linux-kernel - Re: [PATCH net] bonding: use permanent address for MAC swapping if device address is same

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Message-ID: <Z_OFUaeotDYJ31o7@fedora>
Date: Mon, 7 Apr 2025 07:57:05 +0000
From: Hangbin Liu <liuhangbin@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, Jay Vosburgh <jv@...sburgh.net>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Simon Horman <horms@...nel.org>, Cosmin Ratiu <cratiu@...dia.com>,
	linux-kernel@...r.kernel.org, Liang Li <liali@...hat.com>
Subject: Re: [PATCH net] bonding: use permanent address for MAC swapping if
 device address is same

On Thu, Apr 03, 2025 at 04:02:33PM +0200, Paolo Abeni wrote:
> On 3/19/25 9:09 AM, Hangbin Liu wrote:
> > Similar with a951bc1e6ba5 ("bonding: correct the MAC address for "follow"
> > fail_over_mac policy"). The fail_over_mac follow mode requires the formerly
> > active slave to swap MAC addresses with the newly active slave during
> > failover. However, the slave's MAC address can be same under certain
> > conditions:
> > 
> > 1) ip link set eth0 master bond0
> >    bond0 adopts eth0's MAC address (MAC0).
> > 
> > 1) ip link set eth1 master bond0
> >    eth1 is added as a backup with its own MAC (MAC1).
> > 
> > 3) ip link set eth0 nomaster
> >    eth0 is released and restores its MAC (MAC0).
> >    eth1 becomes the active slave, and bond0 assigns MAC0 to eth1.
> 
> It was not immediately clear to me that the mac-dance in the code below
> happens only at failover time.
> 
> I second Jakub's doubt, I think it would be better to change eth0 mac
> address here (possibly to permanent eth1 mac, to preserve some consistency?)

I have talked about one of the duplicate mac issue with Jay before [1]. We
decided to print a warning for that. I will discuss with Jay for this one
in one new patch thread.

[1] https://lore.kernel.org/netdev/Z49yXz1dx2ZzqhC1@fedora

Thanks
Hangbin
> 
> Doing that in ndo_del_slave() should allow bonding to change the mac
> while still owning the old slave and avoid races with user-space.
> 
> WDYT?
> 
> Thanks,
> 
> Paolo
>