From: Dmitry Butskoy <dmitry@butskoy.name>

Taken from http://bugzilla.kernel.org/show_bug.cgi?id=8747

Problem Description:

It is related to the possibility to obtain MSG_ERRQUEUE messages from the udp
and raw sockets, both connected and unconnected.

There is a little typo in net/ipv6/icmp.c code, which prevents such messages
to be delivered to the errqueue of the correspond raw socket, when the socket
is CONNECTED.  The typo is due to swap of local/remote addresses.

Consider __raw_v6_lookup() function from net/ipv6/raw.c. When a raw socket is
looked up usual way, it is something like:

sk = __raw_v6_lookup(sk, nexthdr, daddr, saddr, IP6CB(skb)->iif);

where "daddr" is a destination address of the incoming packet (IOW our local
address), "saddr" is a source address of the incoming packet (the remote end).

But when the raw socket is looked up for some icmp error report, in
net/ipv6/icmp.c:icmpv6_notify() , daddr/saddr are obtained from the echoed
fragment of the "bad" packet, i.e.  "daddr" is the original destination
address of that packet, "saddr" is our local address.  Hence, for
icmpv6_notify() must use "saddr, daddr" in its arguments, not "daddr, saddr"
...

Steps to reproduce:

Create some raw socket, connect it to an address, and cause some error
situation: f.e. set ttl=1 where the remote address is more than 1 hop to reach.
Set IPV6_RECVERR .
Then send something and wait for the error (f.e. poll() with POLLERR|POLLIN).
You should receive "time exceeded" icmp message (because of "ttl=1"), but the
socket do not receive it.

If you do not connect your raw socket, you will receive MSG_ERRQUEUE
successfully.  (The reason is that for unconnected socket there are no actual
checks for local/remote addresses).
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>