[RFC PATCH v15 4/6] arm64: Introduce stack trace reliability checks in the unwinder

[madvenka]

From: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx>

There are some kernel features and conditions that make a stack trace
unreliable. Callers may require the unwinder to detect these cases.
E.g., livepatch.

Introduce a new function called unwind_check_reliability() that will
detect these cases and set a flag in the stack frame. Call
unwind_check_reliability() for every frame in unwind().

Introduce the first reliability check in unwind_check_reliability() - If
a return PC is not a valid kernel text address, consider the stack
trace unreliable. It could be some generated code. Other reliability checks
will be added in the future.

Let unwind() return a boolean to indicate if the stack trace is
reliable.

Signed-off-by: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Mark Brown <broonie@xxxxxxxxxx>
---
 arch/arm64/kernel/stacktrace.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index c749129aba5a..5ef2ce217324 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -44,6 +44,8 @@
  * @final_fp:	 Pointer to the final frame.
  *
  * @failed:      Unwind failed.
+ *
+ * @reliable:    Stack trace is reliable.
  */
 struct unwind_state {
 	unsigned long fp;
@@ -57,6 +59,7 @@ struct unwind_state {
 	struct task_struct *task;
 	unsigned long final_fp;
 	bool failed;
+	bool reliable;
 };
 
 static void unwind_init_common(struct unwind_state *state,
@@ -80,6 +83,7 @@ static void unwind_init_common(struct unwind_state *state,
 	state->prev_fp = 0;
 	state->prev_type = STACK_TYPE_UNKNOWN;
 	state->failed = false;
+	state->reliable = true;
 
 	/* Stack trace terminates here. */
 	state->final_fp = (unsigned long)task_pt_regs(task)->stackframe;
@@ -242,11 +246,34 @@ static void notrace unwind_next(struct unwind_state *state)
 }
 NOKPROBE_SYMBOL(unwind_next);
 
-static void notrace unwind(struct unwind_state *state,
+/*
+ * Check the stack frame for conditions that make further unwinding unreliable.
+ */
+static void unwind_check_reliability(struct unwind_state *state)
+{
+	if (state->fp == state->final_fp) {
+		/* Final frame; no more unwind, no need to check reliability */
+		return;
+	}
+
+	/*
+	 * If the PC is not a known kernel text address, then we cannot
+	 * be sure that a subsequent unwind will be reliable, as we
+	 * don't know that the code follows our unwind requirements.
+	 */
+	if (!__kernel_text_address(state->pc))
+		state->reliable = false;
+}
+
+static bool notrace unwind(struct unwind_state *state,
 			   stack_trace_consume_fn consume_entry, void *cookie)
 {
-	while (unwind_continue(state, consume_entry, cookie))
+	unwind_check_reliability(state);
+	while (unwind_continue(state, consume_entry, cookie)) {
 		unwind_next(state);
+		unwind_check_reliability(state);
+	}
+	return !state->failed && state->reliable;
 }
 NOKPROBE_SYMBOL(unwind);
 
-- 
2.25.1