Re: [PATCH 2/5] selftests/sgx: Fix function pointer relocation in test enclave.

["Huang, Kai" <kai.huang@intel.com>]

On Mon, 2023-07-24 at 18:58 +0200, Jo Van Bulck wrote:
> Relocate encl_op_array entries at runtime relative to the enclave base to
> ensure correct function pointer when compiling the test enclave with -Os.

Putting aside whether we should consider building the selftests using "-Os", it
would be helpful to explain how can the "-Os" break the existing code, so that
we can review why this fix is reasonable.  Perhaps it's obvious to others but
it's not obvious to me what can go wrong here. 

> 
> Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>
> ---
>  tools/testing/selftests/sgx/test_encl.c           | 6 ++++--
>  tools/testing/selftests/sgx/test_encl.lds         | 1 +
>  tools/testing/selftests/sgx/test_encl_bootstrap.S | 5 +++++
>  3 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/sgx/test_encl.c b/tools/testing/selftests/sgx/test_encl.c
> index c0d6397295e3..4e31a6c3d673 100644
> --- a/tools/testing/selftests/sgx/test_encl.c
> +++ b/tools/testing/selftests/sgx/test_encl.c
> @@ -119,9 +119,11 @@ static void do_encl_op_nop(void *_op)
>  
>  }
>  
> +uint64_t get_enclave_base(void);
> +
>  void encl_body(void *rdi,  void *rsi)
>  {
> -	const void (*encl_op_array[ENCL_OP_MAX])(void *) = {
> +	static void (*encl_op_array[ENCL_OP_MAX])(void *) = {
>  		do_encl_op_put_to_buf,
>  		do_encl_op_get_from_buf,
>  		do_encl_op_put_to_addr,
> @@ -135,5 +137,5 @@ void encl_body(void *rdi,  void *rsi)
>  	struct encl_op_header *op = (struct encl_op_header *)rdi;
>  
>  	if (op->type < ENCL_OP_MAX)
> -		(*encl_op_array[op->type])(op);
> +		(*(get_enclave_base() + encl_op_array[op->type]))(op);
>  }
> diff --git a/tools/testing/selftests/sgx/test_encl.lds b/tools/testing/selftests/sgx/test_encl.lds
> index a1ec64f7d91f..ca659db2a534 100644
> --- a/tools/testing/selftests/sgx/test_encl.lds
> +++ b/tools/testing/selftests/sgx/test_encl.lds
> @@ -10,6 +10,7 @@ PHDRS
>  SECTIONS
>  {
>  	. = 0;
> +        __enclave_base = .;
>  	.tcs : {
>  		*(.tcs*)
>  	} : tcs
> diff --git a/tools/testing/selftests/sgx/test_encl_bootstrap.S b/tools/testing/selftests/sgx/test_encl_bootstrap.S
> index 03ae0f57e29d..6126dbd7ad1c 100644
> --- a/tools/testing/selftests/sgx/test_encl_bootstrap.S
> +++ b/tools/testing/selftests/sgx/test_encl_bootstrap.S
> @@ -86,6 +86,11 @@ encl_entry_core:
>  	mov	$4, %rax
>  	enclu
>  
> +	.global get_enclave_base
> +get_enclave_base:
> +	lea __enclave_base(%rip), %rax
> +	ret
> +
>  	.section ".data", "aw"
>  
>  encl_ssa_tcs1: