linux-kernel - Re: [PATCH v3 5/6] x86/ftrace: Use text

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Date:   Mon, 21 Oct 2019 10:31:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jessica Yu <jeyu@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Lawrence <joe.lawrence@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, live-patching@...r.kernel.org
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Fri, Oct 18, 2019 at 03:40:58PM +0200, Petr Mladek wrote:
> On Fri 2019-10-18 15:03:42, Jessica Yu wrote:
> > +++ Miroslav Benes [16/10/19 15:29 +0200]:
> > > On Wed, 16 Oct 2019, Miroslav Benes wrote:
> > > Thinking about it more... crazy idea. I think we could leverage these new
> > > ELF .text per vmlinux/module sections for the reinvention I was talking
> > > about. If we teach module loader to relocate (and apply alternatives and
> > > so on, everything in arch-specific module_finalize()) not the whole module
> > > in case of live patch modules, but separate ELF .text sections, it could
> > > solve the issue with late module patching we have. It is a variation on
> > > Steven's idea. When live patch module is loaded, only its section for
> > > present modules would be processed. Then whenever a to-be-patched module
> > > is loaded, its .text section in all present patch module would be
> > > processed.
> > > 
> > > The upside is that almost no work would be required on patch modules
> > > creation side. The downside is that klp_modinfo must stay. Module loader
> > > needs to be hacked a lot in both cases. So it remains to be seen which
> > > idea is easier to implement.
> > > 
> > > Jessica, do you think it would be feasible?
> > 
> > I think that does sound feasible. I'm trying to visualize how that
> > would look. I guess there would need to be various livepatching hooks
> > called during the different stages (apply_relocate_add(),
> > module_finalize(), module_enable_ro/x()).
> > 
> > So maybe something like the following?
> > 
> > When a livepatch module loads:
> >    apply_relocate_add()
> >        klp hook: apply .klp.rela.$objname relocations *only* for
> >        already loaded modules
> >    module_finalize()
> >        klp hook: apply .klp.arch.$objname changes for already loaded modules
> >    module_enable_ro()
> >        klp hook: only enable ro/x for .klp.text.$objname for already
> >        loaded modules
> 
> Just for record. We should also set ro for the not-yet used
> .klp.text.$objname at this stage so that it can't be modified
> easily "by accident".
> 
> 
> > When a to-be-patched module loads:
> >    apply_relocate_add()
> >        klp hook: for each patch module that patches the coming
> >        module, apply .klp.rela.$objname relocations for this object
> >    module_finalize()
> >        klp hook: for each patch module that patches the coming
> >        module, apply .klp.arch.$objname changes for this object
> >    module_enable_ro()
> >        klp hook: for each patch module, apply ro/x permissions for
> >        .klp.text.$objname for this object
> > 
> > Then, in klp_module_coming, we only need to do the callbacks and
> > enable the patch, and get rid of the module_disable_ro->apply
> > relocs->module_enable_ro block.
> > 
> > Does that sound like what you had in mind or am I totally off?
> 
> Makes sense to me.
> 
> Well, I wonder if it is really any better from what we have now.

AFAICT, this would still have a lot of the same problems we have today.
It has a lot of complexity.  It needs arch-specific livepatch code and
sections, and introduces special cases in the module code.

I'd much prefer the proposal from LPC to have per-module live patches.
It's simpler and has less things that can go wrong IMO.

-- 
Josh