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

    /Jarkko

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