linux-kernel - Re: [PATCH v2 3/3] livepatch: Cleanup module page permission changes

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Date:	Fri, 6 Nov 2015 14:42:46 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	"Cyril B." <cbay@...aysdata.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH v2 3/3] livepatch: Cleanup module page permission changes

On Fri 2015-11-06 06:12:47, Josh Poimboeuf wrote:
> On Fri, Nov 06, 2015 at 11:40:55AM +0100, Petr Mladek wrote:
> > On Thu 2015-11-05 15:18:05, Josh Poimboeuf wrote:
> > > Calling set_memory_rw() and set_memory_ro() for every iteration of the
> > > loop in klp_write_object_relocations() is messy, inefficient, and
> > > error-prone.
> > > 
> > > Change all the read-only pages to read-write before the loop and convert
> > > them back to read-only again afterwards.
> > > 
> > > The {un}set_module_core_ro_nx() functions are used to change the
> > > page permissions.  Toggling NX isn't necessary in this case, but it's
> > > not highly performance sensitive code so it should be fine.
> > 
> > Hmm, the name (un)set_module_core_ro_nx() still sounds a bit strange,
> > especially the "ro_nx" suffix.
> 
> > Alternative solution would be to create
> > 
> >    set_module_text_rw()
> >    set_module_text_ro()
> > 
> > There already exists
> > 
> >    set_all_modules_text_rw()
> >    set_all_modules_text_ro()
> > 
> > They modify only the ro/rw flags. IMHO, the name is more descriptive
> > They are used by ftrace for very similar purpose.
> 
> That wouldn't be enough.  Relocations can occur not only in text, but
> also in data.  That includes read-only data.

I see. This just shows how this all is confusing. Or maybe I am just
dumb :-)

> The (un)set_module_core_ro_nx() naming was taken from the names of
> existing module functions (unset_module_{core,init}_ro_nx()).  They
> enable/disable the CONFIG_DEBUG_SET_MODULE_RONX feature on the core part
> of the module.  The name makes sense to me, though I'm certainly open to
> other ideas.

I think that we should not mix

   set_*_ro()
   set_*_rw()

with

   set_*_ro*()
   unset_*_ro*()

naming schemes. What about adding into the public API?

  set_module_ro()
  set_module_rw()

It should modify everything: init, core, text, and data but only
the ro/rw flags.

Sigh, we went quite far from the few lines patch :-/

> > They modify also the init section. But we might want to touch it
> > as well. klp_module_notify() is called too late now. But once we
> > have a more complex consistency model, we will need to reject
> > the module when the patching fails. We will need to call the
> > livepatch init earlier, close to ftrace_module_init(mod).
> > Then the init section might be interesting as well.
> 
> Init section functions don't have the __fentry() call, so they can't be
> patched.  If that were to change in the future, we could use the
> (un)set_module_init_ro_nx() functions, which already exist.

I see.

Best Regards,
Petr
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