linux-kernel - Re: [PATCH v4] livepatch: introduce shadow variable API

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Date:   Fri, 18 Aug 2017 16:04:45 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Nicolai Stange <nstange@...e.de>
Cc:     Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH v4] livepatch: introduce shadow variable API

On Fri 2017-08-18 15:44:29, Nicolai Stange wrote:
> Joe Lawrence <joe.lawrence@...hat.com> writes:
> 
> <snip>
> > +
> > +/**
> > + * klp_shadow_get() - retrieve a shadow variable data pointer
> > + * @obj:	pointer to parent object
> > + * @id:		data identifier
> > + *
> > + * Return: the shadow variable data element, NULL on failure.
> > + */
> > +void *klp_shadow_get(void *obj, unsigned long id)
> > +{
> > +	struct klp_shadow *shadow;
> > +
> > +	rcu_read_lock();
> > +
> > +	hash_for_each_possible_rcu(klp_shadow_hash, shadow, node,
> > +				   (unsigned long)obj) {
> > +
> > +		if (klp_shadow_match(shadow, obj, id)) {
> > +			rcu_read_unlock();
> > +			return shadow->data;
> 
> I had to think a moment about what protects shadow from getting freed by
> a concurrent detach after that rcu_read_unlock(). Then I noticed that if
> obj and the livepatch are alive, then so is shadow, because there
> obviously hasn't been any reason to detach it.
> 
> So maybe it would be nice to have an additional comment at
> klp_shadow_detach() that it's the API user's responsibility not to use a
> shadow instance after detaching it...

Good point. In fact, it might make sense to rename the functions:

     attach -> create
     detach -> destroy

The name detach suggests that the variable is just not connected to
the parent object but that it is still accessible/usable.

Best Regards,
Petr