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

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Message-ID: <20151113071553.GE13513@packer-debian-8-amd64.digitalocean.com>
Date:	Fri, 13 Nov 2015 02:15:54 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...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

+++ Josh Poimboeuf [12/11/15 14:32 -0600]:
>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().

I was trying to accomodate Miroslav's dislike of klp_reloc_sec :-)
But now that we are sure that we need multiple __klp_rela sections
per object, I think an array of indices might still be somewhat
complicated (we need to figure out its size and everything, a list
here would be easier). I think it might be fine to leave things the
way they are here.

Jessica
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