[PATCH] udp6: Fix udp fragmentation for tunnel traffic.

[Pravin B Shelar <pshelar@nicira.com>]

udp6 over GRE tunnel does not work after to GRE tso changes. GRE
tso handler passes inner packet but keeps track of outer header
start in SKB_GSO_CB(skb)->mac_offset.  udp6 fragment need to
take care of outer header, which start at the mac_offset, while
adding fragment header.
This bug is introduced by commit 68c3316311 (GRE: Add TCP
segmentation offload for GRE).

Reported-by: Dmitry Kravkov <dkravkov@gmail.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
---
 include/linux/skbuff.h |   15 +++++++++++++++
 net/ipv6/udp_offload.c |   20 ++++++++++++--------
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 5f93119..f7fdf71 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2758,6 +2758,21 @@ static inline int skb_tnl_header_len(const struct sk_buff *inner_skb)
 		SKB_GSO_CB(inner_skb)->mac_offset;
 }
 
+static inline int gso_pskb_expand_head(struct sk_buff *skb, int extra)
+{
+	int new_headroom, headroom;
+	int ret;
+
+	headroom = skb_headroom(skb);
+	ret = pskb_expand_head(skb, extra, 0, GFP_ATOMIC);
+	if (ret)
+		return ret;
+
+	new_headroom = skb_headroom(skb);
+	SKB_GSO_CB(skb)->mac_offset += (new_headroom - headroom);
+	return 0;
+}
+
 static inline bool skb_is_gso(const struct sk_buff *skb)
 {
 	return skb_shinfo(skb)->gso_size;
diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index 76d401a..5d1b8d7 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -46,11 +46,12 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 	unsigned int mss;
 	unsigned int unfrag_ip6hlen, unfrag_len;
 	struct frag_hdr *fptr;
-	u8 *mac_start, *prevhdr;
+	u8 *packet_start, *prevhdr;
 	u8 nexthdr;
 	u8 frag_hdr_sz = sizeof(struct frag_hdr);
 	int offset;
 	__wsum csum;
+	int tnl_hlen;
 
 	mss = skb_shinfo(skb)->gso_size;
 	if (unlikely(skb->len <= mss))
@@ -84,9 +85,11 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 	skb->ip_summed = CHECKSUM_NONE;
 
 	/* Check if there is enough headroom to insert fragment header. */
-	if ((skb_mac_header(skb) < skb->head + frag_hdr_sz) &&
-	    pskb_expand_head(skb, frag_hdr_sz, 0, GFP_ATOMIC))
-		goto out;
+	tnl_hlen = skb_tnl_header_len(skb);
+	if (skb_headroom(skb) < (tnl_hlen + frag_hdr_sz)) {
+		if (gso_pskb_expand_head(skb, tnl_hlen + frag_hdr_sz))
+			goto out;
+	}
 
 	/* Find the unfragmentable header and shift it left by frag_hdr_sz
 	 * bytes to insert fragment header.
@@ -94,11 +97,12 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 	unfrag_ip6hlen = ip6_find_1stfragopt(skb, &prevhdr);
 	nexthdr = *prevhdr;
 	*prevhdr = NEXTHDR_FRAGMENT;
-	unfrag_len = skb_network_header(skb) - skb_mac_header(skb) +
-		     unfrag_ip6hlen;
-	mac_start = skb_mac_header(skb);
-	memmove(mac_start-frag_hdr_sz, mac_start, unfrag_len);
+	unfrag_len = (skb_network_header(skb) - skb_mac_header(skb)) +
+		     unfrag_ip6hlen + tnl_hlen;
+	packet_start = (u8 *) skb->head + SKB_GSO_CB(skb)->mac_offset;
+	memmove(packet_start-frag_hdr_sz, packet_start, unfrag_len);
 
+	SKB_GSO_CB(skb)->mac_offset -= frag_hdr_sz;
 	skb->mac_header -= frag_hdr_sz;
 	skb->network_header -= frag_hdr_sz;
 
-- 
1.7.1