Subject: Fw: [Bug 54491] New: UFO (UDP fragmentation offload) does not work if the payload size specified is within the range ( (MTU-28) to (MTU-8) )

From: Stephen Hemminger <[email-masked]>
Date: Tue, 26 Feb 2013 07:46:06 -0800

Begin forwarded message:

Date: Tue, 26 Feb 2013 00:52:33 -0800
From: "[email-masked]" <[email-masked]>
To: "[email-masked]" <[email-masked]>
Subject: [Bug 54491] New: UFO (UDP fragmentation offload) does not work if the payload size specified is within the range ( (MTU-28)  to (MTU-8) )

https://bugzilla.kernel.org/show_bug.cgi?id=54491

           Summary: UFO (UDP fragmentation offload) does not work if the
                    payload size specified is within the range ( (MTU-28)
                    to (MTU-8) )
           Product: Networking
           Version: 2.5
    Kernel Version: 3.3.4
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: IPV4
        AssignedTo: [email-masked]
        ReportedBy: [email-masked]
        Regression: No

For IPv4/UDP if the payload size specified is within the range ( (MTU-28) 
to (MTU-8) ) packet gets segmented in kernel even if the following netdev
features are set:

NETIF_F_SG 
NETIF_F_GEN_CSUM 
NETIF_F_UFO 
NETIF_F_TSO 
NETIF_F_GSO 
NETIF_F_GSO_ROBUST

Analysis:
udp_sendmsg() computes packet length by adding UDP header size to payload size:
“ulen += sizeof(struct udphdr)”
Then the function calls ip_make_skb(), however IP header size is not added to
the packet length.
ip_make_skb() in turn calls __ip_append_data(). The same packet length is used
here. 
__ip_append_data() compares the packet length with MTU. The packet length still
does not include IP header size (20 bytes).
In result, when payload size “is within the range ( (MTU-28) to (MTU-8) )”
standard branch is taken rather than ip_ufo_append_data(). Gso_size is not
computed, therefore IP fragmentation is triggered from ip_finish_output().

The issue was reproduced on 3.4.3 as well

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