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

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Date:   Tue, 23 Mar 2021 18:27:53 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     Mark Rutland <mark.rutland@....com>, jpoimboe@...hat.com,
        jthierry@...hat.com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a
 stack trace unreliable

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

> > 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?

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.

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