kprobes/trace: Fix kprobe selftest for newer gcc

Commit 265a5b7e ("kprobes/trace: Fix kprobe selftest for gcc 4.6")
has added __used attribute to kprobe_trace_selftest_target to ensure
that the method is listed in kallsyms table.

However, even though the method remains in the kernel image, the actual
call is optimized away as there are no side effects and the return value
is never checked.

Add a return value check and a 'noinline' attribute to ensure that an
inlined copy of the method is not used by the caller. Also add checks
that verify that the kprobe was really hit, as at the moment the tests
show positive results despite the test method being optimized away.

Finally, add __init annotations to find_trace_probe_file() and
kprobe_trace_selftest_target() as they are only called from within an
__init method.

Link: http://lkml.kernel.org/r/1481293178-3128-2-git-send-email-marcin.nowakowski@imgtec.comAcked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_kprobe.c

@@ -1361,18 +1361,18 @@ fs_initcall(init_kprobe_trace);
 
 
 #ifdef CONFIG_FTRACE_STARTUP_TEST
-
 /*
  * The "__used" keeps gcc from removing the function symbol
- * from the kallsyms table.
+ * from the kallsyms table. 'noinline' makes sure that there
+ * isn't an inlined version used by the test method below
  */
-static __used int kprobe_trace_selftest_target(int a1, int a2, int a3,
-					       int a4, int a5, int a6)
+static __used __init noinline int
+kprobe_trace_selftest_target(int a1, int a2, int a3, int a4, int a5, int a6)
 {
 	return a1 + a2 + a3 + a4 + a5 + a6;
 }
 
-static struct trace_event_file *
+static struct __init trace_event_file *
 find_trace_probe_file(struct trace_kprobe *tk, struct trace_array *tr)
 {
 	struct trace_event_file *file;
@@ -1450,12 +1450,25 @@ static __init int kprobe_trace_self_tests_init(void)
 
 	ret = target(1, 2, 3, 4, 5, 6);
 
+	/*
+	 * Not expecting an error here, the check is only to prevent the
+	 * optimizer from removing the call to target() as otherwise there
+	 * are no side-effects and the call is never performed.
+	 */
+	if (ret != 21)
+		warn++;
+
 	/* Disable trace points before removing it */
 	tk = find_trace_kprobe("testprobe", KPROBE_EVENT_SYSTEM);
 	if (WARN_ON_ONCE(tk == NULL)) {
 		pr_warn("error on getting test probe.\n");
 		warn++;
 	} else {
+		if (trace_kprobe_nhit(tk) != 1) {
+			pr_warn("incorrect number of testprobe hits\n");
+			warn++;
+		}
+
 		file = find_trace_probe_file(tk, top_trace_array());
 		if (WARN_ON_ONCE(file == NULL)) {
 			pr_warn("error on getting probe file.\n");
@@ -1469,6 +1482,11 @@ static __init int kprobe_trace_self_tests_init(void)
 		pr_warn("error on getting 2nd test probe.\n");
 		warn++;
 	} else {
+		if (trace_kprobe_nhit(tk) != 1) {
+			pr_warn("incorrect number of testprobe2 hits\n");
+			warn++;
+		}
+
 		file = find_trace_probe_file(tk, top_trace_array());
 		if (WARN_ON_ONCE(file == NULL)) {
 			pr_warn("error on getting probe file.\n");