Re: [PATCH 1/2] arm64: Implement arch_stack_walk_reliable

On Tue, Mar 18, 2025 at 04:38:20PM -0700, Song Liu wrote:
> On Tue, Mar 18, 2025 at 4:00 PM Josh Poimboeuf <[email protected]> wrote:
> >   - even in the -ENOENT case the unreliable bit has already been set
> >     right before the call to kunwind_next_frame_record_meta().
> 
> For this one, do you mean we set state->common.unreliable, but
> failed to propagate it to data.unreliable?

Hm, I hadn't noticed that.  That code is quite the maze.

It's unfortunate there are two separate 'unreliable' variables.  It
looks like consume_state() is the only way they get synced?

How does that work if kunwind_next() returns an error and skips
consume_state()?  Or if kunwind_recover_return_address() returns an
error to kunwind_next()?

What I actually meant was the following:

  do_kunwind()
    kunwind_next()
      kunwind_next_frame_record()
        state->common.unreliable = true;
	kunwind_next_frame_record_meta()
	  return -ENOENT;

Notice that in the success case (-ENOENT), unreliable has already been
set.

Actually I think it would be much simpler to just propagate -ENOENT down
the call chain.  Then no 'unreliable' bits needed.

Like so (instead of original patch):

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index c9fe3e7566a6..5713fad567c5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -276,6 +276,7 @@ config ARM64
 	select HAVE_SOFTIRQ_ON_OWN_STACK
 	select USER_STACKTRACE_SUPPORT
 	select VDSO_GETRANDOM
+	select HAVE_RELIABLE_STACKTRACE
 	help
 	  ARM 64-bit (AArch64) Linux support.
 
@@ -2509,4 +2510,3 @@ endmenu # "CPU Power Management"
 source "drivers/acpi/Kconfig"
 
 source "arch/arm64/kvm/Kconfig"
-
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 1d9d51d7627f..e227da842bc3 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -277,22 +277,28 @@ kunwind_next(struct kunwind_state *state)
 
 typedef bool (*kunwind_consume_fn)(const struct kunwind_state *state, void *cookie);
 
-static __always_inline void
+static __always_inline int
 do_kunwind(struct kunwind_state *state, kunwind_consume_fn consume_state,
 	   void *cookie)
 {
-	if (kunwind_recover_return_address(state))
-		return;
+	int ret;
+
+	ret = kunwind_recover_return_address(state);
+	if (ret)
+		return ret;
 
 	while (1) {
 		int ret;
 
 		if (!consume_state(state, cookie))
-			break;
+			return -EINVAL;
+
 		ret = kunwind_next(state);
-		if (ret < 0)
-			break;
+		if (ret)
+			return ret;
 	}
+
+	return -EINVAL;
 }
 
 /*
@@ -324,7 +330,7 @@ do_kunwind(struct kunwind_state *state, kunwind_consume_fn consume_state,
 			: stackinfo_get_unknown();		\
 	})
 
-static __always_inline void
+static __always_inline int
 kunwind_stack_walk(kunwind_consume_fn consume_state,
 		   void *cookie, struct task_struct *task,
 		   struct pt_regs *regs)
@@ -352,7 +358,7 @@ kunwind_stack_walk(kunwind_consume_fn consume_state,
 
 	if (regs) {
 		if (task != current)
-			return;
+			return -EINVAL;
 		kunwind_init_from_regs(&state, regs);
 	} else if (task == current) {
 		kunwind_init_from_caller(&state);
@@ -360,7 +366,7 @@ kunwind_stack_walk(kunwind_consume_fn consume_state,
 		kunwind_init_from_task(&state, task);
 	}
 
-	do_kunwind(&state, consume_state, cookie);
+	return do_kunwind(&state, consume_state, cookie);
 }
 
 struct kunwind_consume_entry_data {
@@ -387,6 +393,25 @@ noinline noinstr void arch_stack_walk(stack_trace_consume_fn consume_entry,
 	kunwind_stack_walk(arch_kunwind_consume_entry, &data, task, regs);
 }
 
+noinline noinstr int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
+			void *cookie, struct task_struct *task)
+{
+	int ret;
+	struct kunwind_consume_entry_data data = {
+		.consume_entry = consume_entry,
+		.cookie = cookie,
+	};
+
+	ret = kunwind_stack_walk(arch_kunwind_consume_entry, &data, task, NULL);
+	if (ret) {
+		if (ret == -ENOENT)
+			return 0;
+		return ret;
+	}
+
+	return -EINVAL;
+}
+
 struct bpf_unwind_consume_entry_data {
 	bool (*consume_entry)(void *cookie, u64 ip, u64 sp, u64 fp);
 	void *cookie;