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

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

[Florian Westphal]

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: de960aa9ab4d ("net: add and use skb_gso_transport_seglen()")
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 net/core/skbuff.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 30c7d35..57e225c 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);
-- 
1.8.1.5

Re: [PATCH] net: core: don't account for udp header size when computing seglen

[David Miller]

From: Florian Westphal <fw@strlen.de>
Date: Tue,  8 Apr 2014 22:51:28 +0200

> 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: de960aa9ab4d ("net: add and use skb_gso_transport_seglen()")
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Reported-by: Tobias Brunner <tobias@strongswan.org>
> Signed-off-by: Florian Westphal <fw@strlen.de>

I agree completely with the fix, but not necessarily with the Fixes:
tag.

All that commit did was move the code to a common location, it
always had this bug.

I suppose you could list the subsequent commit, the one that started
using this helper in the IP forwarding code, as what this "Fixes".

Thanks Florian.