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

[Nathaniel McCallum]

That seems like overkill to me. I'm just asking for one additional mov
instruction. :)

On Tue, Aug 18, 2020 at 11:06 AM Jarkko Sakkinen
<jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 18, 2020 at 05:52:41PM +0300, Jarkko Sakkinen wrote:
> > On Mon, Aug 10, 2020 at 03:23:17PM -0700, Sean Christopherson wrote:
> > > > 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.
> > >
> > > What bits of the x86-64 ABI require writing the handler in assembly?  There
> > > are certainly restrictions on what the handler can do without needing an
> > > assembly trampoline, but I was under the impression that vanilla C code is
> > > compatible with the exit handler patch.  Is Rust more picky about calling
> > > convention?
> > >
> > > Side topic, the documentation for vdso_sgx_enter_enclave_t is wrong, it
> > > states the EFLAGS.DF is not cleared before invoking the handler, but that's
> > > a lie.
> >
> > If handler requires the use of setjmp/longjmp API for sudden exits, that
> > is considered bad even with C++, as it is not compatible with stack
> > unwinding. The handler has a lot of constraints for its environment, and
> > is somewhat unappealing to use.
> >
> > That's why I started today thinking a possibility of using a bpf program
> > as a middle-man. BPF programs can be used to execute code by the kernel
> > in behalf of user in a domain defined sandbox. The execution context is
> > just a buffer passed in R1 to the BPF interpreter. It can be defined by
> > application.
>
> Something like
>
> 1. An exception is triggered.
> 2. Kernel executes an eBPF program behalf of the caller, if one was
>    given.
> 3. vDSO calls a fixed exit handler that based on the outcome calls
>    ERESUME/EENTER.
>
> Possibly an ioctl could be used to attach an eBPF program to an
> enclave and vDSO would only get a context struct.
>
> /Jarkko
>