[PATCH] arm64: ftrace: use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR

[Mark Rutland <mark.rutland@arm.com>]

When CONFIG_FUNCTION_GRAPH_TRACER is selected, and the function graph:
tracer is in use, unwind_frame() may erroneously asscociate a traced
function with an incorrect return address. This can happen when starting
an unwind from a pt_regs, or when unwinding across an exception
boundary.

The underlying problem is that ftrace_graph_get_ret_stack() takes an
index offset from the most recent entry added to the fgraph return
stack. We start an unwind at offset 0, and increment the offset each
time we encounter a rewritten return address (i.e. when we see
`return_to_handler`). This is broken in two cases:

1) Between creating a pt_regs and starting the unwind, function calls
   may place entries on the stack, leaving an abitrary offset which we
   can only determine by performing a full unwind from the caller of the
   unwind code (and relying on none of the unwind code being
   instrumented). While this initial unwind is open-coded in
   dump_backtrace(), this is not performed for other unwinders such as
   perf_callchain_kernel().

2) When unwinding across an exception boundary (whether continuing an
   unwind or starting a new unwind from regs), we currently always skip
   the LR of the interrupted context. Where this was live and contained
   a rewritten address, we won't consume the corresponding fgraph ret
   stack entry, leaving subsequent entries off-by-one.

To fix this, we need to be able to uniquely identify each rewritten
return address such that we can map this back to the original reeturn
address. We can do so using HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, and
associating each rewritten return address with a unique location on the
stack. As the return address is passed in the LR (and so is not
guaranteed a unique location in memory), we use the FP upon entry to the
function (i.e. the address of the caller's frame record) as the return
address pointer. Any nested call will have a different FP value as the
caller must create its own frame record and update FP to point to this.

This doesn't fix the issue of skipping a live LR at an exception
boundary, which is a more general problem and requires more substantial
rework. Note that were we to consume the LR in all cases this would in
warnings where the interrupted context's LR contains
`return_to_handler`, but the FP has been altered, e.g.

| func:
|	<--- ftrace entry ---> 	// logs FP & LR, rewrites FP
| 	STP	FP, LR, [SP, #-16]!
| 	MOV	FP, SP
| 	<--- INTERRUPT --->

... as ftrace_graph_get_ret_stack() fill not find a matching entry,
triggering the WARN_ON_ONCE() in unwind_frame().

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/include/asm/ftrace.h     | 11 +++++++++++
 arch/arm64/include/asm/stacktrace.h |  6 ------
 arch/arm64/kernel/ftrace.c          |  6 +++---
 arch/arm64/kernel/stacktrace.c      | 12 +++++-------
 4 files changed, 19 insertions(+), 16 deletions(-)

I spotted this latent issue when reviewing Madhavan's stack tracing rework:

  https://lore.kernel.org/r/20211015025847.17694-1-madvenka@linux.microsoft.com

This supercedes the initial version I posted as an inline reply in:

  https://lore.kernel.org/r/20211025164925.GB2001@C02TD0UTHF1T.local

I believe we will need this as a prerequisite for other cleanups in that
series. As it fixes an existing bug I'm posting it on its own.

Mark.

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 91fa4baa1a93..c96d47cb8f46 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -12,6 +12,17 @@
 
 #define HAVE_FUNCTION_GRAPH_FP_TEST
 
+/*
+ * HAVE_FUNCTION_GRAPH_RET_ADDR_PTR means that the architecture can provide a
+ * "return address pointer" which can be used to uniquely identify a return
+ * address which has been overwritten.
+ *
+ * On arm64 we use the address of the caller's frame record, which remains the
+ * same for the lifetime of the instrumented function, unlike the return
+ * address in the LR.
+ */
+#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
+
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #else
diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 8aebc00c1718..9a319eca5cab 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -46,9 +46,6 @@ struct stack_info {
  * @prev_type:   The type of stack this frame record was on, or a synthetic
  *               value of STACK_TYPE_UNKNOWN. This is used to detect a
  *               transition from one stack to another.
- *
- * @graph:       When FUNCTION_GRAPH_TRACER is selected, holds the index of a
- *               replacement lr value in the ftrace graph stack.
  */
 struct stackframe {
 	unsigned long fp;
@@ -56,9 +53,6 @@ struct stackframe {
 	DECLARE_BITMAP(stacks_done, __NR_STACK_TYPES);
 	unsigned long prev_fp;
 	enum stack_type prev_type;
-#ifdef CONFIG_FUNCTION_GRAPH_TRACER
-	int graph;
-#endif
 };
 
 extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 7f467bd9db7a..3e5d0ed63eb7 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -249,8 +249,6 @@ int __init ftrace_dyn_arch_init(void)
  * on the way back to parent. For this purpose, this function is called
  * in _mcount() or ftrace_caller() to replace return address (*parent) on
  * the call stack to return_to_handler.
- *
- * Note that @frame_pointer is used only for sanity check later.
  */
 void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
 			   unsigned long frame_pointer)
@@ -268,8 +266,10 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
 	 */
 	old = *parent;
 
-	if (!function_graph_enter(old, self_addr, frame_pointer, NULL))
+	if (!function_graph_enter(old, self_addr, frame_pointer,
+	    (void *)frame_pointer)) {
 		*parent = return_hooker;
+	}
 }
 
 #ifdef CONFIG_DYNAMIC_FTRACE
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 8982a2b78acf..749b680b4999 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -38,9 +38,6 @@ void start_backtrace(struct stackframe *frame, unsigned long fp,
 {
 	frame->fp = fp;
 	frame->pc = pc;
-#ifdef CONFIG_FUNCTION_GRAPH_TRACER
-	frame->graph = 0;
-#endif
 
 	/*
 	 * Prime the first unwind.
@@ -116,17 +113,18 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	if (tsk->ret_stack &&
 		(ptrauth_strip_insn_pac(frame->pc) == (unsigned long)return_to_handler)) {
-		struct ftrace_ret_stack *ret_stack;
+		unsigned long orig_pc;
 		/*
 		 * 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.
 		 */
-		ret_stack = ftrace_graph_get_ret_stack(tsk, frame->graph++);
-		if (WARN_ON_ONCE(!ret_stack))
+		orig_pc = ftrace_graph_ret_addr(tsk, NULL, frame->pc,
+						(void *)frame->fp);
+		if (WARN_ON_ONCE(frame->pc == orig_pc))
 			return -EINVAL;
-		frame->pc = ret_stack->ret;
+		frame->pc = orig_pc;
 	}
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
-- 
2.11.0


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