[PATCH v5 10/10] LoongArch/ftrace: Fix unwind state when option func_stack_trace

[Qing Zhang]

Ftrace plays like function head exception, prologue analysis will stop soon
because PC is at entry.

   90000000004c5a54 <callee>:
   90000000004c5a54:	03400000  andi   $zero, $zero, 0x0   ==>move  t0, ra
   90000000004c5a58:	03400000  andi   $zero, $zero, 0x0   ==>bl callsite
==>90000000004c5a5c:	02fcc063  addi.d $sp, $sp, -208(0xf30)
   ...

When encountering ftrace_call, save trace function ra at PT_ERA, save
parent ra at PT_R1, At this time, pc is the position after the two nops
of callee. There is no conventional prologue operation between this
position and the function entry, so we need to reset the first flag to
make the caller continue to unwind.

testing method:
  echo path_openat > ./set_ftrace_filter
  echo 1 > ./options/func_stack_trace
  echo function > ./current_tracer

Reported-by: Jeff Xie <xiehuan09@xxxxxxxxx>
Tested-by: Jinyang He <hejinyang@xxxxxxxxxxx>
Tested-by: Jeff Xie <xiehuan09@xxxxxxxxx>
Signed-off-by: Qing Zhang <zhangqing@xxxxxxxxxxx>
---
 arch/loongarch/include/asm/unwind.h     |  2 +-
 arch/loongarch/kernel/unwind_prologue.c | 35 +++++++++++++++++++++----
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/arch/loongarch/include/asm/unwind.h b/arch/loongarch/include/asm/unwind.h
index f66b07c3e6a1..f2b52b9ea93d 100644
--- a/arch/loongarch/include/asm/unwind.h
+++ b/arch/loongarch/include/asm/unwind.h
@@ -20,7 +20,7 @@ struct unwind_state {
 	char type; /* UNWINDER_XXX */
 	struct stack_info stack_info;
 	struct task_struct *task;
-	bool first, error;
+	bool first, error, is_ftrace;
 	int graph_idx;
 	unsigned long sp, pc, ra;
 };
diff --git a/arch/loongarch/kernel/unwind_prologue.c b/arch/loongarch/kernel/unwind_prologue.c
index f77f3b6f3f06..4fb4923b68cc 100644
--- a/arch/loongarch/kernel/unwind_prologue.c
+++ b/arch/loongarch/kernel/unwind_prologue.c
@@ -14,9 +14,7 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
 
 	if (unwind_done(state))
 		return 0;
-	else if (state->type)
-		return state->pc;
-	else if (state->first)
+	else if (state->type || state->first)
 		return state->pc;
 
 	return *(unsigned long *)(state->sp);
@@ -42,16 +40,41 @@ static bool unwind_by_guess(struct unwind_state *state)
 	return false;
 }
 
+static inline void unwind_state_fixup(struct unwind_state *state)
+{
+#ifdef CONFIG_FUNCTION_TRACER
+	static unsigned long ftrace_case = (unsigned long)ftrace_call + 4;
+
+	if (state->pc == ftrace_case)
+		state->is_ftrace = true;
+#endif
+}
+
 static bool unwind_by_prologue(struct unwind_state *state)
 {
 	struct stack_info *info = &state->stack_info;
 	union loongarch_instruction *ip, *ip_end;
 	unsigned long frame_size = 0, frame_ra = -1;
 	unsigned long size, offset, pc = state->pc;
+	struct pt_regs *regs;
 
 	if (state->sp >= info->end || state->sp < info->begin)
 		return false;
 
+	if (state->is_ftrace) {
+		/*
+		 * As we meet ftrace_regs_entry, reset first flag like first doing
+		 * tracing, Prologue analysis will stop soon because PC is at entry.
+		 */
+		regs = (struct pt_regs *)state->sp;
+		state->pc = regs->csr_era;
+		state->ra = regs->regs[1];
+		state->sp = regs->regs[3];
+		state->first = true;
+		state->is_ftrace = false;
+		return true;
+	}
+
 	if (!kallsyms_lookup_size_offset(pc, &size, &offset))
 		return false;
 
@@ -97,7 +120,7 @@ static bool unwind_by_prologue(struct unwind_state *state)
 
 	state->pc = *(unsigned long *)(state->sp + frame_ra);
 	state->sp = state->sp + frame_size;
-	return !!__kernel_text_address(state->pc);
+	goto out;
 
 first:
 	state->first = false;
@@ -106,7 +129,9 @@ static bool unwind_by_prologue(struct unwind_state *state)
 
 	state->pc = state->ra;
 
-	return !!__kernel_text_address(state->ra);
+out:
+	unwind_state_fixup(state);
+	return !!__kernel_text_address(state->pc);
 }
 
 void unwind_start(struct unwind_state *state, struct task_struct *task,
-- 
2.36.1