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

[Josh Poimboeuf]

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).

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

-- 
Josh