linux-kernel - Re: [RFC PATCH 0/5] Arch-independent livepatch

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Date:	Wed, 11 Nov 2015 10:28:10 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Jessica Yu <jeyu@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] Arch-independent livepatch

On Wed, Nov 11, 2015 at 03:00:44PM +0100, Miroslav Benes wrote:
> > Background: Why does livepatch need to write its own relocations?
> > ==
> > A typical livepatch module contains patched versions of functions that can
> > reference non-exported global symbols and non-included local symbols.
> > Relocations referencing these types of symbols cannot be left in as-is
> > since the kernel module loader cannot resolve them and will therefore
> > reject the livepatch module. Furthermore, we cannot apply relocations that
> > affect modules not loaded yet at run time (e.g. a patch to a driver). The
> > current kpatch build system therefore solves this problem by embedding
> > special "dynrela" (dynamic reloc) sections in the resulting patch module
> > elf output. Using these dynrela sections, livepatch can correctly resolve
> > symbols while taking into account its scope and what module the symbol
> > belongs to, and then manually apply the dynamic relocations.
> 
> I'll only add that we solve the problem with kallsyms calls in kGraft. It 
> can get really cumbersome from time to time, so this work would simplify 
> our effort as well.

I haven't yet reviewed the code in-depth, but overall I think this patch
set is a good idea.  It simplifies livepatch itself as well as the patch
creation process (for both kGraft and kpatch-build), so it's a win-win.

-- 
Josh
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