netlink: introduce type-checking attribute iteration for nlmsg

Add the nlmsg_for_each_attr_type() macro to simplify iteration over
attributes of a specific type in a Netlink message.

Convert existing users in vxlan and nfsd to use the new macro.

Suggested-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Carolina Jubran <[e-mail]>
Signed-off-by: Mark Bloch <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/vxlan/vxlan_vnifilter.c

···
 	if (!(vxlan->cfg.flags & VXLAN_F_VNIFILTER))
 		return -EOPNOTSUPP;
 
-	nlmsg_for_each_attr(attr, nlh, sizeof(*tmsg), rem) {
-		switch (nla_type(attr)) {
-		case VXLAN_VNIFILTER_ENTRY:
-			err = vxlan_process_vni_filter(vxlan, attr,
-						       nlh->nlmsg_type, extack);
-			break;
-		default:
-			continue;
-		}
+	nlmsg_for_each_attr_type(attr, VXLAN_VNIFILTER_ENTRY, nlh,
+				 sizeof(*tmsg), rem) {
+		err = vxlan_process_vni_filter(vxlan, attr, nlh->nlmsg_type,
+					       extack);
 		vnis++;
 		if (err)
 			break;

fs/nfsd/nfsctl.c

···
 		return -EINVAL;
 
 	/* count number of SERVER_THREADS values */
-	nlmsg_for_each_attr(attr, info->nlhdr, GENL_HDRLEN, rem) {
-		if (nla_type(attr) == NFSD_A_SERVER_THREADS)
-			nrpools++;
-	}
+	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_THREADS, info->nlhdr,
+				 GENL_HDRLEN, rem)
+		nrpools++;
 
 	mutex_lock(&nfsd_mutex);
 
···
 	}
 
 	i = 0;
-	nlmsg_for_each_attr(attr, info->nlhdr, GENL_HDRLEN, rem) {
-		if (nla_type(attr) == NFSD_A_SERVER_THREADS) {
-			nthreads[i++] = nla_get_u32(attr);
-			if (i >= nrpools)
-				break;
-		}
+	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_THREADS, info->nlhdr,
+				 GENL_HDRLEN, rem) {
+		nthreads[i++] = nla_get_u32(attr);
+		if (i >= nrpools)
+			break;
 	}
 
 	if (info->attrs[NFSD_A_SERVER_GRACETIME] ||
···
 	for (i = 0; i <= NFSD_SUPPORTED_MINOR_VERSION; i++)
 		nfsd_minorversion(nn, i, NFSD_CLEAR);
 
-	nlmsg_for_each_attr(attr, info->nlhdr, GENL_HDRLEN, rem) {
+	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_PROTO_VERSION, info->nlhdr,
+				 GENL_HDRLEN, rem) {
 		struct nlattr *tb[NFSD_A_VERSION_MAX + 1];
 		u32 major, minor = 0;
 		bool enabled;
-
-		if (nla_type(attr) != NFSD_A_SERVER_PROTO_VERSION)
-			continue;
 
 		if (nla_parse_nested(tb, NFSD_A_VERSION_MAX, attr,
 				     nfsd_version_nl_policy, info->extack) < 0)
···
 	 * Walk the list of server_socks from userland and move any that match
 	 * back to sv_permsocks
 	 */
-	nlmsg_for_each_attr(attr, info->nlhdr, GENL_HDRLEN, rem) {
+	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_SOCK_ADDR, info->nlhdr,
+				 GENL_HDRLEN, rem) {
 		struct nlattr *tb[NFSD_A_SOCK_MAX + 1];
 		const char *xcl_name;
 		struct sockaddr *sa;
-
-		if (nla_type(attr) != NFSD_A_SERVER_SOCK_ADDR)
-			continue;
 
 		if (nla_parse_nested(tb, NFSD_A_SOCK_MAX, attr,
 				     nfsd_sock_nl_policy, info->extack) < 0)
···
 		svc_xprt_destroy_all(serv, net);
 
 	/* walk list of addrs again, open any that still don't exist */
-	nlmsg_for_each_attr(attr, info->nlhdr, GENL_HDRLEN, rem) {
+	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_SOCK_ADDR, info->nlhdr,
+				 GENL_HDRLEN, rem) {
 		struct nlattr *tb[NFSD_A_SOCK_MAX + 1];
 		const char *xcl_name;
 		struct sockaddr *sa;
 		int ret;
-
-		if (nla_type(attr) != NFSD_A_SERVER_SOCK_ADDR)
-			continue;
 
 		if (nla_parse_nested(tb, NFSD_A_SOCK_MAX, attr,
 				     nfsd_sock_nl_policy, info->extack) < 0)

include/net/netlink.h

···
  *   nlmsg_for_each_msg()		loop over all messages
  *   nlmsg_validate()			validate netlink message incl. attrs
  *   nlmsg_for_each_attr()		loop over all attributes
+ *   nlmsg_for_each_attr_type()		loop over all attributes with the
+ *					given type
  *
  * Misc:
  *   nlmsg_report()			report back to application?
···
 #define nlmsg_for_each_attr(pos, nlh, hdrlen, rem) \
 	nla_for_each_attr(pos, nlmsg_attrdata(nlh, hdrlen), \
 			  nlmsg_attrlen(nlh, hdrlen), rem)
+
+/**
+ * nlmsg_for_each_attr_type - iterate over a stream of attributes
+ * @pos: loop counter, set to the current attribute
+ * @type: required attribute type for @pos
+ * @nlh: netlink message header
+ * @hdrlen: length of the family specific header
+ * @rem: initialized to len, holds bytes currently remaining in stream
+ */
+#define nlmsg_for_each_attr_type(pos, type, nlh, hdrlen, rem) \
+	nlmsg_for_each_attr(pos, nlh, hdrlen, rem) \
+		if (nla_type(pos) == type)
 
 /**
  * nlmsg_put - Add a new netlink message to an skb