linux-kernel - Re: [RFC PATCH v8 3/4] arm64: Introduce stack trace reliability checks in the unwinder

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Message-ID: <YSe50PuWM/mjNwAj@sirena.org.uk>
Date:   Thu, 26 Aug 2021 16:57:04 +0100
From:   Mark Brown <broonie@...nel.org>
To:     madvenka@...ux.microsoft.com
Cc:     mark.rutland@....com, jpoimboe@...hat.com, ardb@...nel.org,
        nobuta.keiya@...itsu.com, sjitindarsingh@...il.com,
        catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
        pasha.tatashin@...een.com, jthierry@...hat.com,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v8 3/4] arm64: Introduce stack trace reliability
 checks in the unwinder

On Thu, Aug 12, 2021 at 02:06:02PM -0500, madvenka@...ux.microsoft.com wrote:

> +	if (frame->need_reliable && !unwind_is_reliable(frame)) {
> +		/* Cannot unwind to the next frame reliably. */
> +		frame->failed = true;
> +		return false;
> +	}

This means we only collect reliability information in the case
where we're specifically doing a reliable stacktrace.  For
example when printing stack traces on the console it might be
useful to print a ? or something if the frame is unreliable as a
hint to the reader that the information might be misleading.
Could we therefore change the flag here to a reliability one and
our need_reliable check so that we always run
unwind_is_reliable()?

I'm not sure if we need to abandon the trace on first error when
doing a reliable trace but I can see it's a bit safer so perhaps
better to do so.  If we don't abandon then we don't require the
need_reliable check at all.

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