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SubjectRe: 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.
netfilter: ctnetlink: fix loop in ctnetlink_get_conntrack()

From: Pablo Neira Ayuso <pablo@netfilter.org>

This patch fixes a loop in ctnetlink_get_conntrack() that can be
triggered if you use the same socket to receive events and to
perform a GET operation. Under heavy load, netlink_unicast()
may return -EAGAIN, this error code is reserved in nfnetlink for
the module load-on-demand. Instead, we return -ENOBUFS which is
the appropriate error code that has to be propagated to
user-space.

Reported-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
net/netfilter/nf_conntrack_netlink.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index b729ace..a84fa6f 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -973,7 +973,8 @@ ctnetlink_get_conntrack(struct sock *ctnl, struct sk_buff *skb,
free:
kfree_skb(skb2);
out:
- return err;
+ /* this avoids a loop in nfnetlink. */
+ return err == -EAGAIN ? -ENOBUFS : err;
}

#ifdef CONFIG_NF_NAT_NEEDED
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