Subject: Re: [PATCH 2/4] bond_alb: don't rewrite bridged non-local MACs

From: Jay Vosburgh <[email-masked]>
Date: Wed, 19 May 2021 15:31:59 -0700

Jarod Wilson <[email-masked]> wrote:

>With a virtual machine behind a bridge on top of a bond, outgoing traffic
>should retain the VM's source MAC. That works fine most of the time, until
>doing a failover, and then the MAC gets rewritten to the bond slave's MAC,
>and the return traffic gets dropped. If we don't rewrite the MAC there, we
>don't lose any traffic.

	Please have the log message here specify that this applies only
to balance-alb mode, and, the usual nomenclature for bonding patches is
"[PATCH] bonding:"; for this case, I'd suggest "balance-alb:" right
afterwards to be clear that it's only for alb mode.  I didn't really
spot the "bond_alb" tag for what it was on first read, and it is the
only indication that this change is specific to alb mode other than the
patch itself.

>Cc: Jay Vosburgh <[email-masked]>
>Cc: Veaceslav Falico <[email-masked]>
>Cc: Andy Gospodarek <[email-masked]>
>Cc: "David S. Miller" <[email-masked]>
>Cc: Jakub Kicinski <[email-masked]>
>Cc: Thomas Davis <[email-masked]>
>Cc: [email-masked]
>Signed-off-by: Jarod Wilson <[email-masked]>
>---
> drivers/net/bonding/bond_alb.c | 23 ++++++++++++++++++++++-
> 1 file changed, 22 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/net/bonding/bond_alb.c b/drivers/net/bonding/bond_alb.c
>index 3455f2cc13f2..ce8257c7cbea 100644
>--- a/drivers/net/bonding/bond_alb.c
>+++ b/drivers/net/bonding/bond_alb.c
>@@ -1302,6 +1302,26 @@ void bond_alb_deinitialize(struct bonding *bond)
> 		rlb_deinitialize(bond);
> }
> 
>+static bool bond_alb_bridged_mac(struct bonding *bond, struct ethhdr *eth_data)
>+{
>+	struct list_head *iter;
>+	struct slave *slave;
>+
>+	if (BOND_MODE(bond) != BOND_MODE_ALB)
>+		return false;
>+
>+	/* Don't modify source MACs that do not originate locally
>+	 * (e.g.,arrive via a bridge).
>+	 */
>+	if (!netif_is_bridge_port(bond->dev))
>+		return false;

	I believe this logic will fail if the plumbing is, e.g., bond ->
vlan -> bridge, as netif_is_bridge_port() would not return true for the
bond in that case.

	Making this reliable is tricky at best, and may be impossible to
be correct for all possible cases.  As such, I think the comment above
should reflect the limited scope of what is actually being checked here
(i.e., the bond itself is directly a bridge port).

	-J

>+
>+	if (bond_slave_has_mac_rx(bond, eth_data->h_source))
>+		return false;
>+
>+	return true;
>+}
>+
> static netdev_tx_t bond_do_alb_xmit(struct sk_buff *skb, struct bonding *bond,
> 				    struct slave *tx_slave)
> {
>@@ -1316,7 +1336,8 @@ static netdev_tx_t bond_do_alb_xmit(struct sk_buff *skb, struct bonding *bond,
> 	}
> 
> 	if (tx_slave && bond_slave_can_tx(tx_slave)) {
>-		if (tx_slave != rcu_access_pointer(bond->curr_active_slave)) {
>+		if (tx_slave != rcu_access_pointer(bond->curr_active_slave) &&
>+		    !bond_alb_bridged_mac(bond, eth_data)) {
> 			ether_addr_copy(eth_data->h_source,
> 					tx_slave->dev->dev_addr);
> 		}
>-- 
>2.30.2
>

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