net: core: don't account for udp header size when computing seglen

In case of tcp, gso_size contains the tcpmss.

For UFO (udp fragmentation offloading) skbs, gso_size is the fragment
payload size, i.e. we must not account for udp header size.

Otherwise, when using virtio drivers, a to-be-forwarded UFO GSO packet
will be needlessly fragmented in the forward path, because we think its
individual segments are too large for the outgoing link.

Fixes: fe6cc55f3a9a053 ("net: ip, ipv6: handle gso skbs in forwarding path")
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 5 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 30c7d35dd862..57e225c8914e 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3937,12 +3937,14 @@ EXPORT_SYMBOL_GPL(skb_scrub_packet);
 unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
 {
         const struct skb_shared_info *shinfo = skb_shinfo(skb);
-        unsigned int hdr_len;
 
         if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)))
-                hdr_len = tcp_hdrlen(skb);
-        else
-                hdr_len = sizeof(struct udphdr);
-        return hdr_len + shinfo->gso_size;
+                return tcp_hdrlen(skb) + shinfo->gso_size;
+
+        /* UFO sets gso_size to the size of the fragmentation
+         * payload, i.e. the size of the L4 (UDP) header is already
+         * accounted for.
+         */
+        return shinfo->gso_size;
 }
 EXPORT_SYMBOL_GPL(skb_gso_transport_seglen);