Fix stack_redzone test to exit before actual program corruption

Change-Id: I644511ecec8626fe3bdb833bf6bae930fd0fced9

test/helpers/hwtest_common.py

@@ -101,6 +101,5 @@ def get_special_binaries(dls_version: str) -> Set[PpuHwTest]:
             expected_exit_code=stack_protection * -559038737),
         PpuHwTest(
             join(TEST_BINARY_PATH, f"test_stack_redzone_{dls_version}.binary"),
-            expected_exit_code=stack_redzone * 12,
-            expect_timeout=not stack_redzone)
+            expected_exit_code=12 if stack_redzone else 2)
     }

test/test_stack_redzone.cpp

+#include <stdint.h>
+
+#include "libnux/helper.h"
+
+extern uint8_t stack_redzone;
+
 /*
     The stack grows infinitely. With '-fstack-limit-symbol=stack_redzone',
     the program exits with error code 12 (= ENOMEM), whereas without this option,
-    it doesn't finish, since the program memory region gets partially overwritten.
+    it wouldn't finish, since the program memory region would get partially overwritten, which
+    is however detected beforehand and a dedicated return value 2 is generated.
 */
-int start(void)
+int32_t start(void)
 {
-	__attribute__((unused)) int volatile i = 5;
-	start();
-	return 0;
+	uintptr_t stack_ptr;
+	// return 2 if stack redzone does not work, which is defined here as the stack pointer is
+	// smaller equal than the stack_redzone symbol location.
+	// clang-format off
+	asm volatile (
+		// load stack pointer
+		"mr %[stack_ptr], 1\n"
+		: [stack_ptr] "=r" (stack_ptr) ::
+	);
+	// clang-format on
+	if (stack_ptr < uintptr_t(&stack_redzone)) {
+		return 2;
+	}
+	int x = 5;
+	do_not_optimize_away(x);
+	return start();
 }