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

[Song Liu]

Hi Josh,

Thanks for the review!

On Tue, Mar 18, 2025 at 11:45 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> On Fri, Mar 07, 2025 at 05:27:41PM -0800, Song Liu wrote:
> > With proper exception boundary detection, it is possible to implment
> > arch_stack_walk_reliable without sframe.
> >
> > Note that, arch_stack_walk_reliable does not guarantee getting reliable
> > stack in all scenarios. Instead, it can reliably detect when the stack
> > trace is not reliable, which is enough to provide reliable livepatching.
> >
> > This version has been inspired by Weinan Liu's patch [1].
> >
> > [1] https://lore.kernel.org/live-patching/20250127213310.2496133-7-wnliu@xxxxxxxxxx/
> > Signed-off-by: Song Liu <song@xxxxxxxxxx>
>
> This looks incomplete.  The reliable unwinder needs to be extra
> paranoid.  There are several already-checked-for errors in the unwinder
> that don't actually set the unreliable bit.
>
> There are likely other failure modes it should also be checking for.
> For example I don't see where it confirms that the unwind completed to
> the end of the stack (which is typically at a certain offset).

If I understand the comment correctly, this should be handled by the
meta data type FRAME_META_TYPE_FINAL.

>
> See for example all the error conditions in the x86 version of
> arch_stack_walk_reliable() and in arch/x86/kernel/unwind_frame.c.

I guess I missed this part:

diff --git i/arch/arm64/kernel/stacktrace.c w/arch/arm64/kernel/stacktrace.c
index 69d0567a0c38..3bb8e3ea4c4b 100644
--- i/arch/arm64/kernel/stacktrace.c
+++ w/arch/arm64/kernel/stacktrace.c
@@ -268,6 +268,8 @@ kunwind_next(struct kunwind_state *state)
        case KUNWIND_SOURCE_TASK:
        case KUNWIND_SOURCE_REGS_PC:
                err = kunwind_next_frame_record(state);
+               if (err && err != -ENOENT)
+                       state->common.unreliable = true;
                break;
        default:
                err = -EINVAL;


With this part, we should cover all these cases? Did I miss something
else?

Thanks,
Song