Re: [PATCH 4/5] selftests/sgx: Ensure expected enclave data buffer size and placement.

[Huang, Kai]

On Mon, 2023-07-24 at 18:58 +0200, Jo Van Bulck wrote:
> Do not declare the enclave data buffer static to ensure it is not optimized
> away by the compiler, even when not used entirely by the test enclave code.

The "encl_buffer" array is initialized to 1 explicitly, which means it should be
in .data section.  As Jarkko commented, can you add more information about in
what cases it can be optimized away?

> Use -fPIE to make the compiler access the non-static buffer with
> RIP-relative addressing. 
> 

I am not quite following how does "RIP-relative addressing" fix any problem?  Is
there any hard relationship between "RIP-relative addressing" and the problem
that you are having?

> Place the enclave data buffer in a separate
> section that is explicitly placed at the start of the .data segment in the
> linker script, as expected by the external tests manipulating page
> permissions.

So this change is not to fix the problem that "the compiler may optimize away
the encl_buffer array", correct?

> 
> Signed-off-by: Jo Van Bulck <jo.vanbulck@xxxxxxxxxxxxxx>
> ---
>  tools/testing/selftests/sgx/Makefile      | 2 +-
>  tools/testing/selftests/sgx/test_encl.c   | 5 +++--
>  tools/testing/selftests/sgx/test_encl.lds | 1 +
>  3 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/sgx/Makefile b/tools/testing/selftests/sgx/Makefile
> index 50aab6b57da3..c5483445ba28 100644
> --- a/tools/testing/selftests/sgx/Makefile
> +++ b/tools/testing/selftests/sgx/Makefile
> @@ -13,7 +13,7 @@ endif
>  
>  INCLUDES := -I$(top_srcdir)/tools/include
>  HOST_CFLAGS := -Wall -Werror -g $(INCLUDES) -fPIC -z noexecstack
> -ENCL_CFLAGS := -Wall -Werror -static -nostdlib -nostartfiles -fPIC \
> +ENCL_CFLAGS := -Wall -Werror -static -nostdlib -nostartfiles -fPIE \
>  	       -fno-stack-protector -mrdrnd $(INCLUDES)
>  
>  TEST_CUSTOM_PROGS := $(OUTPUT)/test_sgx
> diff --git a/tools/testing/selftests/sgx/test_encl.c b/tools/testing/selftests/sgx/test_encl.c
> index aba301abefb8..5c274e517d13 100644
> --- a/tools/testing/selftests/sgx/test_encl.c
> +++ b/tools/testing/selftests/sgx/test_encl.c
> @@ -7,9 +7,10 @@
>  /*
>   * Data buffer spanning two pages that will be placed first in .data
>   * segment. Even if not used internally the second page is needed by
> - * external test manipulating page permissions.
> + * external test manipulating page permissions. Do not declare this
> + * buffer as static, so the compiler cannot optimize it out.
>   */
> -static uint8_t encl_buffer[8192] = { 1 };
> +uint8_t __attribute__((section(".data.encl_buffer"))) encl_buffer[8192];
>  
>  enum sgx_enclu_function {
>  	EACCEPT = 0x5,
> diff --git a/tools/testing/selftests/sgx/test_encl.lds b/tools/testing/selftests/sgx/test_encl.lds
> index ca659db2a534..79b1e41d8d24 100644
> --- a/tools/testing/selftests/sgx/test_encl.lds
> +++ b/tools/testing/selftests/sgx/test_encl.lds
> @@ -24,6 +24,7 @@ SECTIONS
>  	} : text
>  
>  	.data : {
> +		*(.data.encl_buffer)
>  		*(.data*)
>  	} : data
>