Re: [PATCH v36 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Thu, Aug 06, 2020 at 10:55:43AM -0400, Nathaniel McCallum wrote:
> In a past revision of this patch, I had requested a void *misc
> parameter that could be passed through vdso_sgx_enter_enclave_t into
> sgx_enclave_exit_handler_t. This request encountered some push back
> and I dropped the issue. However, I'd like to revisit it or something
> similar.
> 
> One way to create a generic interface to SGX is to pass a structure
> that captures the relevant CPU state from the handler so that it can
> be evaluated in C code before reentry. Precedent for this approach can
> be found in struct kvm_run[0]. Currently, however, there is no way to
> pass a pointer to such a structure directly into the handler.
> 
> This can be done implicitly by wrapping the struct
> sgx_enclave_exception in another structure and then using techniques
> like container_of() to find another field. However, this is made more
> difficult by the fact that the sgx_enclave_exit_handler_t is not
> really using the x86_64 sysv calling convention. Therefore, the
> sgx_enclave_exit_handler_t MUST be written in assembly. This also
> implies that we can't use techniques like container_of() and must
> calculate all the offsets manually, which is tedious, error prone and
> fragile.

If instead of having a callback, kernel would run an optional bpf
program, then a state structure could passed by the kernel to the
execution context of the bpf program.

That would also sort out difficulty of writing exit handlers.

/Jarkko