netdev - Re: [PATCH net-next v3 05/24] ovpn: implement interface creation/destruction via netlink

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Date: Thu, 9 May 2024 10:30:51 +0200
From: Antonio Quartulli <antonio@...nvpn.net>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.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 05/24] ovpn: implement interface
 creation/destruction via netlink

On 09/05/2024 03:09, Jakub Kicinski wrote:
> On Wed, 8 May 2024 11:49:07 +0200 Antonio Quartulli wrote:
>>>> +		netdev_err(dev, "%s: cannot add ifname to reply\n", __func__);
>>>
>>> Probably not worth it, can't happen given the message size
>>
>> Personally I still prefer to check the return value of functions that
>> may fail, because somebody may break the assumption (i.e. message large
>> enough by design) without realizing that this call was relying on that.
>>
>> If you want, I could still add a comment saying that we don't expect
>> this to happen.
> 
> In a few other places we put a WARN_ON_ONCE() on messages size errors.
> That way syzbot usually catches the miscalculation rather quickly.
> But no strong objections if you prefer the print.

I am fine as long as we have some check.
If WARN_ON_ONCE() helps syzbot, then I'll go with it.

>    
>>>> +		genlmsg_cancel(msg, hdr);
>>>> +		nlmsg_free(msg);
>>>> +		return -EMSGSIZE;
>>>> +	}
>>>> +
>>>> +	genlmsg_end(msg, hdr);
>>>> +
>>>> +	return genlmsg_reply(msg, info);
>>>>    }
>>>>    
>>>>    int ovpn_nl_del_iface_doit(struct sk_buff *skb, struct genl_info *info)
>>>>    {
>>>> -	return -ENOTSUPP;
>>>> +	struct ovpn_struct *ovpn = info->user_ptr[0];
>>>> +
>>>> +	rtnl_lock();
>>>> +	ovpn_iface_destruct(ovpn);
>>>> +	dev_put(ovpn->dev);
>>>> +	rtnl_unlock();
>>>> +
>>>> +	synchronize_net();
>>>
>>> Why? 🤔️
>>
>>
>> hmm I was under the impression that we should always call this function
>> when destroying an interface to make sure that packets that already
>> entered the network stack can be properly processed before the interface
>> is gone for good.
>>
>> Maybe this is not the right place? Any hint?
> 
> The unregistration of the netdevice should take care of syncing packets
> in flight, AFAIU.

I have another call to synchronize_net() in ovpn_iface_destruct() after 
calling unregister_netdevice().

Sabrina was actually questioning that call too.

First of all I now realize that we are calling it twice, but from what I 
am understanding, I think we can just ditch any invocation and let core 
do the right thing.

I'll remove it and do some tests.


-- 
Antonio Quartulli
OpenVPN Inc.