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

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Message-ID: <20151106171453.GH28254@treble.redhat.com>
Date:	Fri, 6 Nov 2015 11:14:53 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Petr Mladek <pmladek@...e.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, Nov 06, 2015 at 02:42:46PM +0100, Petr Mladek wrote:
> 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.

Even that naming is not without its problems.  For example,
set_module_ro() is false advertising -- it wouldn't change *all* module
memory to be read-only.  (It wouldn't touch the r/w data areas.)

But I don't really care what the interfaces are called.  It's really
Rusty's call.  I just stuck to the existing naming convention in the
module code with the set/unset ro_nx stuff.

-- 
Josh
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