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 rewrite a dedicated trace_direct_tramp() for arm64.

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

Author: Xu Kuohai

arch/arm64/include/asm/ftrace.h

@@ -126,6 +126,16 @@ static inline bool arch_syscall_match_sym_name(const char *sym,
 	 */
 	return !strcmp(sym + 8, name);
 }
+
+#ifdef CONFIG_FTRACE_SELFTEST
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+
+#define trace_direct_tramp trace_direct_tramp
+extern void trace_direct_tramp(void);
+
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
+#endif /* CONFIG_FTRACE_SELFTEST */
+
 #endif /* ifndef __ASSEMBLY__ */
 
 #endif /* __ASM_FTRACE_H */

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

@@ -785,8 +785,10 @@ static struct fgraph_ops fgraph_ops __initdata  = {
 };
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+#ifndef trace_direct_tramp
 noinline __noclone static void trace_direct_tramp(void) { }
 #endif
+#endif
 
 /*
  * Pretty much the same than for the function tracer from which the selftest