netdev - Re: [PATCH net-next v3 15/24] ovpn: implement peer lookup logic

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Date: Wed, 29 May 2024 22:19:04 +0200
From: Antonio Quartulli <antonio@...nvpn.net>
To: Sabrina Dubroca <sd@...asysnail.net>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Sergey Ryazanov <ryazanov.s.a@...il.com>, Paolo Abeni <pabeni@...hat.com>,
 Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew@...n.ch>,
 Esben Haabendal <esben@...nix.com>
Subject: Re: [PATCH net-next v3 15/24] ovpn: implement peer lookup logic

On 29/05/2024 18:42, Sabrina Dubroca wrote:
> 2024-05-28, 22:09:37 +0200, Antonio Quartulli wrote:
>> On 28/05/2024 18:42, Sabrina Dubroca wrote:
>>> 2024-05-06, 03:16:28 +0200, Antonio Quartulli wrote:
>>>> @@ -303,10 +427,28 @@ static struct ovpn_peer *ovpn_peer_get_by_id_p2p(struct ovpn_struct *ovpn,
>>>>    struct ovpn_peer *ovpn_peer_get_by_id(struct ovpn_struct *ovpn, u32 peer_id)
>>>>    {
>>>> -	struct ovpn_peer *peer = NULL;
>>>> +	struct ovpn_peer *tmp, *peer = NULL;
>>>> +	struct hlist_head *head;
>>>> +	u32 index;
>>>>    	if (ovpn->mode == OVPN_MODE_P2P)
>>>> -		peer = ovpn_peer_get_by_id_p2p(ovpn, peer_id);
>>>> +		return ovpn_peer_get_by_id_p2p(ovpn, peer_id);
>>>> +
>>>> +	index = ovpn_peer_index(ovpn->peers.by_id, &peer_id, sizeof(peer_id));
>>>> +	head = &ovpn->peers.by_id[index];
>>>> +
>>>> +	rcu_read_lock();
>>>> +	hlist_for_each_entry_rcu(tmp, head, hash_entry_id) {
>>>> +		if (tmp->id != peer_id)
>>>> +			continue;
>>>> +
>>>> +		if (!ovpn_peer_hold(tmp))
>>>> +			continue;
>>>
>>> Can there ever be multiple peers with the same id? (ie, is it worth
>>> continuing the loop if this fails? the same question probably applies
>>> to ovpn_peer_get_by_transp_addr as well)
>>
>> Well, not at the same time, but theoretically we could re-use the ID of a
>> peer that is being released (i.e. still in the list but refcnt at 0) because
>> it won't be returned by this lookup.
>>
>> This said, I truly believe it's impossible for a peer to have refcnt 0 and
>> still being in the list:
>> Either
>> * delete on the peer was not yet called, thus peer is in the list and the
>> last reference wasn't yet dropped
>> * delete on the peer was called, thus peer cannot be in the list anymore and
>> refcnt may or may not be 0...
> 
> Ok, thanks. Let's just keep this code.

ok

> 
> 
>>>> +/**
>>>> + * ovpn_nexthop_from_rt6 - look up the IPv6 nexthop for the given destination
>>>
>>> I'm a bit confused by this talk about "destination" when those two
>>> functions are then used with the source address from the packet, from
>>> a function called "get_by_src".
>>
>> well, in my brain a next hop can exists only when I want to reach a certain
>> destination. Therefore, at a low level, the terms nextop and destination
>> always need to go hand in hand.
>>
>> This said, when implementing RPF (Reverse Path Filtering) I need to imagine
>> that I want to route to the source IP of the incoming packet. If the nexthop
>> I looked up matches the peer the packet came from, then everything is fine.
>>
>> makes sense?
> 
> Yeah, that's fair.
> 
>>
>> [FTR I have already renamed/changed get_by_src into check_by_src, because I
>> don't need to truly extract a peer and get a reference, but I only need to
>> perform the aforementioned comparison.]
> 
> Ok.
> 
>>>> + * @ovpn: the private data representing the current VPN session
>>>> + * @dst: the destination to be looked up
>>>> + *
>>>> + * Looks up in the IPv6 system routing table the IO of the nexthop to be used
>>>
>>> "the IO"?
>>
>> typ0: "the IP"
>>
>>>
>>>> + * to reach the destination passed as argument. IF no nexthop can be found, the
>>>> + * destination itself is returned as it probably has to be used as nexthop.
>>>> + *
>>>> + * Return: the IP of the next hop if found or the dst itself otherwise
>>>
>>> "the dst" tends to refer to a dst_entry, maybe "or @dst otherwise"?
>>
>> it refers to @dst (the function argument). That's basically the case where
>> the destination is "onlink" and thus it is the nexthop (basically the
>> destination is the connected peer).
> 
> I understand that, it's just the wording "the dst" that I'm
> nitpicking. s/dst/addr/ would help easily-confused people like me (for
> both "the dst" and my confusion with source vs destination in
> caller/callee), but I can live with this.

Oh ok, now I understand your concern.
I will reword this part a bit and add a comment in the caller to clarify 
why we invoke nexthop_from_rt4/6 passing the source address as param.

Cheers,

> 

-- 
Antonio Quartulli
OpenVPN Inc.