[NET]: Add skb_iter functions.

There's no convenient function to walk the data of an skbuff; provide
one.  Netfilter extensions in particular can use this to examine
packet contents without needing an entire copy.

I originally wrote an 'skb_walk(skb, fn, data)' function, but open
iterators are easier to use for complex cases.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@redhat.com>

include/linux/skbuff.h

@@ -1108,6 +1108,22 @@ extern void	       skb_split(struct sk_buff *skb,
 extern void skb_init(void);
 extern void skb_add_mtu(int mtu);
 
+struct skb_iter {
+	/* Iteration functions set these */
+	unsigned char *data;
+	unsigned int len;
+
+	/* Private to iteration */
+	unsigned int nextfrag;
+	struct sk_buff *fraglist;
+};
+
+/* Keep iterating until skb_iter_next returns false. */
+extern void skb_iter_first(const struct sk_buff *skb, struct skb_iter *i);
+extern int skb_iter_next(const struct sk_buff *skb, struct skb_iter *i);
+/* Call this if aborting loop before !skb_iter_next */
+extern void skb_iter_abort(const struct sk_buff *skb, struct skb_iter *i);
+
 #ifdef CONFIG_NETFILTER
 static inline void nf_conntrack_put(struct nf_ct_info *nfct)
 {

net/core/skbuff.c

@@ -929,6 +929,70 @@ int skb_copy_bits(const struct sk_buff *skb, int offset, void *to, int len)
 	return -EFAULT;
 }
 
+/* Keep iterating until skb_iter_next returns false. */
+void skb_iter_first(const struct sk_buff *skb, struct skb_iter *i)
+{
+	i->len = skb_headlen(skb);
+	i->data = (unsigned char *)skb->data;
+	i->nextfrag = 0;
+	i->fraglist = NULL;
+}
+
+int skb_iter_next(const struct sk_buff *skb, struct skb_iter *i)
+{
+	/* Unmap previous, if not head fragment. */
+	if (i->nextfrag)
+		kunmap_skb_frag(i->data);
+
+	if (i->fraglist) {
+	fraglist:
+		/* We're iterating through fraglist. */
+		if (i->nextfrag < skb_shinfo(i->fraglist)->nr_frags) {
+			i->data = kmap_skb_frag(&skb_shinfo(i->fraglist)
+						->frags[i->nextfrag]);
+			i->len = skb_shinfo(i->fraglist)->frags[i->nextfrag]
+				.size;
+			i->nextfrag++;
+			return 1;
+		}
+		/* Fragments with fragments?  Too hard! */
+		BUG_ON(skb_shinfo(i->fraglist)->frag_list);
+		i->fraglist = i->fraglist->next;
+		if (!i->fraglist)
+			goto end;
+
+		i->len = skb_headlen(i->fraglist);
+		i->data = i->fraglist->data;
+		i->nextfrag = 0;
+		return 1;
+	}
+
+	if (i->nextfrag < skb_shinfo(skb)->nr_frags) {
+		i->data = kmap_skb_frag(&skb_shinfo(skb)->frags[i->nextfrag]);
+		i->len = skb_shinfo(skb)->frags[i->nextfrag].size;
+		i->nextfrag++;
+		return 1;
+	}
+
+	i->fraglist = skb_shinfo(skb)->frag_list;
+	if (i->fraglist)
+		goto fraglist;
+
+end:
+	/* Bug trap for callers */
+	i->data = NULL;
+	return 0;
+}
+
+void skb_iter_abort(const struct sk_buff *skb, struct skb_iter *i)
+{
+	/* Unmap previous, if not head fragment. */
+	if (i->data && i->nextfrag)
+		kunmap_skb_frag(i->data);
+	/* Bug trap for callers */
+	i->data = NULL;
+}
+
 /* Checksum skb data. */
 
 unsigned int skb_checksum(const struct sk_buff *skb, int offset,
@@ -1399,3 +1463,6 @@ EXPORT_SYMBOL(skb_queue_tail);
 EXPORT_SYMBOL(skb_unlink);
 EXPORT_SYMBOL(skb_append);
 EXPORT_SYMBOL(skb_split);
+EXPORT_SYMBOL(skb_iter_first);
+EXPORT_SYMBOL(skb_iter_next);
+EXPORT_SYMBOL(skb_iter_abort);