[PATCH v11 4/5] arm64: Introduce stack trace reliability checks in the unwinder

[Mark Brown]

On Tue, Nov 23, 2021 at 01:37:22PM -0600, madvenka at linux.microsoft.com wrote:

> Introduce arch_stack_walk_reliable() for ARM64. This works like
> arch_stack_walk() except that it returns -EINVAL if the stack trace is not
> reliable.

> Until all the reliability checks are in place, arch_stack_walk_reliable()
> may not be used by livepatch. But it may be used by debug and test code.

Probably also worth noting that this doesn't select
HAVE_RELIABLE_STACKTRACE which is what any actual users are going to use
to identify if the architecture has the feature.  I would have been
tempted to add arch_stack_walk() as a separate patch but equally having
the user code there (even if it itself can't yet be used...) helps with
reviewing the actual unwinder so I don't mind.

> +static void unwind_check_reliability(struct task_struct *task,
> +				     struct stackframe *frame)
> +{
> +	if (frame->fp == (unsigned long)task_pt_regs(task)->stackframe) {
> +		/* Final frame; no more unwind, no need to check reliability */
> +		return;
> +	}

If the unwinder carries on for some reason (the code for that is
elsewhere and may be updated separately...) then this will start
checking again.  I'm not sure if this is a *problem* as such but the
thing about this being the final frame coupled with not actually
explicitly stopping the unwind here makes me think this should at least
be clearer, the comment begs the question about what happens if
something decides it is not in fact the final frame.
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