mptcp: add a new helper subflow_req_create_thmac

This patch adds a new helper named subflow_req_create_thmac, which is
extracted from subflow_token_join_request. It initializes subflow_req's
local_nonce and thmac fields, those are the more expensive to populate.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/subflow.c

@@ -64,10 +64,23 @@ static bool mptcp_can_accept_new_subflow(const struct mptcp_sock *msk)
 }
 
 /* validate received token and create truncated hmac and nonce for SYN-ACK */
+static void subflow_req_create_thmac(struct mptcp_subflow_request_sock *subflow_req)
+{
+	struct mptcp_sock *msk = subflow_req->msk;
+	u8 hmac[SHA256_DIGEST_SIZE];
+
+	get_random_bytes(&subflow_req->local_nonce, sizeof(u32));
+
+	subflow_generate_hmac(msk->local_key, msk->remote_key,
+			      subflow_req->local_nonce,
+			      subflow_req->remote_nonce, hmac);
+
+	subflow_req->thmac = get_unaligned_be64(hmac);
+}
+
 static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
 {
 	struct mptcp_subflow_request_sock *subflow_req = mptcp_subflow_rsk(req);
-	u8 hmac[SHA256_DIGEST_SIZE];
 	struct mptcp_sock *msk;
 	int local_id;
 
@@ -84,13 +97,6 @@ static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
 	}
 	subflow_req->local_id = local_id;
 
-	get_random_bytes(&subflow_req->local_nonce, sizeof(u32));
-
-	subflow_generate_hmac(msk->local_key, msk->remote_key,
-			      subflow_req->local_nonce,
-			      subflow_req->remote_nonce, hmac);
-
-	subflow_req->thmac = get_unaligned_be64(hmac);
 	return msk;
 }
 
@@ -186,6 +192,8 @@ static int subflow_init_req(struct request_sock *req,
 		if (!subflow_req->msk)
 			return -EPERM;
 
+		subflow_req_create_thmac(subflow_req);
+
 		if (unlikely(req->syncookie)) {
 			if (mptcp_can_accept_new_subflow(subflow_req->msk))
 				subflow_init_req_cookie_join_save(subflow_req, skb);