netdev - Re: [PATCH batadv 4/5] batman-adv: remove global hardif list

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Message-ID: <4860101.CbtlEUcBR6@sven-desktop>
Date: Sat, 31 May 2025 11:56:34 +0200
From: Sven Eckelmann <sven@...fation.org>
To: Marek Lindner <marek.lindner@...lbox.org>,
 Simon Wunderlich <sw@...onwunderlich.de>,
 Antonio Quartulli <antonio@...delbit.com>,
 Matthias Schiffer <mschiffer@...verse-factory.net>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 b.a.t.m.a.n@...ts.open-mesh.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 Matthias Schiffer <mschiffer@...verse-factory.net>
Subject: Re: [PATCH batadv 4/5] batman-adv: remove global hardif list

On Monday, 19 May 2025 22:46:31 CEST Matthias Schiffer wrote:
>  struct batadv_hard_iface *
> -batadv_hardif_get_by_netdev(const struct net_device *net_dev)
> +batadv_hardif_get_by_netdev(struct net_device *net_dev)
>  {
>         struct batadv_hard_iface *hard_iface;
> +       struct net_device *mesh_iface;
>  
> -       rcu_read_lock();
> -       list_for_each_entry_rcu(hard_iface, &batadv_hardif_list, list) {
> -               if (hard_iface->net_dev == net_dev &&
> -                   kref_get_unless_zero(&hard_iface->refcount))
> -                       goto out;
> -       }
> +       mesh_iface = netdev_master_upper_dev_get(net_dev);
> +       if (!mesh_iface || !batadv_meshif_is_valid(mesh_iface))
> +               return NULL;
>  
> -       hard_iface = NULL;
> +       hard_iface = netdev_lower_dev_get_private(mesh_iface, net_dev);
> +       if (!kref_get_unless_zero(&hard_iface->refcount))
> +               return NULL;
>  
> -out:
> -       rcu_read_unlock();
>         return hard_iface;
>  }

This code is now relying on rtnl_lock() (see `ASSERT_RTNL` in 
`netdev_master_upper_dev_get` and most likely some comments somwhere about the 
lists used by `netdev_lower_dev_get_private`). But `batadv_tt_local_add` is 
using this function without holding this lock all the time. For example during
packet processing.

See for example `batadv_tt_local_add` calls in `batadv_interface_tx`. This 
will happen when `skb->skb_iif` is not 0 (so it was forwarded).

Please double check this - I have not actually tested it but just went through 
the code.

And saying this, the `batadv_hardif_get_by_netdev` call was also used to 
retrieve additional information about alll kind of interfaces - even when they 
are not used by batman-adv directly. For example for figuring out if it is a 
wifi interface(for the TT wifi flag). With you change here, you are basically 
breaking this functionality because you now require that the netdev is a lower 
interface of batman-adv. Therefore, things like:

                   ┌──────┐             
       ┌───────────┼br-lan├──────┐      
       │           └──────┘      │      
       │                         │      
       │                         │      
     ┌─▼─┐                    ┌──▼─┐    
     │ap0│                    │bat0│    
     └───┘                    └──┬─┘    
                                 │      
                                 │      
                              ┌──▼──┐   
                              │mesh0│   
                              └─────┘   

Is not handled anymore correctly in TT because ap0 is not a lower interface of 
any batadv mesh interface. And as result, the ap-isolation feature of TT
will break.

Kind regards,
	Sven
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