netdev - [PATCH v2 0/1] Issue description and debug

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Date: Tue, 19 Sep 2023 17:38:02 +0800
From: Heng Guo <heng.guo@...driver.com>
To: <davem@...emloft.net>, <sahern@...nel.org>, <edumazet@...gle.com>,
        <kuba@...nel.org>, <pabeni@...hat.com>
CC: <netdev@...r.kernel.org>, <filip.pudak@...driver.com>,
        <heng.guo@...driver.com>
Subject: [PATCH v2 0/1] Issue description and debug

Hi maintainers,

The IPSTATS_MIB_OUTFORWDATAGRAMS is counted after fragment check.

Reproduce environment:
network with 3 VM linuxs is connected as below:
VM1<---->VM2(latest kernel 6.5.0-rc7)<---->VM3
VM1: eth0 ip: 192.168.122.207 MTU 1800
VM2: eth0 ip: 192.168.122.208, eth1 ip: 192.168.123.224 MTU 1500
VM3: eth0 ip: 192.168.123.240 MTU 1800

Reproduce:
VM1 send 1600 bytes UDP data to VM3 using tools scapy with flags='DF'.
scapy command:
send(IP(dst="192.168.123.240",flags='DF')/UDP()/str('0'*1600),count=1,
inter=1.000000)

Result:
Before IP data is sent.
----------------------------------------------------------------------
root@...ux86-64:~# cat /proc/net/snmp
Ip: Forwarding DefaultTTL InReceives InHdrErrors InAddrErrors
ForwDatagrams InUnknownProtos InDiscards InDelivers OutRequests
 OutDiscards OutNoRoutes ReasmTimeout ReasmReqdss
Ip: 1 64 6 0 2 2 0 0 2 4 0 0 0 0 0 0 0 0 0
......
root@...ux86-64:~#
----------------------------------------------------------------------
After IP data is sent.
----------------------------------------------------------------------
root@...ux86-64:~# cat /proc/net/snmp
Ip: Forwarding DefaultTTL InReceives InHdrErrors InAddrErrors
 ForwDatagrams InUnknownProtos InDiscards InDelivers OutRequests
 OutDiscards OutNoRoutes ReasmTimeout ReasmReqdss
Ip: 1 64 7 0 2 2 0 0 2 5 0 0 0 0 0 0 0 1 0
......
root@...ux86-64:~#
----------------------------------------------------------------------
ForwDatagrams is always keeping 2 without increment.

Issue description and patch:
ip_exceeds_mtu() in ip_forward() drops this IP datagram because skb len
(1600 sending by scapy) is over MTU(1500 in VM2) and "DF" is set.
According to RFC 4293 "3.2.3. IP Statistics Tables",
  +-------+------>------+----->-----+----->-----+
  | InForwDatagrams (6) | OutForwDatagrams (6)  |
  |                     V                       +->-+ OutFragReqds
  |                 InNoRoutes                  |   | (packets)
  / (local packet (3)                           |   |
  |  IF is that of the address                  |   +--> OutFragFails
  |  and may not be the receiving IF)           |   |    (packets)
the IPSTATS_MIB_OUTFORWDATAGRAMS should be counted before fragment
check.
The existing implementation, instead, would incease the counter after
fragment check: ip_exceeds_mtu() in ipv4 and ip6_pkt_too_big() in ipv6.
So do patch to move IPSTATS_MIB_OUTFORWDATAGRAMS counter to ip_forward()
for ipv4 and ip6_forward() for ipv6.

Test result with patch:
Before IP data is sent.
----------------------------------------------------------------------
root@...ux86-64:~# cat /proc/net/snmp
Ip: Forwarding DefaultTTL InReceives InHdrErrors InAddrErrors
 ForwDatagrams InUnknownProtos InDiscards InDelivers OutRequests
 OutDiscards OutNoRoutes ReasmTimeout ReasmReqdss
Ip: 1 64 6 0 2 2 0 0 2 4 0 0 0 0 0 0 0 0 0
......
root@...ux86-64:~#
----------------------------------------------------------------------
After IP data is sent.
----------------------------------------------------------------------
root@...ux86-64:~# cat /proc/net/snmp
Ip: Forwarding DefaultTTL InReceives InHdrErrors InAddrErrors
 ForwDatagrams InUnknownProtos InDiscards InDelivers OutRequests
 OutDiscards OutNoRoutes ReasmTimeout ReasmReqdss
Ip: 1 64 7 0 2 3 0 0 2 5 0 0 0 0 0 0 0 1 0
......
root@...ux86-64:~#
----------------------------------------------------------------------
ForwDatagrams is updated from 2 to 3.

Thanks,
Heng



v1->v2: fix build warning
Reported-by: kernel test robot <lkp@...el.com>
Closes:
https://lore.kernel.org/oe-kbuild-all/202309141701.7OFxGdE5-lkp@intel.com/

Heng Guo (1):
  net: ipv4,ipv6: fix IPSTATS_MIB_OUTFORWDATAGRAMS increment after
    fragment check

 net/ipv4/ip_forward.c | 4 ++--
 net/ipv6/ip6_output.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

-- 
2.25.1