linux-kernel - Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability checks

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Message-ID: <20210414123548.GC4535@sirena.org.uk>
Date:   Wed, 14 Apr 2021 13:35:48 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....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,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2 0/4] arm64: Implement stack trace reliability
 checks

On Wed, Apr 14, 2021 at 05:23:38AM -0500, Madhavan T. Venkataraman wrote:
> On 4/13/21 6:02 AM, Mark Brown wrote:
> > On Mon, Apr 12, 2021 at 02:55:35PM -0500, Madhavan T. Venkataraman wrote:

> >> 3. We are going to assume that the reliable unwinder is only for livepatch purposes
> >>    and will only be invoked on a task that is not currently running. The task either
> > 
> > The reliable unwinder can also be invoked on itself.

> I have not called out the self-directed case because I am assuming that the reliable unwinder
> is only used for livepatch. So, AFAICT, this is applicable to the task that performs the
> livepatch operation itself. In this case, there should be no unreliable functions on the
> self-directed stack trace (otherwise, livepatching would always fail).

Someone might've added a probe of some kind which upsets things so
there's a possibility things might fail.  Like you say there's no way a
system in such a state can succesfully apply a live patch but we might
still run into that situation.

> >> I suggest we do (3) first. Then, review the assembly functions to do (1). Then, review the
> >> remaining ones to see which ones must be blacklisted, if any.

> > I'm not clear what the concrete steps you're planning to do first are
> > there - your 3 seems like a statement of assumptions.  For flagging
> > functions I do think it'd be safer to default to assuming that all
> > SYM_CODE functions can't be unwound reliably rather than only explicitly
> > listing ones that cause problems.

> They are not assumptions. They are true statements. But I probably did not do a good
> job of explaining. But Josh sent out a patch that updates the documentation that
> explains what I said a lot better.

You say true statements, I say assumptions :)

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