netlink: add nla_policy_len()

It calculates the max. length of a Netlink policy, which is usefull
for allocating Netlink buffers roughly the size of the actual
message.
Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/net/netlink.h

@@ -230,6 +230,7 @@ extern int		nla_validate(struct nlattr *head, int len, int maxtype,
 extern int		nla_parse(struct nlattr *tb[], int maxtype,
 				  struct nlattr *head, int len,
 				  const struct nla_policy *policy);
+extern int		nla_policy_len(const struct nla_policy *, int);
 extern struct nlattr *	nla_find(struct nlattr *head, int len, int attrtype);
 extern size_t		nla_strlcpy(char *dst, const struct nlattr *nla,
 				    size_t dstsize);

net/netlink/attr.c

@@ -132,6 +132,32 @@ errout:
 	return err;
 }
 
+/**
+ * nla_policy_len - Determin the max. length of a policy
+ * @policy: policy to use
+ * @n: number of policies
+ *
+ * Determines the max. length of the policy.  It is currently used
+ * to allocated Netlink buffers roughly the size of the actual
+ * message.
+ *
+ * Returns 0 on success or a negative error code.
+ */
+int
+nla_policy_len(const struct nla_policy *p, int n)
+{
+	int i, len = 0;
+
+	for (i = 0; i < n; i++) {
+		if (p->len)
+			len += nla_total_size(p->len);
+		else if (nla_attr_minlen[p->type])
+			len += nla_total_size(nla_attr_minlen[p->type]);
+	}
+
+	return len;
+}
+
 /**
  * nla_parse - Parse a stream of attributes into a tb buffer
  * @tb: destination array with maxtype+1 elements
@@ -456,6 +482,7 @@ int nla_append(struct sk_buff *skb, int attrlen, const void *data)
 }
 
 EXPORT_SYMBOL(nla_validate);
+EXPORT_SYMBOL(nla_policy_len);
 EXPORT_SYMBOL(nla_parse);
 EXPORT_SYMBOL(nla_find);
 EXPORT_SYMBOL(nla_strlcpy);