netdev - Re: [Patch net] net_sched: fix a memory leak of filter chain

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Date:   Wed, 6 Sep 2017 10:25:51 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [Patch net] net_sched: fix a memory leak of filter chain

On Wed, Sep 6, 2017 at 12:38 AM, Jiri Pirko <jiri@...nulli.us> wrote:
> Wed, Sep 06, 2017 at 07:03:10AM CEST, xiyou.wangcong@...il.com wrote:
>>tcf_chain_destroy() is called by tcf_block_put() and tcf_chain_put().
>>tcf_chain_put() is refcn'ed and paired with tcf_chain_get(),
>>but tcf_block_put() is not, it should be paired with tcf_block_get()
>>and we still need to decrease the refcnt. However, tcf_block_put()
>>is special, it stores the chains too, we have to detach them if
>>it is not the last user.
>
> You don't describe the original issue, or I am missing that from your
> description.

The original issue is the mismatch of tcf_block_put() and tcf_block_get()
w.r.t. refcnt. Think it in this way: if you call tcf_bock_put() immediately
after tcf_block_get(), would you get effectively a nop?


>
>
>>
>>What's more, index 0 is not special at all, it should be treated
>>like other chains. This also makes the code more readable.
>
> [...]
>
>
>>@@ -246,10 +246,7 @@ EXPORT_SYMBOL(tcf_chain_get);
>>
>> void tcf_chain_put(struct tcf_chain *chain)
>> {
>>-      /* Destroy unused chain, with exception of chain 0, which is the
>>-       * default one and has to be always present.
>>-       */
>>-      if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
>>+      if (--chain->refcnt == 0)
>
> The refcounting is only done for actions holding reference to the chain.
> You still need to check is the filter chain is not empty.
> See tc_ctl_tfilter.

With my patch refcnt is done for block too, if you notice the
tcf_chain_put() in tcf_block_put().


>
> Also, chain 0 is created by default on a block creation. It has to be
> present always for a reason. Please see tcf_block_get. The pointer to
> chain 0 is assigned to the qdisc filter list pointer.

Sure, this is why block holds a refcnt to chain (not just chain 0) with
my patch, aka why the initial refcnt is 1 rather than 0.