linux-kernel - Re: refactoring livepatch documentation was Re: [PATCH 1/2] docs/livepatch: Add new compiler considerations doc

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Date:   Wed, 2 Sep 2020 16:00:55 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Joe Lawrence <joe.lawrence@...hat.com>
cc:     Petr Mladek <pmladek@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        nstange@...e.de
Subject: Re: refactoring livepatch documentation was Re: [PATCH 1/2]
 docs/livepatch: Add new compiler considerations doc

[side note: So not only that my INBOX is a mess after the summer. I also 
lost some emails apparently. I'm really sorry about that. ]

CCing Nicolai too.

> Hi Petr, Josh,
> 
> The compiler optimization pitfall document can wait for refactored livepatch
> documentation if that puts it into better context, particularly for newbies.
> I don't mind either way.  FWIW, I don't profess to be an authoritative source
> its content -- we've dealt some of these issues in kpatch, so it was
> interesting to see how they affect livepatches that don't rely on binary
> comparison.
> 
> 
> Toward the larger goal, I've changed the thread subject to talk about how we
> may rearrange and supplement our current documentation.  This is a first pass
> at a possible refactoring...
> 
> 
> 1. Provide a better index page to connect the other files/docs, like
> https://www.kernel.org/doc/html/latest/core-api/index.html but obviously not
> that extensive.  Right now we have only a Table of Contents tree without any
> commentary.
> 
> 2. Rearrange and refactor sections:
> 
> livepatch.rst
>   Keep just about everything
>   Add a history section to explain ksplice, kgraft, kpatch for the
>     uninitiated?
>   Add a section on source based vs. binary diff livepatch creation,
>     this may be worth its own top-level section
> 
> Livepatch API
>   Basic API
>   Callbacks
>   Shadow variables
>   Cumulative patches
>   System state
> 
> KLP Relocations
>   Right now this is a bit academic AFAIK kpatch is the only tool
>   currently making use of them.  So maybe this document becomes a
>   more general purpose doc explaining how to reference unexported
>   symbols?  (ie, how does kgraft currently do it, particularly
>   w/kallsyms going unexported?)

Yes, we rely on kallsyms_lookup_name() pretty much right now and once we 
hit the problem with the next kernel version upgrade, we'll have to fix 
it.
 
>   Eventually this could contain klp-convert howto if it ever gets
>   merged.
> 
> Compiler considerations
>   TBD
> 
> I suppose this doesn't create a "Livepatching creation for dummies" guide, but
> my feeling is that there are so many potential (hidden) pitfalls that such
> guide would be dangerous.

It does not create the guide, but it looks like a good basis. I agree with 
Josh here. It might be difficult at the beginning, but the outcome could 
be great even for a newbie and I think we should aim for that.
 
> If someone were to ask me today how to start building a livepatch, I would
> probably point them at the samples to demonstrate the basic concept and API,
> but then implore them to read through the documentation to understand how
> quickly complicated it can become.

True.

We discuss the need to properly document our internal process every once 
in a while and there is always something more important to deal with, but 
it is high time to finally start with that.

Miroslav