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Date:   Mon, 21 Oct 2019 10:31:01 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Jessica Yu <jeyu@kernel.org>, Miroslav Benes <mbenes@suse.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joe Lawrence <joe.lawrence@redhat.com>, x86@kernel.org,
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        bristot@redhat.com, jbaron@akamai.com,
        torvalds@linux-foundation.org, tglx@linutronix.de,
        mingo@kernel.org, namit@vmware.com, hpa@zytor.com, luto@kernel.org,
        ard.biesheuvel@linaro.org, live-patching@vger.kernel.org
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()
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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 talki=
ng
> > > about. If we teach module loader to relocate (and apply alternatives =
and
> > > so on, everything in arch-specific module_finalize()) not the whole m=
odule
> > > in case of live patch modules, but separate ELF .text sections, it co=
uld
> > > 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 mod=
ule
> > > is loaded, its .text section in all present patch module would be
> > > processed.
> > >=20
> > > The upside is that almost no work would be required on patch modules
> > > creation side. The downside is that klp_modinfo must stay. Module loa=
der
> > > needs to be hacked a lot in both cases. So it remains to be seen whic=
h
> > > idea is easier to implement.
> > >=20
> > > Jessica, do you think it would be feasible?
> >=20
> > 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()).
> >=20
> > So maybe something like the following?
> >=20
> > 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 mo=
dules
> >    module_enable_ro()
> >        klp hook: only enable ro/x for .klp.text.$objname for already
> >        loaded modules
>=20
> Just for record. We should also set ro for the not-yet used
> .klp.text.$objname at this stage so that it can't be modified
> easily "by accident".
>=20
>=20
> > 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
> >=20
> > 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.
> >=20
> > Does that sound like what you had in mind or am I totally off?
>=20
> Makes sense to me.
>=20
> 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.

--=20
Josh