netdev - Re: [PATCH net v2 2/3] net: sched: fix action overwrite reference counting

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Date:   Wed, 7 Apr 2021 16:50:08 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...dia.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jakub Kicinski <kuba@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Davide Caratti <dcaratti@...hat.com>
Subject: Re: [PATCH net v2 2/3] net: sched: fix action overwrite reference counting

On Wed, Apr 7, 2021 at 8:36 AM Vlad Buslov <vladbu@...dia.com> wrote:
>
> Action init code increments reference counter when it changes an action.
> This is the desired behavior for cls API which needs to obtain action
> reference for every classifier that points to action. However, act API just
> needs to change the action and releases the reference before returning.
> This sequence breaks when the requested action doesn't exist, which causes
> act API init code to create new action with specified index, but action is
> still released before returning and is deleted (unless it was referenced
> concurrently by cls API).
>
> Reproduction:
>
> $ sudo tc actions ls action gact
> $ sudo tc actions change action gact drop index 1
> $ sudo tc actions ls action gact
>

I didn't know 'change' could actually create an action when
it does not exist. So it sets NLM_F_REPLACE, how could it
replace a non-existing one? Is this the right behavior or is it too
late to change even if it is not?

> Extend tcf_action_init() to accept 'init_res' array and initialize it with
> action->ops->init() result. In tcf_action_add() remove pointers to created
> actions from actions array before passing it to tcf_action_put_many().

In my last comments, I actually meant whether we can avoid this
'init_res[]' array. Since here you want to check whether an action
returned by tcf_action_init_1() is a new one or an existing one, how
about checking its refcnt? Something like:

  act = tcf_action_init_1(...);
  if (IS_ERR(act)) {
    err = PTR_ERR(act);
    goto err;
  }
  if (refcount_read(&act->tcfa_refcnt) == 1) {
    // we know this is a newly allocated one
  }

Thanks.