[PATCH 6/8] ftrace: Fix dead loop caused by direct call in ftrace selftest

[Florent Revest]

From: Xu Kuohai <xukuohai at huawei.com>

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 use a dedicated trace_direct_tramp() for arm64.

Reported-by: Li Huafei <lihuafei1 at huawei.com>
Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
Acked-by: Steven Rostedt (Google) <rostedt at goodmis.org>
Signed-off-by: Florent Revest <revest at chromium.org>
---
 arch/arm64/include/asm/ftrace.h  |  7 +++++++
 arch/arm64/kernel/entry-ftrace.S | 10 ++++++++++
 kernel/trace/trace_selftest.c    |  2 ++
 3 files changed, 19 insertions(+)

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 1c2672bbbf37..cf6d9c42ff36 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -168,6 +168,13 @@ static inline bool arch_syscall_match_sym_name(const char *sym,
 	 */
 	return !strcmp(sym + 8, name);
 }
+
+#if defined(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS) && \
+    defined(CONFIG_FTRACE_SELFTEST)
+extern void ftrace_dummy_tramp(void);
+#define trace_direct_tramp ftrace_dummy_tramp
+#endif
+
 #endif /* ifndef __ASSEMBLY__ */
 
 #endif /* __ASM_FTRACE_H */
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
index 350ed81324ac..9869debd22fb 100644
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S
@@ -118,6 +118,16 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
 	ret	x9
 SYM_CODE_END(ftrace_caller)
 
+#if defined(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS) && \
+    defined(CONFIG_FTRACE_SELFTEST)
+SYM_CODE_START(ftrace_dummy_tramp)
+	bti c
+	mov x10, x30
+	mov x30, x9
+	ret x10
+SYM_CODE_END(ftrace_dummy_tramp)
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
+
 #else /* CONFIG_DYNAMIC_FTRACE_WITH_ARGS */
 
 /*
diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 06218fc9374b..f9f5d4e8ab50 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -789,11 +789,13 @@ static struct fgraph_ops fgraph_ops __initdata  = {
 #define CALL_DEPTH_ACCOUNT ""
 #endif
 
+#ifndef trace_direct_tramp
 noinline __noclone static void trace_direct_tramp(void)
 {
 	asm(CALL_DEPTH_ACCOUNT);
 }
 #endif
+#endif
 
 /*
  * Pretty much the same than for the function tracer from which the selftest
-- 
2.39.1.519.gcb327c4b5f-goog