netfilter: ipset: send error message manually

When a message carries multiple commands and one of them triggers
an error, we have to report to the userspace which one was that.
The line number of the command plays this role and there's an attribute
reserved in the header part of the message to be filled out with the error
line number. In order not to modify the original message received from
the userspace, we construct a new, complete netlink error message and
modifies the attribute there, then send it.
Netlink is notified not to send its ACK/error message.

Signed-off-by: Jozsef Kadlecsik <[e-mail]
Signed-off-by: Patrick McHardy <[e-mail]>

net/netfilter/ipset/ip_set_core.c

···
 };
 
 static int
-call_ad(struct sk_buff *skb, struct ip_set *set,
+call_ad(struct sock *ctnl, struct sk_buff *skb, struct ip_set *set,
 	struct nlattr *tb[], enum ipset_adt adt,
 	u32 flags, bool use_lineno)
 {
···
 		return 0;
 	if (lineno && use_lineno) {
 		/* Error in restore/batch mode: send back lineno */
-		struct nlmsghdr *nlh = nlmsg_hdr(skb);
+		struct nlmsghdr *rep, *nlh = nlmsg_hdr(skb);
+		struct sk_buff *skb2;
+		struct nlmsgerr *errmsg;
+		size_t payload = sizeof(*errmsg) + nlmsg_len(nlh);
 		int min_len = NLMSG_SPACE(sizeof(struct nfgenmsg));
 		struct nlattr *cda[IPSET_ATTR_CMD_MAX+1];
-		struct nlattr *cmdattr = (void *)nlh + min_len;
+		struct nlattr *cmdattr;
 		u32 *errline;
+
+		skb2 = nlmsg_new(payload, GFP_KERNEL);
+		if (skb2 == NULL)
+			return -ENOMEM;
+		rep = __nlmsg_put(skb2, NETLINK_CB(skb).pid,
+				  nlh->nlmsg_seq, NLMSG_ERROR, payload, 0);
+		errmsg = nlmsg_data(rep);
+		errmsg->error = ret;
+		memcpy(&errmsg->msg, nlh, nlh->nlmsg_len);
+		cmdattr = (void *)&errmsg->msg + min_len;
 
 		nla_parse(cda, IPSET_ATTR_CMD_MAX,
 			  cmdattr, nlh->nlmsg_len - min_len,
···
 		errline = nla_data(cda[IPSET_ATTR_LINENO]);
 
 		*errline = lineno;
+
+		netlink_unicast(ctnl, skb2, NETLINK_CB(skb).pid, MSG_DONTWAIT);
+		/* Signal netlink not to send its ACK/errmsg.  */
+		return -EINTR;
 	}
 
 	return ret;
···
 				     attr[IPSET_ATTR_DATA],
 				     set->type->adt_policy))
 			return -IPSET_ERR_PROTOCOL;
-		ret = call_ad(skb, set, tb, IPSET_ADD, flags, use_lineno);
+		ret = call_ad(ctnl, skb, set, tb, IPSET_ADD, flags,
+			      use_lineno);
 	} else {
 		int nla_rem;
 
···
 			    nla_parse_nested(tb, IPSET_ATTR_ADT_MAX, nla,
 					     set->type->adt_policy))
 				return -IPSET_ERR_PROTOCOL;
-			ret = call_ad(skb, set, tb, IPSET_ADD,
+			ret = call_ad(ctnl, skb, set, tb, IPSET_ADD,
 				      flags, use_lineno);
 			if (ret < 0)
 				return ret;
···
 				     attr[IPSET_ATTR_DATA],
 				     set->type->adt_policy))
 			return -IPSET_ERR_PROTOCOL;
-		ret = call_ad(skb, set, tb, IPSET_DEL, flags, use_lineno);
+		ret = call_ad(ctnl, skb, set, tb, IPSET_DEL, flags,
+			      use_lineno);
 	} else {
 		int nla_rem;
 
···
 			    nla_parse_nested(tb, IPSET_ATTR_ADT_MAX, nla,
 					     set->type->adt_policy))
 				return -IPSET_ERR_PROTOCOL;
-			ret = call_ad(skb, set, tb, IPSET_DEL,
+			ret = call_ad(ctnl, skb, set, tb, IPSET_DEL,
 				      flags, use_lineno);
 			if (ret < 0)
 				return ret;