[PATCH net v3] bonding: fix slave_cnt leak on XDP error paths

[Hangbin Liu <hangbin.liu@linux.dev>]

From: Hangbin Liu <liuhangbin@kylinos.cn>

When bond_enslave() succeeds up to the XDP setup stage, slave_cnt is
already incremented. If XDP setup subsequently fails, the error paths
jump directly to err_sysfs_del, bypassing the slave_cnt decrement.

This causes slave_cnt to drift upward on each failed enslaving attempt,
which would lead to unbalanced traffic distribution with round-robin
mode.

Fix it by moving the slave_cnt increase after XDP setup.

Fixes: 9e2ee5c7e7c3 ("net, bonding: Add XDP support to the bonding driver")
Signed-off-by: Hangbin Liu <liuhangbin@kylinos.cn>
---
Changes in v3:
- move the slave_cnt increasement before bond_update_slave_arr() (selftest)
- run all bonding selftests on debug kernel to make sure no regression (Jakub)
- Link to v2: https://lore.kernel.org/r/20260903-bond_slave_cnt-v2-1-02e27304ca36@kylinos.cn

Changes in v2:
- move the slave_cnt increasement after XDP setup (Nikolay Aleksandrov)
- balance-xor mode is not affected, not mention it (Nikolay Aleksandrov)
- Link to v1: https://lore.kernel.org/r/20260902-bond_slave_cnt-v1-1-36e95bf4a6ff@kylinos.cn
---
 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 a9bff7663eec..1514a8ba0888 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -2284,7 +2284,6 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 		}
 	}
 
-	WRITE_ONCE(bond->slave_cnt, bond->slave_cnt + 1);
 	netdev_compute_master_upper_features(bond->dev, true);
 	bond_set_carrier(bond);
 
@@ -2332,6 +2331,9 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 			bpf_prog_inc(bond->xdp_prog);
 	}
 
+	/* Increase the slave count before rebuilding the slave arrays. */
+	WRITE_ONCE(bond->slave_cnt, bond->slave_cnt + 1);
+
 	/* broadcast mode uses the all_slaves to loop through slaves. */
 	if (bond_mode_can_use_xmit_hash(bond) ||
 	    BOND_MODE(bond) == BOND_MODE_BROADCAST)

---
base-commit: 38b6be101006d3e7af972999f45d4f1e8250587a
change-id: 20260807-bond_slave_cnt-88e78c5f0ab9

Best regards,
-- 
Hangbin Liu <liuhangbin@kylinos.cn>