ftrace: Fix deadloop caused by direct call in ftrace selftest

After direct call is enabled for arm64, ftrace selftest enters a
dead loop:

<trace_selftest_dynamic_test_func>:
00  bti     c
01  mov     x9, x30                            <trace_direct_tramp>:
02  bl      <trace_direct_tramp>    ---------->     ret
                                                     |
                                         lr/x30 is 03, return to 03
                                                     |
03  mov     w0, #0x0   <-----------------------------|
     |                                               |
     |                   dead loop!                  |
     |                                               |
04  ret   ---- lr/x30 is still 03, go back to 03 ----|

The reason is that when the direct caller trace_direct_tramp() returns
to the patched function trace_selftest_dynamic_test_func(), lr is still
the address after the instrumented instruction in the patched function,
so when the patched function exits, it returns to itself!

To fix this issue, we need to restore lr before trace_direct_tramp()
exits, so make trace_direct_tramp() a weak symbol and rewrite it for
arm64.

To detect this issue directly, call DYN_FTRACE_TEST_NAME() before
register_ftrace_graph().

Reported-by: Li Huafei <lihuafei1@huawei.com>
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>

Author: Xu Kuohai

arch/arm64/kernel/entry-ftrace.S

@@ -357,3 +357,13 @@ SYM_CODE_START(return_to_handler)
 	ret
 SYM_CODE_END(return_to_handler)
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
+
+#ifdef CONFIG_FTRACE_SELFTEST
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+SYM_FUNC_START(trace_direct_tramp)
+	mov	x10, x30
+	mov	x30, x9
+	ret	x10
+SYM_FUNC_END(trace_direct_tramp)
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
+#endif /* CONFIG_FTRACE_SELFTEST */

kernel/trace/trace_selftest.c

@@ -785,7 +785,7 @@ static struct fgraph_ops fgraph_ops __initdata  = {
 };
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
-noinline __noclone static void trace_direct_tramp(void) { }
+void __weak trace_direct_tramp(void) { }
 #endif
 
 /*
@@ -868,6 +868,8 @@ trace_selftest_startup_function_graph(struct tracer *trace,
 	if (ret)
 		goto out;
 
+	DYN_FTRACE_TEST_NAME();
+
 	ret = register_ftrace_graph(&fgraph_ops);
 	if (ret) {
 		warn_failed_init_tracer(trace, ret);