linux-kernel - Re: [PATCH 1/1 linux-next] netfilter: conntrack: fix kmemleak false positive

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Date:   Wed, 21 Sep 2016 23:02:53 +0200
From:   Florian Westphal <fw@...len.de>
To:     Fabian Frederick <fabf@...net.be>
Cc:     Florian Westphal <fw@...len.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 1/1 linux-next] netfilter: conntrack: fix kmemleak false
 positive

Fabian Frederick <fabf@...net.be> wrote:
> Since commit f330a7fdbe16
> ("netfilter: conntrack: get rid of conntrack timer")
> 
> closed connections remain longer in /proc/net/nf_conntrack
> 
> Running current kernel; just after boot:
> cat /proc/net/nf_conntrack | wc -l = 5
> 4 minutes required to clean up the table.

We should reap the stale entries while iterating, just like
we do for ctnetlink interface.

Can you try this patch?

diff --git a/net/netfilter/nf_conntrack_standalone.c b/net/netfilter/nf_conntrack_standalone.c
--- a/net/netfilter/nf_conntrack_standalone.c
+++ b/net/netfilter/nf_conntrack_standalone.c
@@ -212,6 +212,11 @@ static int ct_seq_show(struct seq_file *s, void *v)
 	if (unlikely(!atomic_inc_not_zero(&ct->ct_general.use)))
 		return 0;
 
+	if (nf_ct_should_gc(ct)) {
+		nf_ct_kill(ct);
+		goto release;
+	}
+
 	/* we only want to print DIR_ORIGINAL */
 	if (NF_CT_DIRECTION(hash))
 		goto release;

> Going back to kernel version before commit above there are
> no connections after some seconds.
> 
> Referring to the commit changelog this was an expected behaviour but
> it results in temporary kmemleak reports:

I don't see kmemleak complaints on my test vm, I'm reluctant to
turn it off.

Can you explain why we see such 'false positive'?

The conntracks should still be referenced, as they
are in main table.

> unreferenced object 0xffff88003b0e6600 (size 248):
>   comm "rsyslogd", pid 1595, jiffies 4294741312 (age 7.343s)
>   ...
>   backtrace:
>     [<ffffffff818f0163>] kmemleak_alloc+0x23/0x40
>     [<ffffffff8117abf9>] kmem_cache_alloc+0xd9/0x180
>     [<ffffffff817533a8>] __nf_conntrack_alloc.isra.50+0x48/0x170
>     [<ffffffff81754262>] nf_conntrack_in+0x3a2/0x5f0
>     [<ffffffff817bc9e0>] ipv4_conntrack_local+0x40/0x50
>     [<ffffffff8174f17d>] nf_iterate+0x5d/0x70
>     [<ffffffff8174f1ef>] nf_hook_slow+0x5f/0xb0
>     [<ffffffff8176ffbd>] __ip_local_out+0xad/0xe0
>     [<ffffffff81770007>] ip_local_out+0x17/0x40
>     [<ffffffff817711f4>] ip_send_skb+0x14/0x40
>     [<ffffffff81797591>] udp_send_skb+0x91/0x260
>     [<ffffffff817985e5>] udp_sendmsg+0x2f5/0x950
>     [<ffffffff817a4f80>] inet_sendmsg+0x60/0x90
>     [<ffffffff816fe7d3>] sock_sendmsg+0x33/0x40
>     [<ffffffff816fed0e>] SYSC_sendto+0xee/0x160
>     [<ffffffff816ff719>] SyS_sendto+0x9/0x10
> 
> (248 bytes being an nf_conn structure)
> 
> Those structures being cleared in gc_worker() later on we can't talk
> about unreferenced object so this patch uses kmemleak_not_leak() to
> prevent those warnings.

If thats the case, why is kmemleak complaining? Are you sure this
is a false positive?