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

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Date: Wed, 8 May 2024 18:09:46 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Antonio Quartulli <antonio@...nvpn.net>
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 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.
  
> >> +		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.