linux-kernel - Re: [PATCH 1/3] livepatch: introduce shadow variable API

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Message-ID: <20170609183425.qn4n45mwqewxavmb@treble>
Date:   Fri, 9 Jun 2017 13:34:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH 1/3] livepatch: introduce shadow variable API

On Fri, Jun 09, 2017 at 11:36:27AM -0400, Joe Lawrence wrote:
> On 06/08/2017 12:49 PM, Josh Poimboeuf wrote:
> > On Thu, Jun 01, 2017 at 02:25:24PM -0400, Joe Lawrence wrote:
> >> Add three exported API for livepatch modules:
> >>
> >>   void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data);
> >>   void klp_shadow_detach(void *obj, char *var);
> >>   void *klp_shadow_get(void *obj, char *var);
> >>
> >> that implement "shadow" variables, which allow callers to associate new
> >> shadow fields to existing data structures.
> >>
> >> Signed-off-by: Joe Lawrence <joe.lawrence@...hat.com>
> > 
> > Overall the patch looks good to me.  It's a simple API but we've found
> > it to be very useful for certain patches.
> > 
> > One comment below:
> > 
> >> +void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data)
> >> +{
> >> +	unsigned long flags;
> >> +	struct klp_shadow *shadow;
> >> +
> >> +	shadow = kmalloc(sizeof(*shadow), gfp);
> >> +	if (!shadow)
> >> +		return NULL;
> >> +
> >> +	shadow->obj = obj;
> >> +
> >> +	shadow->var = kstrdup(var, gfp);
> >> +	if (!shadow->var) {
> >> +		kfree(shadow);
> >> +		return NULL;
> >> +	}
> >> +
> >> +	shadow->data = data;
> >> +
> >> +	spin_lock_irqsave(&klp_shadow_lock, flags);
> >> +	hash_add_rcu(klp_shadow_hash, &shadow->node, (unsigned long)obj);
> >> +	spin_unlock_irqrestore(&klp_shadow_lock, flags);
> >> +
> >> +	return shadow->data;
> >> +}
> >> +EXPORT_SYMBOL_GPL(klp_shadow_attach);
> > 
> > I wonder if we should worry about people misusing the API by calling
> > klp_shadow_attach() for a shadow variable that already exists.  Maybe we
> > should add a check and return NULL if it already exists.
> > 
> 
> I don't think the API (the shadow or the underlying hash table calls)
> currently protects against double-adds...  adding a check to do so would
> probably need to occur with the klp_shadow_lock to protect against
> concurrent detach calls.
> 
> I could implement this protection in a v2, or leave it up to the caller.
>  What do you think?

Yeah, I don't have a strong opinion either way.  It's fine with me to
leave it as it is and trust the patch author not to mess it up.

-- 
Josh