netdev - Re: [PATCH net-next v20 12/25] ovpn: implement TCP transport

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Message-ID: <Z8SqiJedxrFhGuB9@hog>
Date: Sun, 2 Mar 2025 19:59:20 +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>,
	David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net-next v20 12/25] ovpn: implement TCP transport

2025-02-27, 02:21:37 +0100, Antonio Quartulli wrote:
> Moreover export tcp_release_cb by means of EXPORT_SYMBOL instead of
> EXPORT_IPV6_MOD, so that other modules can use it, even if IPV6 is
> not compiled in.

Is that really needed? You're saving tcp.sk_cb.prot, so you could just
call peer->tcp.sk_cb.prot->release_cb? (with a bit of care since it's
called after peer_put)

[I don't know what the maintainers' preference is wrt "re-exporting"
symbols that got moved to EXPORT_IPV6_MOD]


[...]
> +static void ovpn_tcp_send_sock(struct ovpn_peer *peer, struct sock *sk)
> +{
> +	struct sk_buff *skb = peer->tcp.out_msg.skb;
> +
> +	if (!skb)
> +		return;
> +
> +	if (peer->tcp.tx_in_progress)
> +		return;
> +
> +	peer->tcp.tx_in_progress = true;
> +
> +	do {
> +		int ret = skb_send_sock_locked(sk, skb,
> +					       peer->tcp.out_msg.offset,
> +					       peer->tcp.out_msg.len);
> +		if (unlikely(ret < 0)) {
> +			if (ret == -EAGAIN)
> +				goto out;
> +
> +			net_warn_ratelimited("%s: TCP error to peer %u: %d\n",
> +					     netdev_name(peer->ovpn->dev),
> +					     peer->id, ret);
> +
> +			/* in case of TCP error we can't recover the VPN
> +			 * stream therefore we abort the connection
> +			 */
> +			ovpn_peer_del(peer,
> +				      OVPN_DEL_PEER_REASON_TRANSPORT_ERROR);

I don't think this works:

ovpn_peer_del -> unlock_ovpn -> ovpn_socket_release -> might_sleep

but we can get to ovpn_tcp_send_sock in a few contexts that are not
allowed to sleep:

ovpn_tcp_send_skb -> ovpn_tcp_send_sock_skb -> ovpn_tcp_send_sock
__sk_flush_backlog -> release_cb = ovpn_tcp_release -> ovpn_tcp_send_sock_skb
release_sock       -> release_cb = ovpn_tcp_release -> ovpn_tcp_send_sock_skb


(I checked all other paths leading to unlock_ovpn/ovpn_socket_release,
this is the only one I could find that is not allowed to sleep. So it
would likely be easier to push this peer_del (or even just the
handling of release_list) into some other sleepable context than
trying to reshuffle all the other paths)

-- 
Sabrina