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

On Wed, Sep 6, 2017 at 12:38 AM, Jiri Pirko [off-list ref] wrote:

Wed, Sep 06, 2017 at 07:03:10AM CEST, xiyou.wangcong@gmail.com wrote:

quoted

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?

quoted

What's more, index 0 is not special at all, it should be treated
like other chains. This also makes the code more readable.

[...]

quoted

@@ -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.