netdev - [PATCH net 1/2] bonding: set random address only when slaves already exist

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Message-ID: <20250820091009.393785-1-liuhangbin@gmail.com>
Date: Wed, 20 Aug 2025 09:10:08 +0000
From: Hangbin Liu <liuhangbin@...il.com>
To: netdev@...r.kernel.org
Cc: 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>,
	Paolo Abeni <pabeni@...hat.com>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Simon Horman <horms@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	linux-kselftest@...r.kernel.org,
	Hangbin Liu <liuhangbin@...il.com>,
	Qiuling Ren <qren@...hat.com>
Subject: [PATCH net 1/2] bonding: set random address only when slaves already exist

Commit 5c3bf6cba791 ("bonding: assign random address if device address is
same as bond") fixed an issue where, after releasing the first slave and
re-adding it to the bond with fail_over_mac=follow, both the active and
backup slaves could end up with duplicate MAC addresses. To avoid this,
the new slave was assigned a random address.

However, if this happens when adding the very first slave, the bond’s
hardware address is set to match the slave’s. Later, during the
fail_over_mac=follow check, the slave’s MAC is randomized because it
naturally matches the bond, which is incorrect.

The issue is normally hidden since the first slave usually becomes the
active one, which restores the bond's MAC address. However, if another
slave is selected as the initial active interface, the issue becomes visible.

Fix this by assigning a random address only when slaves already exist in
the bond.

Fixes: 5c3bf6cba791 ("bonding: assign random address if device address is same as bond")
Reported-by: Qiuling Ren <qren@...hat.com>
Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
---
 drivers/net/bonding/bond_main.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 257333c88710..8832bc9f107b 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -2132,6 +2132,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 		memcpy(ss.__data, bond_dev->dev_addr, bond_dev->addr_len);
 	} else if (bond->params.fail_over_mac == BOND_FOM_FOLLOW &&
 		   BOND_MODE(bond) == BOND_MODE_ACTIVEBACKUP &&
+		   bond_has_slaves(bond) &&
 		   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.
-- 
2.50.1