Date:	Mon, 1 Feb 2016 08:37:00 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	Miroslav Benes <mbenes@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: livepatch: Implement separate coming and going module notifiers

On Fri, Jan 29, 2016 at 05:58:29PM -0500, Jessica Yu wrote:
> +++ Josh Poimboeuf [29/01/16 13:42 -0600]:
> >On Fri, Jan 29, 2016 at 08:25:15PM +0100, Miroslav Benes wrote:
> >>On Fri, 29 Jan 2016, Josh Poimboeuf wrote:
> >>
> >>> On Fri, Jan 29, 2016 at 12:40:14PM -0500, Steven Rostedt wrote:
> >>> > [ Added Rusty, as he's still maintainer of the module code ]
> >>> >
> >>> > On Fri, 29 Jan 2016 11:30:10 -0600
> >>> > Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >>> >
> >>> > > On Fri, Jan 29, 2016 at 05:30:46PM +0100, Miroslav Benes wrote:
> >>> > > > Otherwise than that it looks good. I agree there are advantages to split
> >>> > > > the notifiers. For example we can replace the coming one with the function
> >>> > > > call somewhere in load_module() to improve error handling if the patching
> >>> > > > fails while loading a module. This would be handy with a consistency model
> >>> > > > in the future.
> >>> > >
> >>> > > Yeah, we'll need something like that eventually.  Though we'll need to
> >>> > > make sure that ftrace_module_enable() is still called beforehand, after
> >>> > > setting MODULE_STATE_COMING state, due to the race described in 5156dca.
> >>> > >
> >>> > > Something like:
> >>> > >
> >>> > > [note: klp_module_notify_coming() is replaced with klp_module_enable()]
> >>> > >
> >>> > > diff --git a/kernel/module.c b/kernel/module.c
> >>> > > index 8358f46..aeabd81 100644
> >>> > > --- a/kernel/module.c
> >>> > > +++ b/kernel/module.c
> >>> > > @@ -3371,6 +3371,13 @@ static int complete_formation(struct module *mod, struct load_info *info)
> >>> > >  	mod->state = MODULE_STATE_COMING;
> >>> > >  	mutex_unlock(&module_mutex);
> >>> > >
> >>> > > +	ftrace_module_enable(mod);
> >>> > > +	err = klp_module_enable(mod);
> >>> > > +	if (err) {
> >>> > > +		ftrace_release_mod(mod);
> >>> > > +		return err;
> >>> > > +	}
> >>> > > +
> >>> > >  	blocking_notifier_call_chain(&module_notify_list,
> >>> > >  				     MODULE_STATE_COMING, mod);
> >>> > >  	return 0;
> >>> > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> >>> > > index eca592f..c42cf37 100644
> >>> > > --- a/kernel/trace/ftrace.c
> >>> > > +++ b/kernel/trace/ftrace.c
> >>> > > @@ -5045,9 +5045,6 @@ static int ftrace_module_notify(struct notifier_block *self,
> >>> > >  	struct module *mod = data;
> >>> > >
> >>> > >  	switch (val) {
> >>> > > -	case MODULE_STATE_COMING:
> >>> > > -		ftrace_module_enable(mod);
> >>> > > -		break;
> >>> > >  	case MODULE_STATE_GOING:
> >>> > >  		ftrace_release_mod(mod);
> >>> > >  		break;
> >>> >
> >>> > If we end up doing something like this, I would just say punt and have
> >>> > the ftrace code be hardcoded into the module code and remove the
> >>> > notifiers completely. ftrace (and live kernel patching for that matter)
> >>> > are rather special. They are not a filesystem or driver. They are core
> >>> > utilities and having them called directly from the module code may be
> >>> > prudent and better to understand and control.
> >>>
> >>> Agreed, and we might as well make this change now to avoid more churn
> >>> later.
> >>
> >>It is possible to achieve the same goal even with the notifiers. They are
> >>processed synchronously in complete_formation(). So we can put our klp
> >>hook after that, right? Or better, put it to load_module() after
> >>complete_formation() call. There is an error handling code even today
> >>(that is, parse_args() or mod_sysfs_setup() can fail). Moreover, we'll
> >>have a hook there with Jessica's relocation rework patch set.
> >
> >Well, my feeling is that we should really apply livepatch relocations
> >before allowing any other notifiers to run, in case the relocations
> >affect them.  But it's just a feeling; I don't have any specific
> >examples to justify it (yet).
> 
> So what I'm gathering from this discussion is that we are leaning
> towards completely removing the ftrace notifier in favor of inserting direct
> calls to ftrace_module_enable() and ftrace_release_mod() in the
> module loader, as well as removing the livepatch coming module notifier
> and hard-coding klp_module_enable() (formerly klp_module_notify_coming).
> 
> Since we're already doing all that, might we be able to just completely remove
> the klp notifiers altogether as well, and replace klp_module_notify_going()
> with something like klp_module_disable()? This way everything is symmetrical.
> 
> Then the whole thing might look something like this -
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 8358f46..eccd289 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -979,8 +979,12 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
> 	/* Final destruction now no one is using it. */
> 	if (mod->exit != NULL)
> 		mod->exit();
> 	blocking_notifier_call_chain(&module_notify_list,
> 				     MODULE_STATE_GOING, mod);
> +	klp_module_disable(mod);
> +	ftrace_release_mod(mod);
> +
> 	async_synchronize_full();
> 
> 	/* Store the name of the last unloaded module for diagnostic purposes */
> @@ -3371,6 +3375,13 @@ static int complete_formation(struct module *mod, struct load_info *info)
> 	mod->state = MODULE_STATE_COMING;
> 	mutex_unlock(&module_mutex);
> 
> +	ftrace_module_enable(mod);
> +	err = klp_module_enable(mod); // write all relocations before calling coming notifiers
> +	if (err) {
> +		ftrace_release_mod(mod);
> +		goto out;
> +	}
> +
> 	blocking_notifier_call_chain(&module_notify_list,
> 				     MODULE_STATE_COMING, mod);
> 	return 0;
> 
> The function call ordering here should emulate the same ordering were they
> notifiers instead, with the priorities Josh suggested in the other mail.
> 
> On module load, ftrace_module_enable() is called first (as if it had priority
> INT_MAX), then klp_module_enable() (INT_MAX-1), then the rest of the coming
> notifier call chain. For the GOING part, the going call chain is called first,
> then klp_module_disable() (INT_MIN+1), then ftrace_release_mod() last (INT_MIN).
> 
> Note: There are multiple places where the GOING notifiers are called (i.e. in
> delete_module(), and in the error paths for do_init_module() and
> load_module()), but the calls would look the same there as well.
> 
> Does this all sound OK?

Sounds good to me :-)

-- 
Josh

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