selftests/sgx: Ensure test enclave buffer is entirely preserved

Attach the "used" attribute to instruct the compiler to preserve the static
encl_buffer, even if it appears it is not entirely referenced in the enclave
code, as expected by the external tests manipulating page permissions.
Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/all/a2732938-f3db-a0af-3d68-a18060f66e79@cs.kuleuven.be/
Link: https://lore.kernel.org/all/20231005153854.25566-11-jo.vanbulck%40cs.kuleuven.be

tools/testing/selftests/sgx/defines.h

@@ -13,6 +13,7 @@
 
 #define __aligned(x) __attribute__((__aligned__(x)))
 #define __packed __attribute__((packed))
+#define __used __attribute__((used))
 
 #include "../../../../arch/x86/include/asm/sgx.h"
 #include "../../../../arch/x86/include/asm/enclu.h"

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. Even if not used internally the second page is needed by external
+ * test manipulating page permissions, so mark encl_buffer as "used" to make
+ * sure it is entirely preserved by the compiler.
  */
-static uint8_t encl_buffer[8192] = { 1 };
+static uint8_t __used encl_buffer[8192] = { 1 };
 
 enum sgx_enclu_function {
 	EACCEPT = 0x5,