[PATCH] bonding: allow nesting of bonding device

From: Di Zhu <redacted>

The commit 3c9ef511b9fa ("bonding: avoid adding slave device with
IFF_MASTER flag") fix a crash when add slave device with IFF_MASTER,
but it rejects the scenario of nested bonding device.

As Eric Dumazet described: since there indeed is a usage scenario about
nesting bonding, we should not break it.

So we add a new judgment condition to allow nesting of bonding device.

Fixes: 3c9ef511b9fa ("bonding: avoid adding slave device with IFF_MASTER flag")
Suggested-by: Jay Vosburgh <redacted>
Signed-off-by: Di Zhu <redacted>
---
 drivers/net/bonding/bond_main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 16840c9bc00d..03b1a93d7fea 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c

@@ -1601,7 +1601,9 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	int link_reporting;
 	int res = 0, i;
 
-	if (slave_dev->flags & IFF_MASTER) {
+	if (slave_dev->flags & IFF_MASTER &&
+	    !netif_is_bond_master(slave_dev)) {
+		NL_SET_ERR_MSG(extack, "Device with IFF_MASTER cannot be enslaved");
 		netdev_err(bond_dev,
 			   "Error: Device with IFF_MASTER cannot be enslaved\n");
 		return -EPERM;

-- 
2.23.0