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

[Fabian Frederick]

> On 22 September 2016 at 23:56 Florian Westphal <fw@xxxxxxxxx> wrote:
>
>
> Fabian Frederick <fabf@xxxxxxxxx> wrote:
> > Hello Florian,
> >
> > Â Â Â Â First problem is solved: table gets cleared 3 minutes earlier
> > but I still have kmemleak before running the following:
> >
> > echo scan > /sys/kernel/debug/kmemleak
> > cat /sys/kernel/debug/kmemleak
> > Nothing
> > echo scan > /sys/kernel/debug/kmemleak
> > cat /sys/kernel/debug/kmemleak
> > -> rsyslogd
> >
> > I talked about false positive because everything is cleared later.
>
> Hmm, I fear this is a real bug and not false positive.
>
> Should be possible to confirm this via slabinfo:
>
> grep nf_conntrack /proc/slabinfo
>
> The active objects should match the conntrack count.
> (conntrack -C, or wc -l < /proc/....).

Softirq latencies have been solved today in linux-next: commit
60bf815c0d08
("Revert "softirq: fix tasklet_kill() and its users"")

but nf_conntrack still needs a lot of time to reach 0

Slabinfo always remains the following:

/proc/slabinfo:Â
nf_conntrack     16  Â16  256 Â16

even when /proc/net/nf_conntrack is 0

Going back to kernel version before commit f330a7fdbe16
It's the same with 12 active objects:

/proc/slabinfo:Â
nf_conntrack     12  Â12  320 12

>
> > > > unreferenced object 0xffff88003b0e6600 (size 248):
> > > >Â Âcomm "rsyslogd", pid 1595, jiffies 4294741312 (age 7.343s)
> > > >Â Â...
> > > >Â Âbacktrace:
> > > >Â Â Â[] kmemleak_alloc+0x23/0x40
> > > >Â Â Â[] kmem_cache_alloc+0xd9/0x180
> > > >Â Â Â[] __nf_conntrack_alloc.isra.50+0x48/0x170
> > > >Â Â Â[] nf_conntrack_in+0x3a2/0x5f0
> > > >Â Â Â[] ipv4_conntrack_local+0x40/0x50
> > > >Â Â Â[] nf_iterate+0x5d/0x70
> > > >Â Â Â[] nf_hook_slow+0x5f/0xb0
> > > >Â Â Â[] __ip_local_out+0xad/0xe0
> > > >Â Â Â[] ip_local_out+0x17/0x40
> > > >Â Â Â[] ip_send_skb+0x14/0x40
> > > >Â Â Â[] udp_send_skb+0x91/0x260
> > > >Â Â Â[] udp_sendmsg+0x2f5/0x950
> > > >Â Â Â[] inet_sendmsg+0x60/0x90
> > > >Â Â Â[] sock_sendmsg+0x33/0x40
> > > >Â Â Â[] SYSC_sendto+0xee/0x160
> > > >Â Â Â[] SyS_sendto+0x9/0x10
>
> Hmm, so we leak when allocating conntrack for outgoing packet.
> Do you do any filtering (DROP) in output/postrouting?
iptables -L gives ACCEPT for all.

>
> > > > (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?
>
> Looks like a real bug to me, but I don't see anything obvious so far.
> I'll look at this again tomorrow.