From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
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) )
Date: Tue, 26 Feb 2013 07:46:06 -0800
Message-ID: <20130226074606.4d16ebfb@nehalam.linuxnetplumber.net>
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Begin forwarded message:

Date: Tue, 26 Feb 2013 00:52:33 -0800
=46rom: "bugzilla-daemon@bugzilla.kernel.org" <bugzilla-daemon@bugzilla=
=2Ekernel.org>
To: "stephen@networkplumber.org" <stephen@networkplumber.org>
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=3D54491

           Summary: UFO (UDP fragmentation offload) does not work if th=
e
                    payload size specified is within the range ( (MTU-2=
8)
                    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: shemminger@linux-foundation.org
        ReportedBy: hnk437@gmail.com
        Regression: No


=46or IPv4/UDP if the payload size specified is within the range ( (MTU=
-28)=20
to (MTU-8) ) packet gets segmented in kernel even if the following netd=
ev
features are set:

NETIF_F_SG=20
NETIF_F_GEN_CSUM=20
NETIF_F_UFO=20
NETIF_F_TSO=20
NETIF_F_GSO=20
NETIF_F_GSO_ROBUST

Analysis:
udp_sendmsg() computes packet length by adding UDP header size to paylo=
ad size:
=E2=80=9Culen +=3D sizeof(struct udphdr)=E2=80=9D
Then the function calls ip_make_skb(), however IP header size is not ad=
ded to
the packet length.
ip_make_skb() in turn calls __ip_append_data(). The same packet length =
is used
here.=20
__ip_append_data() compares the packet length with MTU. The packet leng=
th still
does not include IP header size (20 bytes).
In result, when payload size =E2=80=9Cis within the range ( (MTU-28) to=
 (MTU-8) )=E2=80=9D
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

--=20
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