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

[Josh Poimboeuf]

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@xxxxxxxxxx>

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.

-- 
Josh