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

[Antonio Quartulli]

On 02/03/2025 19:59, Sabrina Dubroca wrote:
> 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]

Mh yeah, I like your suggestion as I don't have to change the EXPORT and 
also I don't have to make assumptions on the original value of 
->release_cb as I can just call it.

Will follow your suggestion. Thanks!

> [...]
> > +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)

ACK (and thanks for double checking all other paths!)
I will do some magic on this peer_del() in order to make it sleep 
without issues.

Thanks!

Regards,

>

--
Antonio Quartulli
OpenVPN Inc.