Subject: Re: [PATCHv3 net] bonding: assign random address if device address is same as bond

From: Jay Vosburgh <[email-masked]>
Date: Thu, 24 Apr 2025 16:44:52 -0700

Hangbin Liu <[email-masked]> wrote:

>This change addresses a MAC address conflict issue in failover scenarios,
>similar to the problem described in commit a951bc1e6ba5 ("bonding: correct
>the MAC address for 'follow' fail_over_mac policy").
>
>In fail_over_mac=follow mode, the bonding driver expects the formerly active
>slave to swap MAC addresses with the newly active slave during failover.
>However, under certain conditions, two slaves may end up with the same MAC
>address, which breaks this policy:
>
>1) ip link set eth0 master bond0
>   -> bond0 adopts eth0's MAC address (MAC0).
>
>2) 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.
>
>4) ip link set eth0 master bond0
>   -> eth0 is re-added to bond0, now both eth0 and eth1 have MAC0.
>
>This results in a MAC address conflict and violates the expected behavior
>of the failover policy.
>
>To fix this, we assign a random MAC address to any newly added slave if
>its current MAC address matches that of the bond. The original (permanent)
>MAC address is saved and will be restored when the device is released
>from the bond.
>
>This ensures that each slave has a unique MAC address during failover
>transitions, preserving the integrity of the fail_over_mac=follow policy.
>
>Fixes: 3915c1e8634a ("bonding: Add "follow" option to fail_over_mac")
>Signed-off-by: Hangbin Liu <[email-masked]>

	The code flow is a little clunky in the "if (situation one) else
if (situation two) else goto skip_mac_set" bit, but I don't really have
a better suggestion that isn't clunky in some other way.

	This implementation does keep the already complicated failover
logic from becoming more complicated for this corner case.

	-J

Acked-by: Jay Vosburgh <[email-masked]>

>---
>v3: set random MAC address for the new added link (Jakub Kicinski)
>    change the MAC address during enslave, not failover (Jay Vosburgh)
>v2: use memcmp directly instead of adding a redundant helper (Jakub Kicinski)
>---
> drivers/net/bonding/bond_main.c | 25 ++++++++++++++++++-------
> 1 file changed, 18 insertions(+), 7 deletions(-)
>
>diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>index 8ea183da8d53..b91ed8eb7eb7 100644
>--- a/drivers/net/bonding/bond_main.c
>+++ b/drivers/net/bonding/bond_main.c
>@@ -2118,15 +2118,26 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
> 		 * set the master's mac address to that of the first slave
> 		 */
> 		memcpy(ss.__data, bond_dev->dev_addr, bond_dev->addr_len);
>-		ss.ss_family = slave_dev->type;
>-		res = dev_set_mac_address(slave_dev, (struct sockaddr *)&ss,
>-					  extack);
>-		if (res) {
>-			slave_err(bond_dev, slave_dev, "Error %d calling set_mac_address\n", res);
>-			goto err_restore_mtu;
>-		}
>+	} else if (bond->params.fail_over_mac == BOND_FOM_FOLLOW &&
>+		   BOND_MODE(bond) == BOND_MODE_ACTIVEBACKUP &&
>+		   memcmp(slave_dev->dev_addr, bond_dev->dev_addr, bond_dev->addr_len) == 0) {
>+		/* Set slave to random address to avoid duplicate mac
>+		 * address in later fail over.
>+		 */
>+		eth_random_addr(ss.__data);
>+	} else {
>+		goto skip_mac_set;
> 	}
> 
>+	ss.ss_family = slave_dev->type;
>+	res = dev_set_mac_address(slave_dev, (struct sockaddr *)&ss, extack);
>+	if (res) {
>+		slave_err(bond_dev, slave_dev, "Error %d calling set_mac_address\n", res);
>+		goto err_restore_mtu;
>+	}
>+
>+skip_mac_set:
>+
> 	/* set no_addrconf flag before open to prevent IPv6 addrconf */
> 	slave_dev->priv_flags |= IFF_NO_ADDRCONF;
> 
>-- 
>2.46.0
>

---
	-Jay Vosburgh, [email-masked]