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

[Mark Rutland]

On Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote:
> On 3/23/21 12:02 PM, Mark Rutland wrote:

[...]

> I think that I did a bad job of explaining what I wanted to do. It is not
> for any additional protection at all.
> 
> So, let us say we create a field in the task structure:
> 
> 	u64		unreliable_stack;
> 
> Whenever an EL1 exception is entered or FTRACE is entered and pt_regs get
> set up and pt_regs->stackframe gets chained, increment unreliable_stack.
> On exiting the above, decrement unreliable_stack.
> 
> In arch_stack_walk_reliable(), simply do this check upfront:
> 
> 	if (task->unreliable_stack)
> 		return -EINVAL;
> 
> This way, the function does not even bother unwinding the stack to find
> exception frames or checking for different return addresses or anything.
> We also don't have to worry about code being reorganized, functions
> being renamed, etc. It also may help in debugging to know if a task is
> experiencing an exception and the level of nesting, etc.

As in my other reply, since this is an optimization that is not
necessary for functional correctness, I would prefer to avoid this for
now. We can reconsider that in future if we encounter performance
problems.

Even with this there will be cases where we have to identify
non-unwindable functions explicitly (e.g. the patchable-function-entry
trampolines, where the real return address is in x9), and I'd prefer
that we use one mechanism consistently.

I suspect that in the future we'll need to unwind across exception
boundaries using metadata, and we can treat the non-unwindable metadata
in the same way.

[...]

> > 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.
> 
> 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?

I agree that it would be legitimate to bail out early if we knew there
was going to be an exception somewhere in the trace. Regardless, I think
it's simpler overall to identify non-unwindability during the trace, and
doing that during the trace aligns more closely with the structure that
we'll need to permit unwinding across these boundaries in future, so I'd
prefer we do that rather than trying to optimize for early returns
today.

Thanks,
Mark.