libnetlink: add parse_rtattr_one_nested helper

Sometimes, it is more convenient to get only one specific nested attribute by
type. For example for IFLA_AF_SPEC where type is address family (AF_INET6).
So add this helper for this purpose.
Signed-off-by: Jiri Pirko <jiri@resnulli.us>

include/libnetlink.h

@@ -82,11 +82,15 @@ extern int parse_rtattr(struct rtattr *tb[], int max, struct rtattr *rta, int le
 extern int parse_rtattr_flags(struct rtattr *tb[], int max, struct rtattr *rta,
 			      int len, unsigned short flags);
 extern int parse_rtattr_byindex(struct rtattr *tb[], int max, struct rtattr *rta, int len);
+extern struct rtattr *parse_rtattr_one(int type, struct rtattr *rta, int len);
 extern int __parse_rtattr_nested_compat(struct rtattr *tb[], int max, struct rtattr *rta, int len);
 
 #define parse_rtattr_nested(tb, max, rta) \
 	(parse_rtattr((tb), (max), RTA_DATA(rta), RTA_PAYLOAD(rta)))
 
+#define parse_rtattr_one_nested(type, rta) \
+	(parse_rtattr_one(type, RTA_DATA(rta), RTA_PAYLOAD(rta)))
+
 #define parse_rtattr_nested_compat(tb, max, rta, data, len) \
 	({ data = RTA_PAYLOAD(rta) >= len ? RTA_DATA(rta) : NULL;	\
 		__parse_rtattr_nested_compat(tb, max, rta, len); })

lib/libnetlink.c

@@ -713,6 +713,18 @@ int parse_rtattr_byindex(struct rtattr *tb[], int max, struct rtattr *rta, int l
 	return i;
 }
 
+struct rtattr *parse_rtattr_one(int type, struct rtattr *rta, int len)
+{
+	while (RTA_OK(rta, len)) {
+		if (rta->rta_type == type)
+			return rta;
+		rta = RTA_NEXT(rta, len);
+	}
+	if (len)
+		fprintf(stderr, "!!!Deficit %d, rta_len=%d\n", len, rta->rta_len);
+	return NULL;
+}
+
 int __parse_rtattr_nested_compat(struct rtattr *tb[], int max, struct rtattr *rta,
 			         int len)
 {