linux-kernel - Re: ctnetlink loop

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Message-ID: <4D00B5CD.3050406@netfilter.org>
Date:	Thu, 09 Dec 2010 11:56:13 +0100
From:	Pablo Neira Ayuso <pablo@...filter.org>
To:	netfilter-devel <netfilter-devel@...ts.netfilter.org>,
	netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
CC:	"David S. Miller" <davem@...emloft.net>
Subject: Re: ctnetlink loop

Sorry, I finally found your email reporting this:

> nfnetlink: avoid unbound loop on busy Netlink socket
> 
> I see a problem with how ctnetlink GET requests are being
> processed in the kernel (2.6.32.24) under high load.
> 
> The sympton is Netlink looping around nfnetlink_rcv_msg(), which
> is just because netlink_unicast() came back with EAGAIN when
> trying to write the newly created Netlink skb to the SK receive
> buffer in ctnetlink_get_conntrack().  In this case a (possibly)
> infinit loop is entered.  Mostly infinit I think in case the
> userland party trying to receive those messages may be stuck in
> the sendmsg() call, being unable to read anything if being single
> threaded.
> 
> I tried to reproduce several times, a few times the loop
> disappeared and the box proceeded normally after some minutes.
> I have no explanation for this.
> 
> The attached patch tries to solve it by simple not trying again
> to netlink_unicast() the reply skb and just fail with -ENOBUFS.
> The reasoning is that at the point a Netlink overrun is detected
> it seems counter intuitive to insist on sending one more Netlink
> message.

We still need EAGAIN, and it doesn't necessarily means ENOBUFS for the
general case in nfnetlink.

The following patch covers the case that you're reporting.

View attachment "f.patch" of type "text/x-patch" (1165 bytes)