netdev - Re: [PATCH net-next v18 12/25] ovpn: implement TCP transport

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Message-ID: <Z4qP-x4F-lQiQTRy@hog>
Date: Fri, 17 Jan 2025 18:14:35 +0100
From: Sabrina Dubroca <sd@...asysnail.net>
To: Antonio Quartulli <antonio@...nvpn.net>
Cc: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Donald Hunter <donald.hunter@...il.com>,
	Shuah Khan <shuah@...nel.org>, ryazanov.s.a@...il.com,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Simon Horman <horms@...nel.org>, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, Xiao Liang <shaw.leon@...il.com>
Subject: Re: [PATCH net-next v18 12/25] ovpn: implement TCP transport

2025-01-13, 10:31:31 +0100, Antonio Quartulli wrote:
> +static int ovpn_tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> +			    int flags, int *addr_len)
> +{
> +	int err = 0, off, copied = 0, ret;
> +	struct ovpn_socket *sock;
> +	struct ovpn_peer *peer;
> +	struct sk_buff *skb;
> +
> +	rcu_read_lock();
> +	sock = rcu_dereference_sk_user_data(sk);
> +	if (!sock || !sock->peer) {
> +		rcu_read_unlock();
> +		return -EBADF;
> +	}
> +	/* we take a reference to the peer linked to this TCP socket, because
> +	 * in turn the peer holds a reference to the socket itself.

Not anymore since v12? [*]

I think it's ok here because we're only using peer and sk (not
anything from ovpn_socket), but it is relevant in _sendmsg, which has
the same peer_hold pattern without this comment.

[*]
v11:
 - https://lore.kernel.org/netdev/20241029-b4-ovpn-v11-8-de4698c73a25@openvpn.net/
   ovpn_peer_release -> ovpn_socket_put

v12:
 - https://lore.kernel.org/netdev/20241202-b4-ovpn-v12-9-239ff733bf97@openvpn.net/
   ovpn_peer_release doesn't do ovpn_socket_put

 - https://lore.kernel.org/netdev/20241202-b4-ovpn-v12-7-239ff733bf97@openvpn.net/
   ovpn_socket_put is done directly at ovpn_peer_remove time, before the final peer_put

> +	 * By doing so we also ensure that the peer stays alive along with
> +	 * the socket while executing this function
> +	 */
> +	ovpn_peer_hold(sock->peer);
> +	peer = sock->peer;
> +	rcu_read_unlock();
> +
> +	skb = __skb_recv_datagram(sk, &peer->tcp.user_queue, flags, &off, &err);
> +	if (!skb) {
> +		if (err == -EAGAIN && sk->sk_shutdown & RCV_SHUTDOWN) {
> +			ret = 0;
> +			goto out;
> +		}
> +		ret = err;
> +		goto out;
> +	}
> +
> +	copied = len;
> +	if (copied > skb->len)
> +		copied = skb->len;
> +	else if (copied < skb->len)
> +		msg->msg_flags |= MSG_TRUNC;
> +
> +	err = skb_copy_datagram_msg(skb, 0, msg, copied);
> +	if (unlikely(err)) {
> +		kfree_skb(skb);
> +		ret = err;
> +		goto out;
> +	}
> +
> +	if (flags & MSG_TRUNC)
> +		copied = skb->len;
> +	kfree_skb(skb);
> +	ret = copied;
> +out:
> +	ovpn_peer_put(peer);
> +	return ret;
> +}

[...]
> +static int ovpn_tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
> +{
> +	struct ovpn_socket *sock;
> +	int ret, linear = PAGE_SIZE;
> +	struct ovpn_peer *peer;
> +	struct sk_buff *skb;
> +
> +	rcu_read_lock();
> +	sock = rcu_dereference_sk_user_data(sk);
> +	if (unlikely(!sock || !sock->peer || !ovpn_peer_hold(sock->peer))) {
> +		rcu_read_unlock();
> +		return -EIO;
> +	}
> +	peer = sock->peer;
> +	rcu_read_unlock();
> +
> +	lock_sock(peer->sock->sock->sk);

Isn't that just sk?

> +
> +	if (msg->msg_flags & ~MSG_DONTWAIT) {
> +		ret = -EOPNOTSUPP;
> +		goto peer_free;
> +	}
> +
> +	if (peer->tcp.out_msg.skb) {
> +		ret = -EAGAIN;
> +		goto peer_free;
> +	}
> +
> +	if (size < linear)
> +		linear = size;
> +
> +	skb = sock_alloc_send_pskb(sk, linear, size - linear,
> +				   msg->msg_flags & MSG_DONTWAIT, &ret, 0);
> +	if (!skb) {
> +		net_err_ratelimited("%s: skb alloc failed: %d\n",
> +				    netdev_name(sock->peer->ovpn->dev), ret);

Since we only have a ref on peer (but not on sock), I'd use
peer->... directly instead of sock->peer.

> +		goto peer_free;
> +	}
> +
> +	skb_put(skb, linear);
> +	skb->len = size;
> +	skb->data_len = size - linear;
> +
> +	ret = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, size);
> +	if (ret) {
> +		kfree_skb(skb);
> +		net_err_ratelimited("%s: skb copy from iter failed: %d\n",
> +				    netdev_name(sock->peer->ovpn->dev), ret);

s/sock->//

> +		goto peer_free;
> +	}
> +
> +	ovpn_tcp_send_sock_skb(sock->peer, skb);

s/sock->//

> +	ret = size;
> +peer_free:
> +	release_sock(peer->sock->sock->sk);
> +	ovpn_peer_put(peer);
> +	return ret;
> +}

-- 
Sabrina