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

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Message-ID: <1546564.1755908490@famine>
Date: Fri, 22 Aug 2025 17:21:30 -0700
From: Jay Vosburgh <jv@...sburgh.net>
To: Hangbin Liu <liuhangbin@...il.com>
cc: netdev@...r.kernel.org, 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, Qiuling Ren <qren@...hat.com>
Subject: Re: [PATCH net 1/2] bonding: set random address only when slaves already exist

Hangbin Liu <liuhangbin@...il.com> wrote:

>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 description here seems confusing to me; what does "this"
refer to?  I don't understand the sequence of events that lead to the
issue being fixed here.

	I wonder if there's another bug somewhere, since nominally when
releasing the last interface in the bond, __bond_release_one() should
randomize the bond's MAC address, so it shouldn't match when adding (or
re-adding ?) the first interface to the bond.

	-J

>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
>

---
	-Jay Vosburgh, jv@...sburgh.net