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, 71 insertions, 15 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 2ee15af..cb52cba 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7669,7 +7669,7 @@ static int __init net_dev_init(void)
 	open_softirq(NET_RX_SOFTIRQ, net_rx_action);
 
 	hotcpu_notifier(dev_cpu_callback, 0);
-	dst_init();
+	dst_subsys_init();
 	rc = 0;
 out:
 	return rc;
diff --git a/net/core/dst.c b/net/core/dst.c
index e956ce6..917364f 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -22,6 +22,7 @@
 #include <linux/prefetch.h>
 
 #include <net/dst.h>
+#include <net/dst_metadata.h>
 
 /*
  * Theory of operations:
@@ -158,19 +159,10 @@ const u32 dst_default_metrics[RTAX_MAX + 1] = {
 	[RTAX_MAX] = 0xdeadbeef,
 };
 
-
-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)
 {
-	struct dst_entry *dst;
-
-	if (ops->gc && dst_entries_get_fast(ops) > ops->gc_thresh) {
-		if (ops->gc(ops))
-			return NULL;
-	}
-	dst = kmem_cache_alloc(ops->kmem_cachep, GFP_ATOMIC);
-	if (!dst)
-		return NULL;
 	dst->child = NULL;
 	dst->dev = dev;
 	if (dev)
@@ -200,6 +192,25 @@ void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
 	dst->next = NULL;
 	if (!(flags & DST_NOCOUNT))
 		dst_entries_add(ops, 1);
+}
+EXPORT_SYMBOL(dst_init);
+
+void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
+		int initial_ref, int initial_obsolete, unsigned short flags)
+{
+	struct dst_entry *dst;
+
+	if (ops->gc && dst_entries_get_fast(ops) > ops->gc_thresh) {
+		if (ops->gc(ops))
+			return NULL;
+	}
+
+	dst = kmem_cache_alloc(ops->kmem_cachep, GFP_ATOMIC);
+	if (!dst)
+		return NULL;
+
+	dst_init(dst, ops, dev, initial_ref, initial_obsolete, flags);
+
 	return dst;
 }
 EXPORT_SYMBOL(dst_alloc);
@@ -248,7 +259,11 @@ again:
 		dst->ops->destroy(dst);
 	if (dst->dev)
 		dev_put(dst->dev);
-	kmem_cache_free(dst->ops->kmem_cachep, dst);
+
+	if (dst->flags & DST_METADATA)
+		kfree(dst);
+	else
+		kmem_cache_free(dst->ops->kmem_cachep, dst);
 
 	dst = child;
 	if (dst) {
@@ -327,6 +342,47 @@ void __dst_destroy_metrics_generic(struct dst_entry *dst, unsigned long old)
 }
 EXPORT_SYMBOL(__dst_destroy_metrics_generic);
 
+static struct dst_ops md_dst_ops = {
+	.family =		AF_UNSPEC,
+};
+
+static int dst_md_discard_sk(struct sock *sk, struct sk_buff *skb)
+{
+	WARN_ONCE(1, "Attempting to call output on metadata dst\n");
+	kfree_skb(skb);
+	return 0;
+}
+
+static int dst_md_discard(struct sk_buff *skb)
+{
+	WARN_ONCE(1, "Attempting to call input on metadata dst\n");
+	kfree_skb(skb);
+	return 0;
+}
+
+struct metadata_dst *metadata_dst_alloc(u8 optslen, gfp_t flags)
+{
+	struct metadata_dst *md_dst;
+	struct dst_entry *dst;
+
+	md_dst = kmalloc(sizeof(*md_dst) + optslen, flags);
+	if (!md_dst)
+		return ERR_PTR(-ENOMEM);
+
+	dst = &md_dst->dst;
+	dst_init(dst, &md_dst_ops, NULL, 1, DST_OBSOLETE_NONE,
+		 DST_METADATA | DST_NOCACHE | DST_NOCOUNT);
+
+	dst->input = dst_md_discard;
+	dst->output = dst_md_discard_sk;
+
+	memset(dst + 1, 0, sizeof(*md_dst) + optslen - sizeof(*dst));
+	md_dst->opts_len = optslen;
+
+	return md_dst;
+}
+EXPORT_SYMBOL_GPL(metadata_dst_alloc);
+
 /* Dirty hack. We did it in 2.2 (in __dst_free),
  * we have _very_ good reasons not to repeat
  * this mistake in 2.3, but we have no choice
@@ -391,7 +447,7 @@ static struct notifier_block dst_dev_notifier = {
 	.priority = -10, /* must be called after other network notifiers */
 };
 
-void __init dst_init(void)
+void __init dst_subsys_init(void)
 {
 	register_netdevice_notifier(&dst_dev_notifier);
 }