netlink: factor out extack composition

The ext_ack writing code looks very "organically grown".
Move the calculation of the size and writing out to helpers.
This is more idiomatic and gives us the ability to return early
avoiding the long (and randomly ordered) "if" conditions.
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/netlink/af_netlink.c

@@ -2400,6 +2400,57 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 }
 EXPORT_SYMBOL(__netlink_dump_start);
 
+static size_t
+netlink_ack_tlv_len(struct netlink_sock *nlk, int err,
+		    const struct netlink_ext_ack *extack)
+{
+	size_t tlvlen;
+
+	if (!extack || !(nlk->flags & NETLINK_F_EXT_ACK))
+		return 0;
+
+	tlvlen = 0;
+	if (extack->_msg)
+		tlvlen += nla_total_size(strlen(extack->_msg) + 1);
+	if (extack->cookie_len)
+		tlvlen += nla_total_size(extack->cookie_len);
+
+	/* Following attributes are only reported as error (not warning) */
+	if (!err)
+		return tlvlen;
+
+	if (extack->bad_attr)
+		tlvlen += nla_total_size(sizeof(u32));
+	if (extack->policy)
+		tlvlen += netlink_policy_dump_attr_size_estimate(extack->policy);
+
+	return tlvlen;
+}
+
+static void
+netlink_ack_tlv_fill(struct sk_buff *in_skb, struct sk_buff *skb,
+		     struct nlmsghdr *nlh, int err,
+		     const struct netlink_ext_ack *extack)
+{
+	if (extack->_msg)
+		WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG, extack->_msg));
+	if (extack->cookie_len)
+		WARN_ON(nla_put(skb, NLMSGERR_ATTR_COOKIE,
+				extack->cookie_len, extack->cookie));
+
+	if (!err)
+		return;
+
+	if (extack->bad_attr &&
+	    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
+		     (u8 *)extack->bad_attr >= in_skb->data + in_skb->len))
+		WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
+				    (u8 *)extack->bad_attr - (u8 *)nlh));
+	if (extack->policy)
+		netlink_policy_dump_write_attr(skb, extack->policy,
+					       NLMSGERR_ATTR_POLICY);
+}
+
 void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		 const struct netlink_ext_ack *extack)
 {
@@ -2407,29 +2458,20 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 	struct nlmsghdr *rep;
 	struct nlmsgerr *errmsg;
 	size_t payload = sizeof(*errmsg);
-	size_t tlvlen = 0;
 	struct netlink_sock *nlk = nlk_sk(NETLINK_CB(in_skb).sk);
 	unsigned int flags = 0;
-	bool nlk_has_extack = nlk->flags & NETLINK_F_EXT_ACK;
+	size_t tlvlen;
 
 	/* Error messages get the original request appened, unless the user
 	 * requests to cap the error message, and get extra error data if
 	 * requested.
 	 */
-	if (nlk_has_extack && extack && extack->_msg)
-		tlvlen += nla_total_size(strlen(extack->_msg) + 1);
-
 	if (err && !(nlk->flags & NETLINK_F_CAP_ACK))
 		payload += nlmsg_len(nlh);
 	else
 		flags |= NLM_F_CAPPED;
-	if (err && nlk_has_extack && extack && extack->bad_attr)
-		tlvlen += nla_total_size(sizeof(u32));
-	if (nlk_has_extack && extack && extack->cookie_len)
-		tlvlen += nla_total_size(extack->cookie_len);
-	if (err && nlk_has_extack && extack && extack->policy)
-		tlvlen += netlink_policy_dump_attr_size_estimate(extack->policy);
 
+	tlvlen = netlink_ack_tlv_len(nlk, err, extack);
 	if (tlvlen)
 		flags |= NLM_F_ACK_TLVS;
 
@@ -2446,25 +2488,8 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 	errmsg->error = err;
 	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
 
-	if (nlk_has_extack && extack) {
-		if (extack->_msg) {
-			WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
-					       extack->_msg));
-		}
-		if (err && extack->bad_attr &&
-		    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
-			     (u8 *)extack->bad_attr >= in_skb->data +
-						       in_skb->len))
-			WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
-					    (u8 *)extack->bad_attr -
-					    (u8 *)nlh));
-		if (extack->cookie_len)
-			WARN_ON(nla_put(skb, NLMSGERR_ATTR_COOKIE,
-					extack->cookie_len, extack->cookie));
-		if (extack->policy)
-			netlink_policy_dump_write_attr(skb, extack->policy,
-						       NLMSGERR_ATTR_POLICY);
-	}
+	if (tlvlen)
+		netlink_ack_tlv_fill(in_skb, skb, nlh, err, extack);
 
 	nlmsg_end(skb, rep);