This problem involves MIB counter inaccuracies triggered by
failed UDP checksums.
Problem: ip_local_deliver_finish calls udp_rcv, which calls
udp_queue_rcv_skb.
Unless the sk_filter is set, the checksum of the incoming
UDP datagram is not verified. If there are no other problems
InDatagrams (UDP_MIB_INDATAGRAMS) is then incremented.
Now, if udp_recvmsg is called as a handler for incoming
UDP datagrams, the checksum is verified for the first time
(unless sk_filter was set) and if the checksum fails, the
`goto csum_copy_err' leads to forcibly removing the datagram
_and_ incrementing InErrors (UDP_MIB_INERRORS).
Issue: When problem occurs in the manner described, the datagram
is counted twice: once as InDatagram and once as InErrors.
RFC 2013 defines InDatagrams as counter for delivered datagrams;
these datagrams are counted but never delivered.
How to reproduce: Send UDP datagrams with checksums enabled, use middlebox
which corrupts part of the traffic (e.g. bit errors / NetEm) and
use /proc/net/snmp to watch the counters. The sum of InErrors,
NoPorts and InDatagrams exceeds the real number of sent datagrams
by the number of datagrams which were counted twice and forcibly
removed by udp_recvmsg.
Non-occurrence: The problem does not occur if the sender disabled
UDP checksums (zero field; allowed for IPv4, but not for IPv6),
since then the checksum code returns success.
Fix: Move the `UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS)' statement from
udp_queue_rcv_skb to udp_recvmsg. Now InDatagrams only counts those
datagrams which were really delivered (as per RFC 2013).
Please CC: any correspondence to [EMAIL PROTECTED]
Signed-off-by: <[EMAIL PROTECTED]>
---
diff -Nurp a/net/ipv4/udp.c b/net/ipv4/udp.c
--- a/net/ipv4/udp.c 2006-06-06 17:01:26.000000000 +0100
+++ b/net/ipv4/udp.c 2006-06-06 23:39:45.000000000 +0100
@@ -823,6 +823,7 @@ try_again:
goto out_free;
sock_recv_timestamp(msg, sk, skb);
+ UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS);
/* Copy the address. */
if (sin)
@@ -1032,7 +1033,6 @@ static int udp_queue_rcv_skb(struct sock
kfree_skb(skb);
return -1;
}
- UDP_INC_STATS_BH(UDP_MIB_INDATAGRAMS);
return 0;
}
-
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