linux-kernel - Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal

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Message-ID: <alpine.LSU.2.21.1909051445410.25712@pobox.suse.cz>
Date:   Thu, 5 Sep 2019 14:49:00 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>, jikos@...nel.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module
 removal

On Thu, 5 Sep 2019, Josh Poimboeuf wrote:

> On Thu, Sep 05, 2019 at 02:03:34PM +0200, Miroslav Benes wrote:
> > > >   + I would like to better understand the scope of the current
> > > >     problems. It is about modifying code in the livepatch that
> > > >     depends on position of the related code:
> > > > 
> > > >       + relocations are rather clear; we will need them anyway
> > > > 	to access non-public (static) API from the original code.
> > > > 
> > > >       + What are the other changes?
> > > 
> > > I think the .klp.arch sections are the big ones:
> > > 
> > >   .klp.arch.altinstructions
> > >   .klp.arch.parainstructions
> > >   .klp.arch.jump_labels (doesn't exist yet)
> > > 
> > > And that's just x86...
> > 
> > I may misunderstand, but we have .klp.arch sections because para and 
> > alternatives have to be processed after relocations. And if we cannot get 
> > rid of relocations completely, because of static symbols, then we cannot 
> > get rid of .klp.arch sections either.
> 
> With late module patching gone, the module code can just process the klp
> relocations at the same time it processes normal relocations.
> 
> Then the normal module alt/para/jump_label processing code can be used
> instead of arch_klp_init_object_loaded().

Ah, of course. I obviously cannot grasp the idea of not having late module 
patching :)
 
> Note this also means that Joe's patches can remove copy_module_elf() and
> free_module_elf().  And module_arch_freeing_init() in s390.

Correct.

So yes, it would simplify the code a lot. I am still worried about the 
consequences.

> > > And then of course there's the klp coming/going notifiers which have
> > > also been an additional source of complexity.
> > 
> > True, but I think we (me and Petr) do not consider it as much of a problem 
> > as you.
> 
> It's less of an issue than .klp.arch and all the related code which can
> be removed.

Ok.

Miroslav