Subject: Re: [PATCH net-next v11 06/23] ovpn: introduce the ovpn_peer object

From: Sabrina Dubroca <[email-masked]>
Date: Tue, 5 Nov 2024 14:12:32 +0100

2024-10-30, 21:47:58 +0100, Antonio Quartulli wrote:
> On 30/10/2024 17:37, Sabrina Dubroca wrote:
> > 2024-10-29, 11:47:19 +0100, Antonio Quartulli wrote:
> > > +static void ovpn_peer_release(struct ovpn_peer *peer)
> > > +{
> > > +	ovpn_bind_reset(peer, NULL);
> > > +
> > > +	dst_cache_destroy(&peer->dst_cache);
> > 
> > Is it safe to destroy the cache at this time? In the same function, we
> > use rcu to free the peer, but AFAICT the dst_cache will be freed
> > immediately:
> > 
> > void dst_cache_destroy(struct dst_cache *dst_cache)
> > {
> > [...]
> > 	free_percpu(dst_cache->cache);
> > }
> > 
> > (probably no real issue because ovpn_udp_send_skb gets called while we
> > hold a reference to the peer?)
> 
> Right.
> That was my assumption: release happens on refcnt = 0 only, therefore no
> field should be in use anymore.
> Anything that may still be in use will have its own refcounter.

My worry is that code changes over time, assumptions are forgotten,
and we end up with code that was a bit odd but safe not being safe
anymore.

> > 
> > > +	netdev_put(peer->ovpn->dev, &peer->ovpn->dev_tracker);
> > > +	kfree_rcu(peer, rcu);
> > > +}
> > 
> > 
> > [...]
> > > +static int ovpn_peer_del_p2p(struct ovpn_peer *peer,
> > > +			     enum ovpn_del_peer_reason reason)
> > > +	__must_hold(&peer->ovpn->lock)
> > > +{
> > > +	struct ovpn_peer *tmp;
> > > +
> > > +	tmp = rcu_dereference_protected(peer->ovpn->peer,
> > > +					lockdep_is_held(&peer->ovpn->lock));
> > > +	if (tmp != peer) {
> > > +		DEBUG_NET_WARN_ON_ONCE(1);
> > > +		if (tmp)
> > > +			ovpn_peer_put(tmp);
> > 
> > Does peer->ovpn->peer need to be set to NULL here as well? Or is it
> > going to survive this _put?
> 
> First of all consider that this is truly something that we don't expect to
> happen (hence the WARN_ON).
> If this is happening it's because we are trying to delete a peer that is not
> the one we are connected to (unexplainable scenario in p2p mode).
>
> Still, should we hit this case (I truly can't see how), I'd say "leave
> everything as is - maybe this call was just a mistake".

Yeah, true, let's leave it. Thanks.

-- 
Sabrina