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

[Joe Lawrence]

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@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.
> 

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?

-- Joe