Re: [PATCH net v5 2/2] bonding: fix u32 overflow in compute_gap()

[David Laight <david.laight.linux@gmail.com>]

On Tue, 25 Aug 2026 09:01:30 +0800
Hangbin Liu <hangbin.liu@linux.dev> wrote:

> From: Hangbin Liu <liuhangbin@kylinos.cn>
> 
> The TLB load-tracking fields tx_bytes, load_history, load, and
> unbalanced_load are all u32. At sustained throughput above ~3.2 Gbit/s
> over the 10-second rebalance interval the byte counters wrap, causing
> compute_gap() to produce incorrect gap values and mis-select slaves.
> Such speeds are common on modern NICs under heavy traffic.
> 
> Widen these fields to u64. Use u64_stats_sync to protect the per-cpu
> unbalanced_load_stats against tearing on 32-bit architectures, and
> div_u64() for the 64-bit divisions. The tx_bytes and load_history
> are protected in spin_lock. Also protect the slave load writing in
> bond_alb_monitor() with spin_lock in case of tear on 32-bit.

How about changing the rebalance interval to either 8 or 16 seconds
to avoid the expensive divide?
Alternatively multiply the other side of the comparisons by the interval,
replacing the expensive divide with a cheap multiply.

David

> 
> Rework compute_gap() to use s64 arithmetic throughout. Return LLONG_MIN
> when the speed is unknown.
> 
> Detected by AI code review.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Hangbin Liu <liuhangbin@kylinos.cn>
> ---
>  drivers/net/bonding/bond_alb.c | 52 ++++++++++++++++++++++++++++++------------
>  include/net/bond_alb.h         | 11 +++++----
>  2 files changed, 44 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_alb.c b/drivers/net/bonding/bond_alb.c
> index 0afed2c39231..9a43a1f47893 100644
> --- a/drivers/net/bonding/bond_alb.c
> +++ b/drivers/net/bonding/bond_alb.c
> @@ -6,6 +6,7 @@
>  #include <linux/skbuff.h>
>  #include <linux/netdevice.h>
>  #include <linux/etherdevice.h>
> +#include <linux/ethtool.h>
>  #include <linux/pkt_sched.h>
>  #include <linux/spinlock.h>
>  #include <linux/slab.h>
> @@ -74,8 +75,8 @@ static inline u8 _simple_hash(const u8 *hash_start, int hash_size)
>  static inline void tlb_init_table_entry(struct tlb_client_info *entry, int save_load)
>  {
>  	if (save_load) {
> -		entry->load_history = 1 + entry->tx_bytes /
> -				      BOND_TLB_REBALANCE_INTERVAL;
> +		entry->load_history = 1 + div_u64(entry->tx_bytes,
> +				      BOND_TLB_REBALANCE_INTERVAL);
>  		entry->tx_bytes = 0;
>  	}
>  
> @@ -133,7 +134,7 @@ static int tlb_initialize(struct bonding *bond)
>  	if (!new_hashtbl)
>  		return -ENOMEM;
>  
> -	bond_info->unbalanced_load = alloc_percpu(struct unbalanced_load_stats);
> +	bond_info->unbalanced_load = netdev_alloc_pcpu_stats(struct unbalanced_load_stats);
>  	if (!bond_info->unbalanced_load)
>  		goto out;
>  
> @@ -170,8 +171,14 @@ static void tlb_deinitialize(struct bonding *bond)
>  
>  static long long compute_gap(struct slave *slave)
>  {
> -	return (s64) (slave->speed << 20) - /* Convert to Megabit per sec */
> -	       (s64) (SLAVE_TLB_INFO(slave).load << 3); /* Bytes to bits */
> +	u32 raw_speed = READ_ONCE(slave->speed);
> +
> +	/* It's meaningless to compare gap on unknown speed NIC */
> +	if (raw_speed == (u32)SPEED_UNKNOWN)
> +		return LLONG_MIN;
> +
> +	return ((s64)raw_speed << 20) - /* Convert to bits per sec */
> +	       ((s64)SLAVE_TLB_INFO(slave).load << 3); /* Bytes to bits */
>  }
>  
>  static struct slave *tlb_get_least_loaded_slave(struct bonding *bond)
> @@ -188,7 +195,7 @@ static struct slave *tlb_get_least_loaded_slave(struct bonding *bond)
>  		if (bond_slave_can_tx(slave)) {
>  			long long gap = compute_gap(slave);
>  
> -			if (max_gap < gap) {
> +			if (!least_loaded || max_gap < gap) {
>  				least_loaded = slave;
>  				max_gap = gap;
>  			}
> @@ -1354,8 +1361,14 @@ static netdev_tx_t bond_do_alb_xmit(struct sk_buff *skb, struct bonding *bond,
>  	if (!tx_slave) {
>  		/* unbalanced or unassigned, send through primary */
>  		tx_slave = rcu_dereference(bond->curr_active_slave);
> -		if (bond->params.tlb_dynamic_lb)
> -			this_cpu_add(bond_info->unbalanced_load->tx_bytes, skb->len);
> +		if (bond->params.tlb_dynamic_lb) {
> +			struct unbalanced_load_stats *pcpu_load;
> +
> +			pcpu_load = this_cpu_ptr(bond_info->unbalanced_load);
> +			u64_stats_update_begin(&pcpu_load->syncp);
> +			u64_stats_add(&pcpu_load->tx_bytes, skb->len);
> +			u64_stats_update_end(&pcpu_load->syncp);
> +		}
>  	}
>  
>  	if (tx_slave && bond_slave_can_tx(tx_slave)) {
> @@ -1539,21 +1552,27 @@ netdev_tx_t bond_alb_xmit(struct sk_buff *skb, struct net_device *bond_dev)
>  	return bond_do_alb_xmit(skb, bond, tx_slave);
>  }
>  
> -static u32 reset_unbalanced_load(struct alb_bond_info *bond_info)
> +static u64 reset_unbalanced_load(struct alb_bond_info *bond_info)
>  {
> +	u64 delta, tx_bytes, total_bytes = 0;
>  	struct unbalanced_load_stats *p;
> -	u32 delta, total_bytes = 0;
> +	unsigned int start;
>  	int i;
>  
>  	for_each_possible_cpu(i) {
>  		p = per_cpu_ptr(bond_info->unbalanced_load, i);
> -		total_bytes += READ_ONCE(p->tx_bytes);
> +		do {
> +			start = u64_stats_fetch_begin(&p->syncp);
> +			tx_bytes = u64_stats_read(&p->tx_bytes);
> +		} while (u64_stats_fetch_retry(&p->syncp, start));
> +
> +		total_bytes += tx_bytes;
>  	}
>  
>  	delta = total_bytes - bond_info->prev_total_unbalanced;
>  	bond_info->prev_total_unbalanced = total_bytes;
>  
> -	return delta / BOND_TLB_REBALANCE_INTERVAL;
> +	return div_u64(delta, BOND_TLB_REBALANCE_INTERVAL);
>  }
>  
>  void bond_alb_monitor(struct work_struct *work)
> @@ -1597,8 +1616,13 @@ void bond_alb_monitor(struct work_struct *work)
>  	if (atomic_read(&bond_info->tx_rebalance_counter) >= BOND_TLB_REBALANCE_TICKS) {
>  		bond_for_each_slave_rcu(bond, slave, iter) {
>  			tlb_clear_slave(bond, slave, 1);
> -			if (slave == rcu_access_pointer(bond->curr_active_slave))
> -				SLAVE_TLB_INFO(slave).load = reset_unbalanced_load(bond_info);
> +			if (slave == rcu_access_pointer(bond->curr_active_slave)) {
> +				u64 new_load = reset_unbalanced_load(bond_info);
> +
> +				spin_lock_bh(&bond->mode_lock);
> +				SLAVE_TLB_INFO(slave).load = new_load;
> +				spin_unlock_bh(&bond->mode_lock);
> +			}
>  		}
>  		atomic_set(&bond_info->tx_rebalance_counter, 0);
>  	}
> diff --git a/include/net/bond_alb.h b/include/net/bond_alb.h
> index 6fb09b4fc7e2..32f1981033e4 100644
> --- a/include/net/bond_alb.h
> +++ b/include/net/bond_alb.h
> @@ -57,12 +57,12 @@ struct tlb_client_info {
>  				 * packets to a Client that the Hash function
>  				 * gave this entry index.
>  				 */
> -	u32 tx_bytes;		/* Each Client accumulates the BytesTx that
> +	u64 tx_bytes;		/* Each Client accumulates the BytesTx that
>  				 * were transmitted to it, and after each
>  				 * CallBack the LoadHistory is divided
>  				 * by the balance interval
>  				 */
> -	u32 load_history;	/* This field contains the amount of Bytes
> +	u64 load_history;	/* This field contains the amount of Bytes
>  				 * that were transmitted to this client by
>  				 * the server on the previous balance
>  				 * interval in Bps.
> @@ -118,19 +118,20 @@ struct tlb_slave_info {
>  			 * are the entries that were assigned to use this
>  			 * slave for transmit.
>  			 */
> -	u32 load;	/* Each slave sums the loadHistory of all clients
> +	u64 load;	/* Each slave sums the loadHistory of all clients
>  			 * assigned to it
>  			 */
>  };
>  
>  struct unbalanced_load_stats {
> -	u32			tx_bytes;
> +	u64_stats_t		tx_bytes;
> +	struct u64_stats_sync	syncp;
>  };
>  
>  struct alb_bond_info {
>  	struct tlb_client_info	*tx_hashtbl; /* Dynamically allocated */
>  	struct unbalanced_load_stats __percpu	*unbalanced_load;
> -	u32			prev_total_unbalanced;
> +	u64			prev_total_unbalanced;
>  	atomic_t		tx_rebalance_counter;
>  	int			lp_counter;
>  	/* -------- rlb parameters -------- */
>