linux-kernel - Re: livepatch: reuse module loader code to write relocations

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Message-ID: <20151112203221.GM4038@treble.hsd1.ky.comcast.net>
Date:	Thu, 12 Nov 2015 14:32:21 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	Miroslav Benes <mbenes@...e.cz>,
	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: livepatch: reuse module loader code to write relocations

On Thu, Nov 12, 2015 at 03:22:44PM -0500, Jessica Yu wrote:
> Looking into this more, I think we do need one __klp_rela section per
> function being patched.  Each rela section is linked to the section to
> which the relocations apply via the rela section's sh_info field. In
> SHT_RELA sections, the sh_info field contains the section index to
> which the relocs apply. We cannot have one single combined rela
> section per object as the call to apply_relocate_add() simply won't
> work, because we would have relocs that apply to different functions
> (and hence different sections).
> 
> So I guess instead of a single field in klp_object specifying the
> __klp_rela section index, we could probably just have an array of
> section indices.

Ok, makes sense, sounds like we need multiple klp relas per object.

I still don't quite understand the benefit of caching the klp_rela
section indices.  What problem does it solve?  It seems simpler to just
iterate over all the sections in klp_write_object_relocations().

-- 
Josh
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