linux-kernel - Re: [PATCH v3] Documentation: livepatch: document reliable stacktrace

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Message-ID: <20210115151609.lqrl2yuy2zvrcm47@treble>
Date:   Fri, 15 Jan 2021 09:16:32 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>, linux-doc@...r.kernel.org,
        live-patching@...r.kernel.org, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3] Documentation: livepatch: document reliable stacktrace

On Fri, Jan 15, 2021 at 02:24:46PM +0000, Mark Brown wrote:
> From: Mark Rutland <mark.rutland@....com>
> 
> Add documentation for reliable stacktrace. This is intended to describe
> the semantics and to be an aid for implementing architecture support for
> HAVE_RELIABLE_STACKTRACE.
> 
> Unwinding is a subtle area, and architectures vary greatly in both
> implementation and the set of concerns that affect them, so I've tried
> to avoid making this too specific to any given architecture. I've used
> examples from both x86_64 and arm64 to explain corner cases in more
> detail, but I've tried to keep the descriptions sufficient for those who
> are unfamiliar with the particular architecture.
> 
> I've tried to give rationale for all the recommendations/requirements,
> since that makes it easier to spot nearby issues, or when a check
> happens to catch a few things at once. I believe what I have written is
> sound, but as some of this was reverse-engineered I may have missed
> things worth noting.
> 
> I've made a few assumptions about preferred behaviour, notably:
> 
> * If you can reliably unwind through exceptions, you should (as x86_64
>   does).
> 
> * It's fine to omit ftrace_return_to_handler and other return
>   trampolines so long as these are not subject to patching and the
>   original return address is reported. Most architectures do this for
>   ftrace_return_handler, but not other return trampolines.
> 
> * For cases where link register unreliability could result in duplicate
>   entries in the trace or an inverted trace, I've assumed this should be
>   treated as unreliable. This specific case shouldn't matter to
>   livepatching, but I assume that that we want a reliable trace to have
>   the correct order.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Jiri Kosina <jikos@...nel.org>
> Cc: Joe Lawrence <joe.lawrence@...hat.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Mark Brown <broonie@...nel.org>
> Cc: Miroslav Benes <mbenes@...e.cz>
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: linux-doc@...rt.kernel.org
> Cc: live-patching@...r.kernel.org
> [Updates following review -- broonie]
> Signed-off-by: Mark Brown <broonie@...nel.org>
> ---
> 
> v3:
>  - Incorporated objtool section from Mark.
>  - Deleted confusing notes about using annotations.

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh