linux-kernel - Re: [RFC PATCH v6 3/3] arm64: Create a list of SYM_CODE functions, check return PC against list

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Message-ID: <20210729145210.GP4670@sirena.org.uk>
Date:   Thu, 29 Jul 2021 15:52:10 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     Mark Rutland <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 v6 3/3] arm64: Create a list of SYM_CODE functions,
 check return PC against list

On Thu, Jul 29, 2021 at 09:06:26AM -0500, Madhavan T. Venkataraman wrote:
> On 7/28/21 12:25 PM, Mark Rutland wrote:
> > On Wed, Jun 30, 2021 at 05:33:56PM -0500, madvenka@...ux.microsoft.com wrote:

> > Since some of the above is speculative (e.g. the bit about optprobes),
> > and as code will change over time, I think we should have a much terser
> > comment, e.g.

> > 	/*
> > 	 * As SYM_CODE functions don't follow the usual calling
> > 	 * conventions, we assume by default that any SYM_CODE function
> > 	 * cannot be unwound reliably.
> > 	 *
> > 	 * Note that this includes exception entry/return sequences and
> > 	 * trampoline for ftrace and kprobes.
> > 	 */

> Just to confirm, are you suggesting that I remove the entire large comment
> detailing the various cases and replace the whole thing with the terse comment?
> I did the large comment because of Mark Brown's input that we must be verbose
> about all the cases so that it is clear in the future what the different
> cases are and how we handle them in this code. As the code evolves, the comments
> would evolve.

I do agree with Mark that this has probably gone from one extreme to the
other and could be cut back a lot - originally it didn't reference there
being complicated cases like the trampoline at all IIRC so you needed
external knowledge to figure out that those cases were handled.

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