On Fri, Mar 13, 2020 at 02:32:29PM -0400, Nathaniel McCallum wrote: > On Fri, Mar 13, 2020 at 12:46 PM Sean Christopherson > <sean.j.christopherson@xxxxxxxxx> wrote: > > > > On Fri, Mar 13, 2020 at 11:48:54AM -0400, Nathaniel McCallum wrote: > > > Therefore, I'd like to propose that __vdso_sgx_enter_enclave(): > > > * Preserve %rbx. > > > > At first glance, that looks sane. Being able to call __vdso... from C > > would certainly be nice. > > Agreed. I think ergonomically we want __vdso...() to be called from C > and the handler to be implemented in asm (optionally); without > breaking the ability to call __vdso..() from asm in special cases. > > I think all ergonomic issues get solved by the following: > * Pass a void * into the handler from C through __vdso...(). > * Allow the handler to pop parameters off of the output stack without hacks. > > This allows the handler to pop extra arguments off the stack and write > them into the memory at the void *. Then the handler can be very small > and pass logic back to the caller of __vdso...(). > > Here's what this all means for the enclave. For maximum usability, the > enclave should preserve all callee-saved registers (except %rbx, which > is preserved by __vdso..()). For each ABI rule that the enclave > breaks, you need logic in a handler to fix it. So if you push return > params on the stack, the handler needs to undo that. Or the untrusted runtime needs to wrap the __vdso() to save state that is clobbered by the enclave. Just want to make it crystal clear that using a handler is only required for stack shenanigans. > This doesn't compromise the ability to treat __vsdo...() like ENCLU if > you need the full power. But it does make it significantly easier to > consume when you don't have special needs. So as I see it, __vdso...() > should: > > 1. preserve %rbx > 2. take leaf in %rcx > 3. gain a void* stack param which is passed to the handler Unless I'm misunderstanding the request, this already exists. %rsp at the time of EEXIT is passed to the handler. > 4. sub/add to %rsp rather than save/restore Can you elaborate on why you want to sub/add to %rsp instead of having the enclave unwind the stack? Preserving %rsp across EEXIT/ERESUME seems more in line with function call semantics, which I assume is desirable? E.g. push param3 push param2 push param1 enclu[EEXIT] add $0x18, %rsp > That would make this a very usable and fast interface without > sacrificing any of its current power.
