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

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Message-ID: <alpine.LSU.2.21.1910151611000.13169@pobox.suse.cz>
Date:   Tue, 15 Oct 2019 16:13:41 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Jessica Yu <jeyu@...nel.org>, 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()

[ live-patching ML CCed ]

On Tue, 15 Oct 2019, Peter Zijlstra wrote:

> On Tue, Oct 15, 2019 at 03:07:40PM +0200, Jessica Yu wrote:
> 
> > > The fact that it is executable; also the fact that you do it right after
> > > we mark it ro. Flipping the memory protections back and forth is just
> > > really poor everything.
> > > 
> > > Once this ftrace thing is sorted, we'll change x86 to _refuse_ to make
> > > executable (kernel) memory writable.
> > 
> > Not sure if relevant, but just thought I'd clarify: IIRC,
> > klp_module_coming() is not poking the coming module, but it calls
> > module_enable_ro() on itself (the livepatch module) so it can apply
> > relocations and such on the new code, which lives inside the livepatch
> > module, and it needs to possibly do this numerous times over the
> > lifetime of the patch module for any coming module it is responsible
> > for patching (i.e., call module_enable_ro() on the patch module, not
> > necessarily the coming module). So I am not be sure why
> > klp_module_coming() should be moved before complete_formation(). I
> > hope I'm remembering the details correctly, livepatch folks feel free
> > to chime in if I'm incorrect here.
> 
> You mean it does module_disable_ro() ? That would be broken and it needs
> to be fixed. Can some livepatch person explain what it does and why?

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.

Miroslav