Subject: Re: 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: Cong Wang <[email-masked]>
Date: Wed, 27 Feb 2013 03:20:40 +0000 (UTC)

On Tue, 26 Feb 2013 at 15:46 GMT, Stephen Hemminger <[email-masked]> wrote:
> 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().
>

I haven't looked into this deeply, it sounds like we need this fix:

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 5e12dca..3ccb704 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -845,7 +845,7 @@ static int __ip_append_data(struct sock *sk,
                csummode = CHECKSUM_PARTIAL;
		 
	         cork->length += length;
 -       if (((length > mtu) || (skb &&skb_is_gso(skb))) &&
 +       if (((length + fragheaderlen > mtu) || (skb &&skb_is_gso(skb))) &&
            (sk->sk_protocol == IPPROTO_UDP) &&
	                (rt->dst.dev->features & NETIF_F_UFO) && !rt->dst.header_len) {
                err = ip_ufo_append_data(sk, queue, getfrag, from,length,