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

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Message-ID: <alpine.LNX.2.00.1511131445550.15073@pobox.suse.cz>
Date:	Fri, 13 Nov 2015 14:51:27 +0100 (CET)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
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: livepatch: reuse module loader code to write relocations

On Thu, 12 Nov 2015, Josh Poimboeuf wrote:

> 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.

Ok, it seems so.

> 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().

It was just my need to be efficient and I think it would have made sense 
with only one dynrela section per object. An array of indices is "ugly" so 
I am all for iteration over all the sections in 
klp_write_object_relocations().

Miroslav
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