icecap/seL4/-/blob/morello/libsel4/...