dst: Metadata destinations

Introduces a new dst_metadata which enables to carry per packet metadata
between forwarding and processing elements via the skb->dst pointer.

The structure is set up to be a union. Thus, each separate type of
metadata requires its own dst instance. If demand arises to carry
multiple types of metadata concurrently, metadata dst entries can be
made stackable.

The metadata dst entry is refcnt'ed as expected for now but a non
reference counted use is possible if the reference is forced before
queueing the skb.

In order to allow allocating dsts with variable length, the existing
dst_alloc() is split into a dst_alloc() and dst_init() function. The
existing dst_init() function to initialize the subsystem is being
renamed to dst_subsys_init() to make it clear what is what.

The check before ip_route_input() is changed to ignore metadata dsts
and drop the dst inside the routing function thus allowing to interpret
metadata in a later commit.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 37 insertions, 1 deletions

diff --git a/include/net/dst.h b/include/net/dst.h
index 2bc73f8a00a9..2578811cef51 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -57,6 +57,7 @@ struct dst_entry {
 #define DST_FAKE_RTABLE		0x0040
 #define DST_XFRM_TUNNEL		0x0080
 #define DST_XFRM_QUEUE		0x0100
+#define DST_METADATA		0x0200
 
 	unsigned short		pending_confirm;
 
@@ -356,6 +357,9 @@ static inline int dst_discard(struct sk_buff *skb)
 }
 void *dst_alloc(struct dst_ops *ops, struct net_device *dev, int initial_ref,
 		int initial_obsolete, unsigned short flags);
+void dst_init(struct dst_entry *dst, struct dst_ops *ops,
+	      struct net_device *dev, int initial_ref, int initial_obsolete,
+	      unsigned short flags);
 void __dst_free(struct dst_entry *dst);
 struct dst_entry *dst_destroy(struct dst_entry *dst);
 
@@ -457,7 +461,7 @@ static inline struct dst_entry *dst_check(struct dst_entry *dst, u32 cookie)
 	return dst;
 }
 
-void dst_init(void);
+void dst_subsys_init(void);
 
 /* Flags for xfrm_lookup flags argument. */
 enum {
diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
new file mode 100644
index 000000000000..4f7694f3c7d0
--- /dev/null
+++ b/include/net/dst_metadata.h
@@ -0,0 +1,32 @@
+#ifndef __NET_DST_METADATA_H
+#define __NET_DST_METADATA_H 1
+
+#include <linux/skbuff.h>
+#include <net/ip_tunnels.h>
+#include <net/dst.h>
+
+struct metadata_dst {
+	struct dst_entry		dst;
+	size_t				opts_len;
+};
+
+static inline struct metadata_dst *skb_metadata_dst(struct sk_buff *skb)
+{
+	struct metadata_dst *md_dst = (struct metadata_dst *) skb_dst(skb);
+
+	if (md_dst && md_dst->dst.flags & DST_METADATA)
+		return md_dst;
+
+	return NULL;
+}
+
+static inline bool skb_valid_dst(const struct sk_buff *skb)
+{
+	struct dst_entry *dst = skb_dst(skb);
+
+	return dst && !(dst->flags & DST_METADATA);
+}
+
+struct metadata_dst *metadata_dst_alloc(u8 optslen, gfp_t flags);
+
+#endif /* __NET_DST_METADATA_H */