Function graph tracer modifies a return address (LR) in a stack frame
to hook a function return. This will result in many useless entries
(return_to_handler) showing up in a stack tracer's output.

This patch replaces such entries with originals values preserved in
current->ret_stack[].

Signed-off-by: AKASHI Takahiro <redacted>
---
arch/arm64/include/asm/ftrace.h |    2 ++
arch/arm64/kernel/stacktrace.c  |   21 +++++++++++++++++++++
2 files changed, 23 insertions(+)

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 2b43e20..b7d597c 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h

@@ -29,6 +29,8 @@ struct dyn_arch_ftrace {

extern unsigned long ftrace_graph_call;

+extern void return_to_handler(void);
+
static inline unsigned long ftrace_call_adjust(unsigned long addr)
{
	/*

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index bc0689a..631c49d 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c

@@ -17,6 +17,7 @@
 */

#include <linux/kernel.h>
#include <linux/export.h>
+#include <linux/ftrace.h>
#include <linux/sched.h>
#include <linux/stacktrace.h>

@@ -78,6 +79,9 @@ struct stack_trace_data {

	struct stack_trace *trace;
	unsigned int no_sched_functions;
	unsigned int skip;
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	unsigned int ret_stack_index;
+#endif
};

static int save_trace(struct stackframe *frame, void *d)

@@ -86,6 +90,20 @@ static int save_trace(struct stackframe *frame, void *d)

	struct stack_trace *trace = data->trace;
	unsigned long addr = frame->pc;

+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	if (addr == (unsigned long)return_to_handler - AARCH64_INSN_SIZE) {
+		/*
+		 * This is a case where function graph tracer has
+		 * modified a return address (LR) in a stack frame
+		 * to hook a function return.
+		 * So replace it to an original value.
+		 */
+		frame->pc = addr =
+			current->ret_stack[data->ret_stack_index--].ret
+							- AARCH64_INSN_SIZE;
+	}
+#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
+