datapath: 16bit inner_network_header field in struct ovs_gso_cb

The motivation for this is to create a 16bit hole in struct ovs_gso_cb
which may be used for the inner_protocol field which is needed
for the proposed implementation of compatibility for MPLS GSO segmentation.

This should be safe as inner_network_header is now an offset to
the inner_mac_header rather than skb->head.

As pointed out by Thomas Graf simply making both inner offsets 16bis is not
safe as there have been cases of overflow with "with collapsed TCP frames
on IB when the headroom grew beyond 64K. See commit 50bceae9bd ``tcp:
Reallocate headroom if it would overflow csum_start'' for additional
details."

This patch is based on suggestions by Thomas Graf and Jesse Gross.

Cc: Thomas Graf <[email protected]>
Cc: Jesse Gross <[email protected]>
Signed-off-by: Simon Horman <[email protected]>
Signed-off-by: Jesse Gross <[email protected]>

datapath/linux/compat/gso.h

@@ -11,21 +11,17 @@
 
 struct ovs_gso_cb {
 	struct ovs_skb_cb dp_cb;
-	sk_buff_data_t	inner_network_header;
-	sk_buff_data_t	inner_mac_header;
+	u16		inner_network_header;	/* Offset from
+						 * inner_mac_header */
+	/* 16bit hole */
+	sk_buff_data_t	inner_mac_header;	/* Offset from skb->head */
 	void (*fix_segment)(struct sk_buff *);
 };
 #define OVS_GSO_CB(skb) ((struct ovs_gso_cb *)(skb)->cb)
 
 #define skb_inner_network_header rpl_skb_inner_network_header
 
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
-#define skb_inner_network_header rpl_skb_inner_network_header
-static inline unsigned char *skb_inner_network_header(const struct sk_buff *skb)
-{
-	return skb->head + OVS_GSO_CB(skb)->inner_network_header;
-}
-
 #define skb_inner_mac_header rpl_skb_inner_mac_header
 static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb)
 {
@@ -34,12 +30,6 @@ static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb)
 
 #else
 
-#define skb_inner_network_header rpl_skb_inner_network_header
-static inline unsigned char *skb_inner_network_header(const struct sk_buff *skb)
-{
-	return OVS_GSO_CB(skb)->inner_network_header;
-}
-
 #define skb_inner_mac_header rpl_skb_inner_mac_header
 static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb)
 {
@@ -48,6 +38,13 @@ static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb)
 
 #endif
 
+#define skb_inner_network_header rpl_skb_inner_network_header
+static inline unsigned char *skb_inner_network_header(const struct sk_buff *skb)
+{
+	return skb_inner_mac_header(skb) +
+		OVS_GSO_CB(skb)->inner_network_header;
+}
+
 #define skb_inner_network_offset rpl_skb_inner_network_offset
 static inline int skb_inner_network_offset(const struct sk_buff *skb)
 {
@@ -64,7 +61,8 @@ static inline int skb_inner_mac_offset(const struct sk_buff *skb)
 static inline void skb_reset_inner_headers(struct sk_buff *skb)
 {
 	BUILD_BUG_ON(sizeof(struct ovs_gso_cb) > FIELD_SIZEOF(struct sk_buff, cb));
-	OVS_GSO_CB(skb)->inner_network_header = skb->network_header;
+	OVS_GSO_CB(skb)->inner_network_header = skb->network_header -
+		skb->mac_header;
 	OVS_GSO_CB(skb)->inner_mac_header = skb->mac_header;
 
 	OVS_GSO_CB(skb)->fix_segment = NULL;