netdev - Re: [PATCH net] batman-adv: fix duplicate MAC address check

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Message-ID: <94e1ac2c-46f1-4787-ad50-e4a5ab11011a@universe-factory.net>
Date: Thu, 10 Apr 2025 17:38:30 +0200
From: Matthias Schiffer <mschiffer@...verse-factory.net>
To: Paolo Abeni <pabeni@...hat.com>, Marek Lindner
 <marek.lindner@...lbox.org>, Simon Wunderlich <sw@...onwunderlich.de>,
 Antonio Quartulli <antonio@...delbit.com>,
 Sven Eckelmann <sven@...fation.org>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Simon Horman <horms@...nel.org>, b.a.t.m.a.n@...ts.open-mesh.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] batman-adv: fix duplicate MAC address check

On 10/04/2025 11:38, Paolo Abeni wrote:
> On 4/8/25 6:30 PM, Matthias Schiffer wrote:
>> batadv_check_known_mac_addr() is both too lenient and too strict:
>>
>> - It is called from batadv_hardif_add_interface(), which means that it
>>    checked interfaces that are not used for batman-adv at all. Move it
>>    to batadv_hardif_enable_interface(). Also, restrict it to hardifs of
>>    the same mesh interface; different mesh interfaces should not interact
>>    at all. The batadv_check_known_mac_addr() argument is changed from
>>    `struct net_device` to `struct batadv_hard_iface` to achieve this.
>> - The check only cares about hardifs in BATADV_IF_ACTIVE and
>>    BATADV_IF_TO_BE_ACTIVATED states, but interfaces in BATADV_IF_INACTIVE
>>    state should be checked as well, or the following steps will not
>>    result in a warning then they should:
>>
>>    - Add two interfaces on down state with different MAC addresses to
>>      a mesh as hardifs
>>    - Change the MAC addresses so they confliect
>>    - Set interfaces to up state
>>
>>    Now there will be two active hardifs with the same MAC address, but no
>>    warning. Fix by only ignoring hardifs in BATADV_IF_NOT_IN_USE state.
>>
>> The RCU lock can be dropped, as we're holding RTNL anyways when the
>> function is called.
>>
>> While we're at it, also switch from pr_warn() to netdev_warn().
>>
>> Fixes: c6c8fea29769 ("net: Add batman-adv meshing protocol")
>> Signed-off-by: Matthias Schiffer <mschiffer@...verse-factory.net>
> 
> Even if marked for net I assume this will eventually go first via the
> batman tree.

Yes. Should I have marked this differently?

> 
>> ---
>>
>> Aside: batadv_hardif_add_interface() being called for all existing
>> interfaces and having a global batadv_hardif_list at all is also not
>> very nice, but this will be addressed separately, as changing it will
>> require more refactoring.
>>
>>   net/batman-adv/hard-interface.c | 37 ++++++++++++++++++++-------------
>>   1 file changed, 22 insertions(+), 15 deletions(-)
>>
>> diff --git a/net/batman-adv/hard-interface.c b/net/batman-adv/hard-interface.c
>> index f145f9662653..07b436626afb 100644
>> --- a/net/batman-adv/hard-interface.c
>> +++ b/net/batman-adv/hard-interface.c
>> @@ -506,28 +506,34 @@ batadv_hardif_is_iface_up(const struct batadv_hard_iface *hard_iface)
>>   	return false;
>>   }
>>   
>> -static void batadv_check_known_mac_addr(const struct net_device *net_dev)
>> +static void batadv_check_known_mac_addr(const struct batadv_hard_iface *hard_iface)
>>   {
>> -	const struct batadv_hard_iface *hard_iface;
>> +	const struct net_device *mesh_iface = hard_iface->mesh_iface;
>> +	const struct batadv_hard_iface *tmp_hard_iface;
>>   
>> -	rcu_read_lock();
>> -	list_for_each_entry_rcu(hard_iface, &batadv_hardif_list, list) {
>> -		if (hard_iface->if_status != BATADV_IF_ACTIVE &&
>> -		    hard_iface->if_status != BATADV_IF_TO_BE_ACTIVATED)
>> +	if (!mesh_iface)
>> +		return;
>> +
>> +	list_for_each_entry(tmp_hard_iface, &batadv_hardif_list, list) {
>> +		if (tmp_hard_iface == hard_iface)
>> +			continue;
>> +
>> +		if (tmp_hard_iface->mesh_iface != mesh_iface)
>>   			continue;
>>   
>> -		if (hard_iface->net_dev == net_dev)
>> +		if (tmp_hard_iface->if_status == BATADV_IF_NOT_IN_USE)
>>   			continue;
>>   
>> -		if (!batadv_compare_eth(hard_iface->net_dev->dev_addr,
>> -					net_dev->dev_addr))
>> +		if (!batadv_compare_eth(tmp_hard_iface->net_dev->dev_addr,
>> +					hard_iface->net_dev->dev_addr))
>>   			continue;
>>   
>> -		pr_warn("The newly added mac address (%pM) already exists on: %s\n",
>> -			net_dev->dev_addr, hard_iface->net_dev->name);
>> -		pr_warn("It is strongly recommended to keep mac addresses unique to avoid problems!\n");
>> +		netdev_warn(hard_iface->net_dev,
>> +			    "The newly added mac address (%pM) already exists on: %s\n",
>> +			    hard_iface->net_dev->dev_addr, tmp_hard_iface->net_dev->name);
>> +		netdev_warn(hard_iface->net_dev,
>> +			    "It is strongly recommended to keep mac addresses unique to avoid problems!\n");
>>   	}
>> -	rcu_read_unlock();
>>   }
> 
> I feel like the above code mixes unnecessarily fix and refactor
> (variable rename, different print helper usage).
> 
> I think the fix should be minimal, the refactor should land in a
> different patch for next.

Okay. I'll remove the print helper change for now.

I think the variable rename should be kept, as we now have two 
batadv_hard_iface* vars, so we need to introduce a second name. Naming the 
interface we're working on hard_iface and using tmp_hard_iface for a loop 
variable matches similar code that already exists in batman-adv.

Best,
Matthias

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