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Date:	Fri, 16 Nov 2007 10:59:04 +0000
From:	Gerrit Renker <gerrit@....abdn.ac.uk>
To:	Wang Chen <wangchen@...fujitsu.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] [IPV4] SNMP: Decrement of UDP InDatagrams for bad checksum

| If there is no socket filter, a bad checksum udp packet will be queued and
| ready for user to read. At the same time the InDatagrams is increased.
| So we need to decrement InDatagrams when recvmsg() or poll() notice the
| bad checksum.
| 
| Seems Gerrit had fix it a year ago.(http://lkml.org/lkml/2006/8/28/218)
| But it's not in kernel now. So I make the same patch for 2.6.24.rc2.
| 
The problem is known: packets with failed checksums will still increment
InDatagrams. The solution is also known (as acknowledged above). I
remember the discussion about this kind of solution: the problem is
apparently that these variables may sit on different CPUs, so that first
incrementing, and then later decrementing, would not have the desired
effect.
Desirable would be to only increment InDatagrams after passing all the 
checksum tests (i.e. in udp_recvmsg). This solution was also discussed
on netdev, but people said that this would lead to trouble with the
data_ready handler (notably NFS).

| If this patch ok, I will make the same one for IPv6 UDP.
| 
| [PATCH 1/2] [IPV4] SNMP: Decrement of UDP InDatagrams for bad checksum
| Decrement UDP InDatagrams for bad checksum UDP packet in ready queue.
| 
| Signed-off-by: Wang Chen <wangchen@...fujitsu.com>
| ---
|  include/net/snmp.h |    2 ++
|  include/net/udp.h  |    3 +++
|  net/ipv4/udp.c     |    2 ++
|  3 files changed, 7 insertions(+)
| 
| diff -Nurp linux-2.6.24-rc2.org/include/net/snmp.h linux-2.6.24-rc2/include/net/snmp.h
| --- linux-2.6.24-rc2.org/include/net/snmp.h	2007-11-09 16:37:08.000000000 +0800
| +++ linux-2.6.24-rc2/include/net/snmp.h	2007-11-16 16:57:51.000000000 +0800
| @@ -142,6 +142,8 @@ struct linux_mib {
|  	(per_cpu_ptr(mib[!in_softirq()], raw_smp_processor_id())->mibs[field]++)
|  #define SNMP_DEC_STATS(mib, field) 	\
|  	(per_cpu_ptr(mib[!in_softirq()], raw_smp_processor_id())->mibs[field]--)
| +#define SNMP_DEC_STATS_BH(mib, field)	\
| +	(per_cpu_ptr(mib[0], raw_smp_processor_id())->mibs[field]--)
|  #define SNMP_ADD_STATS_BH(mib, field, addend) 	\
|  	(per_cpu_ptr(mib[0], raw_smp_processor_id())->mibs[field] += addend)
|  #define SNMP_ADD_STATS_USER(mib, field, addend) 	\
| diff -Nurp linux-2.6.24-rc2.org/include/net/udp.h linux-2.6.24-rc2/include/net/udp.h
| --- linux-2.6.24-rc2.org/include/net/udp.h	2007-11-09 16:37:08.000000000 +0800
| +++ linux-2.6.24-rc2/include/net/udp.h	2007-11-16 16:58:41.000000000 +0800
| @@ -148,6 +148,9 @@ DECLARE_SNMP_STAT(struct udp_mib, udp_st
|  #define UDP_INC_STATS_BH(field, is_udplite) 			       do  {  \
|  	if (is_udplite) SNMP_INC_STATS_BH(udplite_statistics, field);         \
|  	else		SNMP_INC_STATS_BH(udp_statistics, field);    }  while(0)
| +#define UDP_DEC_STATS_BH(field, is_udplite)			       do {   \
| +	if (is_udplite) SNMP_DEC_STATS_BH(udplite_statistics, field);         \
| +	else		SNMP_DEC_STATS_BH(udp_statistics, field);    }  while(0)
|  
|  /* /proc */
|  struct udp_seq_afinfo {
| diff -Nurp linux-2.6.24-rc2.org/net/ipv4/udp.c linux-2.6.24-rc2/net/ipv4/udp.c
| --- linux-2.6.24-rc2.org/net/ipv4/udp.c	2007-11-09 16:37:57.000000000 +0800
| +++ linux-2.6.24-rc2/net/ipv4/udp.c	2007-11-16 15:02:44.000000000 +0800
| @@ -897,6 +897,7 @@ out:
|  
|  csum_copy_err:
|  	UDP_INC_STATS_BH(UDP_MIB_INERRORS, is_udplite);
| +	UDP_DEC_STATS_BH(UDP_MIB_INDATAGRAMS, is_udplite);
|  
|  	skb_kill_datagram(sk, skb, flags);
|  
| @@ -1416,6 +1417,7 @@ unsigned int udp_poll(struct file *file,
|  		while ((skb = skb_peek(rcvq)) != NULL &&
|  		       udp_lib_checksum_complete(skb)) {
|  			UDP_INC_STATS_BH(UDP_MIB_INERRORS, is_lite);
| +			UDP_DEC_STATS_BH(UDP_MIB_INDATAGRAMS, is_lite);
|  			__skb_unlink(skb, rcvq);
|  			kfree_skb(skb);
|  		}
| 
| 

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