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

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Date:   Thu, 8 Apr 2021 14:55:03 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Vlad Buslov <vladbu@...dia.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        David Miller <davem@...emloft.net>,
        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 Thu, Apr 8, 2021 at 4:59 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On 2021-04-07 7:50 p.m., Cong Wang wrote:
> > 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?
>
> Thats expected behavior for "change" essentially mapping
> to classical "SET" i.e.
> "create if it doesnt exist, replace if it exists"
> i.e NLM_F_CREATE | NLM_F_REPLACE
>
> In retrospect, "replace" should probably have been just NLM_F_REPLACE
> "replace if it exists, error otherwise".
> Currently there is no distinction between the two.

This is how I interpret "replace" too, but again it is probably too late
to change.

>
> "Add" is classical "CREATE" i.e "create if doesnt exist, otherwise
> error"
>
> It may be feasible to fix "replace" but not sure how many scripts over
> the years are now dependent on that behavior.

Right, we probably have to live with it forever.

Thanks.