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

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Message-ID: <20191016095143.GA3818@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Oct 2019 11:51:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>, 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, jpoimboe@...hat.com,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Wed, Oct 16, 2019 at 11:36:10AM +0200, Jessica Yu wrote:
> +++ Peter Zijlstra [16/10/19 11:23 +0200]:
> > On Wed, Oct 16, 2019 at 08:51:27AM +0200, Miroslav Benes wrote:
> > > On Tue, 15 Oct 2019, Joe Lawrence wrote:
> > > 
> > > > On 10/15/19 10:13 AM, Miroslav Benes wrote:
> > > > > Yes, it does. klp_module_coming() calls module_disable_ro() on all
> > > > > patching modules which patch the coming module in order to call
> > > > > apply_relocate_add(). New (patching) code for a module can be relocated
> > > > > only when the relevant module is loaded.
> > > >
> > > > FWIW, would the LPC blue-sky2 model (ie, Steve's suggestion @ plumber's where
> > > > livepatches only patch a single object and updates are kept on disk to handle
> > > > coming module updates as they are loaded) eliminate those outstanding
> > > > relocations and the need to perform this late permission flipping?
> > > 
> > > Yes, it should, but we don't have to wait for it. PeterZ proposed a
> > > different solution to this specific issue in
> > > https://lore.kernel.org/lkml/20191015141111.GP2359@hirez.programming.kicks-ass.net/
> > > 
> > > It should not be a problem to create a live patch module like that and the
> > > code in kernel/livepatch/ is almost ready. Something like
> > > module_section_disable_ro(mod, section) (and similar for X protection)
> > > should be enough. Module reloads would still require juggling with the
> > > protections, but I think it is all feasible.
> > 
> > Just had a browse around the module code, and while the section info is
> > in load_info, it doesn't get retained for active modules.
> > 
> > So I suppose I'll go add that for KLP enabled things.
> 
> Elf section info does get saved for livepatch modules though, see
> mod->klp_info. And wouldn't this only be needed for livepatch modules?

Right, I just found that, but it is x86 only for some mysterious reason.
And yes, it's KLP only.

I was thikning of adding a KLP only list of {name,addr,size} sections
that start with ".klp" in layout_sections(). Would that not work across
architectures?