Re: [RFC PATCH v5 1/2] arm64: Introduce stack trace reliability checks in the unwinder

On 6/25/21 10:51 AM, Mark Brown wrote:

On Fri, Jun 25, 2021 at 10:39:57AM -0500, Madhavan T. Venkataraman wrote:

quoted

On 6/24/21 9:40 AM, Mark Rutland wrote:

quoted

quoted

At a high-level, I'm on-board with keeping track of this per unwind
step, but if we do that then I want to be abel to use this during
regular unwinds (e.g. so that we can have a backtrace idicate when a
step is not reliable, like x86 does with '?'), and to do that we need to
be a little more accurate.

quoted

The only consumer of frame->reliable is livepatch. So, in retrospect, my
original per-frame reliability flag was an overkill. I was just trying to
provide extra per-frame debug information which is not really a requirement
for livepatch.

It's not a requirement for livepatch but if it's there a per frame
reliability flag would have other uses - for example Mark has mentioned
the way x86 prints a ? next to unreliable entries in oops output for
example, that'd be handy for people debugging issues and would have the
added bonus of ensuring that there's more constant and widespread
exercising of the reliability stuff than if it's just used for livepatch
which is a bit niche.

I agree. That is why I introduced the per-frame flag.

So, let us try a different approach.

First, let us get rid of the frame->reliable flag from this patch series. That flag
can be implemented when all of the pieces are in place for per-frame debug and tracking.

For consumers such as livepatch that don't really care about per-frame stuff, let us
solve it more cleanly via the return value of unwind_frame().

Currently, the return value from unwind_frame() is a tri-state return value which is
somewhat confusing.

	0	means continue unwinding
	-error	means stop unwinding. However,
			-ENOENT means successful termination
			Other values mean an error has happened.

Instead, let unwind_frame() return one of 3 values:

enum {
	UNWIND_CONTINUE,
	UNWIND_CONTINUE_WITH_ERRORS,
	UNWIND_STOP,
};

All consumers will stop unwinding upon seeing UNWIND_STOP.

Livepatch type consumers will stop unwinding upon seeing anything other than UNWIND_CONTINUE.

Debug type consumers can choose to continue upon seeing UNWIND_CONTINUE_WITH_ERRORS.

When we eventually implement per-frame stuff, debug consumers can examine the
frame for more information when they see UNWIND_CONTINUE_WITH_ERRORS.

This way, my patch series does not have a dependency on the per-frame enhancements.

quoted

So, let us separate the two. I will rename frame->reliable to frame->livepatch_safe.
This will apply to the whole stacktrace and not to every frame.

I'd rather keep it as reliable, even with only the livepatch usage I
think it's clearer.

See suggestion above.

quoted

Finally, it might be a good idea to perform reliability checks even in
start_backtrace() so we don't assume that the starting frame is reliable even
if the caller passes livepatch_safe=true. What do you think?

That makes sense to me.

Thanks.

Madhavan

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