Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable

On 3/23/21 1:27 PM, Mark Brown wrote:

On Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote:

quoted

On 3/23/21 12:02 PM, Mark Rutland wrote:

quoted

quoted

3. Figure out exception boundary handling. I'm currently working to
   simplify the entry assembly down to a uniform set of stubs, and I'd
   prefer to get that sorted before we teach the unwinder about
   exception boundaries, as it'll be significantly simpler to reason
   about and won't end up clashing with the rework.

quoted

So, here is where I still have a question. Is it necessary for the unwinder
to know the exception boundaries? Is it not enough if it knows if there are
exceptions present? For instance, using something like num_special_frames
I suggested above?

For reliable stack trace we can live with just flagging things as
unreliable when we know there's an exception boundary somewhere but (as
Mark mentioned elsewhere) being able to actually go through a subset of
exception boundaries safely is likely to help usefully improve the
performance of live patching, and for defensiveness we want to try to
detect during an actual unwind anyway so it ends up being a performance
improvment and double check rather than saving us code.  Better
understanding of what's going on in the presence of exceptions may also
help other users of the unwinder which can use stacks which aren't
reliable get better results.

Actually, I was not suggesting that the counter replace the unwinder
intelligence to recognize exception boundaries. I was only suggesting
the use of the counter for arch_stack_walk_reliable().

But I am fine with not implementing the counter for now.

Madhavan