netdev - Re: [patch net] net: sched: fix memleak for chain zero

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Message-ID: <20170906203323.GA16570@nanopsycho>
Date:   Wed, 6 Sep 2017 22:33:23 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jakub Kicinski <kubakici@...pl>, mlxsw@...lanox.com
Subject: Re: [patch net] net: sched: fix memleak for chain zero

Wed, Sep 06, 2017 at 07:40:02PM CEST, xiyou.wangcong@...il.com wrote:
>On Wed, Sep 6, 2017 at 4:14 AM, Jiri Pirko <jiri@...nulli.us> wrote:
>> From: Jiri Pirko <jiri@...lanox.com>
>>
>> There's a memleak happening for chain 0. The thing is, chain 0 needs to
>> be always present, not created on demand. Therefore tcf_block_get upon
>> creation of block calls the tcf_chain_create function directly. The
>> chain is created with refcnt == 1, which is not correct in this case and
>> causes the memleak. So move the refcnt increment into tcf_chain_get
>> function even for the case when chain needs to be created.
>>
>
>Your approach could work but you just make the code even
>uglier than it is now:
>
>1. The current code is already ugly for special-casing chain 0:
>
>        if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
>                tcf_chain_destroy(chain);
>
>2. With your patch, chain 0 has a different _initial_ refcnt with others.

No. Initial refcnt is the same. ! for every action that holds the chain.
So actually, it returns it back where it should be.


>
>3. Allowing an object (chain 0) exists with refcnt==0

So? That is for every chain that does not have goto_chain action
pointing at. Please read the code.


>
>Compare it with my patch:
>
>1. No special-case for chain 0, the above ugly part is removed
>
>2. Every chain is equal and created with refcnt==1
>
>3. Any chain with refcnt==0 is destroyed