From: Vlad Buslov <vladbu@nvidia.com>
To: Cong Wang <xiyou.wangcong@gmail.com>,
Jamal Hadi Salim <jhs@mojatatu.com>
Cc: "Linux Kernel Network Developers" <netdev@vger.kernel.org>,
"Kumar Kartikeya Dwivedi" <memxor@gmail.com>,
"David Miller" <davem@davemloft.net>,
"Jiri Pirko" <jiri@resnulli.us>,
"Jakub Kicinski" <kuba@kernel.org>,
"Toke Høiland-Jørgensen" <toke@redhat.com>,
"Marcelo Ricardo Leitner" <marcelo.leitner@gmail.com>,
"Davide Caratti" <dcaratti@redhat.com>
Subject: Re: [PATCH net v2 2/3] net: sched: fix action overwrite reference counting
Date: Thu, 8 Apr 2021 10:50:00 +0300 [thread overview]
Message-ID: <ygnhsg419pw7.fsf@nvidia.com> (raw)
In-Reply-To: <CAM_iQpXEGs-Sq-SjNrewEyQJ7p2-KUxL5-eUvWs0XTKGoh7BsQ@mail.gmail.com>
On Thu 08 Apr 2021 at 02:50, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> On Wed, Apr 7, 2021 at 8:36 AM Vlad Buslov <vladbu@nvidia.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?
Origins of setting ovr based on NLM_F_REPLACE are lost since this code
goes back to Linus' Linux-2.6.12-rc2 commit. Jamal, do you know if this
is the expected behavior or just something unintended?
>
>> 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.
Hmm, I don't think this would work in general case. Consider following
cases:
1. Action existed during init as filter action(refcnt=1), init overwrote
it setting refcnt=2, by the time we got to checking tcfa_refcnt filter
has been deleted (refcnt=1) so code will incorrectly assume that it has
created the action.
2. We need this check in tcf_action_add() to release the refcnt in case
of overwriting existing actions, but by that time actions are already
accessible though idr, so even in case when new action has been created
(refcnt=1) it could already been referenced by concurrently created
filter (refcnt=2).
Regards,
Vlad
next prev parent reply other threads:[~2021-04-08 7:50 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-07 15:36 [PATCH net v2 0/3] Action initalization fixes Vlad Buslov
2021-04-07 15:36 ` [PATCH net v2 1/3] Revert "net: sched: bump refcount for new action in ACT replace mode" Vlad Buslov
2021-04-07 15:36 ` [PATCH net v2 2/3] net: sched: fix action overwrite reference counting Vlad Buslov
2021-04-07 23:50 ` Cong Wang
2021-04-08 7:50 ` Vlad Buslov [this message]
2021-04-08 13:43 ` Jamal Hadi Salim
2021-04-08 21:52 ` Cong Wang
2021-04-08 11:59 ` Jamal Hadi Salim
2021-04-08 21:55 ` Cong Wang
2021-04-07 15:36 ` [PATCH net v2 3/3] net: sched: fix err handler in tcf_action_init() Vlad Buslov
2021-04-08 22:02 ` [PATCH net v2 0/3] Action initalization fixes Cong Wang
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