[PATCH 6/8] selftests/sgx: Ensure expected enclave data buffer size and placement

[Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>]

Ensure the compiler respects the size and placement of encl_buffer as
expected by the external tests manipulating page permissions:

1. Declare encl_buffer as global, in order to ensure that it is not
   optimized away by the compiler, even when not used entirely by the test
   enclave code.

2. Place encl_buffer in a separate section that is explicitly placed
   at the start of the .data segment in the linker script to avoid the
   compiler placing it somewhere else in .data.

Link: https://lore.kernel.org/all/a2732938-f3db-a0af-3d68-a18060f66e79@cs.kuleuven.be/
Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>
---
 tools/testing/selftests/sgx/test_encl.c   | 9 +++++----
 tools/testing/selftests/sgx/test_encl.lds | 1 +
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/sgx/test_encl.c b/tools/testing/selftests/sgx/test_encl.c
index 5b758eaf808c..02a9e8c55e82 100644
--- a/tools/testing/selftests/sgx/test_encl.c
+++ b/tools/testing/selftests/sgx/test_encl.c
@@ -5,11 +5,12 @@
 #include "defines.h"
 
 /*
- * 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.
+ * Data buffer spanning two pages that will be placed first in the .data
+ * segment via the linker script. Even if not used internally the second page
+ * is needed by external test manipulating page permissions, so do not declare
+ * encl_buffer as static to make sure it is entirely preserved by the compiler.
  */
-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 27c2527ecbc4..2ec29340ba94 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
 
-- 
2.34.1