Re: [PATCH v2 4/4] livepatch/shadow: Add garbage collection of shadow variables

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Wed, Oct 26, 2022 at 04:41:22PM -0300, Marcos Paulo de Souza wrote:
> The life of shadow variables is not completely trivial to maintain.
> They might be used by more livepatches and more livepatched objects.
> They should stay as long as there is any user.
> 
> In practice, it requires to implement reference counting in callbacks
> of all users. They should register all the user and remove the shadow
> variables only when there is no user left.
> 
> This patch hides the reference counting into the klp_shadow API.
> The counter is connected with the shadow variable @id. It requires
> an API to take and release the reference. The release function also
> calls the related dtor() when defined.
> 
> An easy solution would be to add some get_ref()/put_ref() API.
> But it would need to get called from pre()/post_un() callbacks.
> It might be easy to forget a callback and make it wrong.
> 
> A more safe approach is to associate the klp_shadow_type with
> klp_objects that use the shadow variables. The livepatch core
> code might then handle the reference counters on background.
> 
> The shadow variable type might then be added into a new @shadow_types
> member of struct klp_object. They will get then automatically registered
> and unregistered when the object is being livepatched. The registration
> increments the reference count. Unregistration decreases the reference
> count. All shadow variables of the given type are freed when the reference
> count reaches zero.
> 
> All klp_shadow_alloc/get/free functions also checks whether the requested
> type is registered. It will help to catch missing registration and might
> also help to catch eventual races.

Is there a reason the shadow variable lifetime is tied to klp_object
rather than klp_patch?

I get the feeling the latter would be easier to implement (no reference
counting; also maybe can be auto-detected with THIS_MODULE?) and harder
for the patch author to mess up (by accidentally omitting an object
which uses it).

-- 
Josh