Re: [PATCH net v4 2/2] bonding: fix u32 overflow in compute_gap()

[Nikolay Aleksandrov <razor@blackwall.org>]

On 20/08/2026 08:55, Hangbin Liu 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.
> 
> Rework compute_gap() to use u64 arithmetic throughout. Return 0 when the
> speed is unknown or the slave is already overloaded.
> 
> 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 | 65 ++++++++++++++++++++++++++++++------------
>   include/net/bond_alb.h         | 11 +++----
>   2 files changed, 53 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_alb.c b/drivers/net/bonding/bond_alb.c
> index 0afed2c39231..372db54803d3 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;
>   
> @@ -168,27 +169,38 @@ static void tlb_deinitialize(struct bonding *bond)
>   	spin_unlock_bh(&bond->mode_lock);
>   }
>   
> -static long long compute_gap(struct slave *slave)
> +static u64 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 */
> +	u64 slave_load = SLAVE_TLB_INFO(slave).load << 3; /* Bytes to bits */
> +	u32 raw_speed = READ_ONCE(slave->speed);
> +	u64 speed = (u64)raw_speed << 20; /* Convert to bits per sec */
> +
> +	/* It's meaningless to compare gap on unknown speed NIC */
> +	if (raw_speed == (u32)SPEED_UNKNOWN)
> +		return 0;
> +
> +	/* Skip slave which is over loaded */
> +	if (speed <= slave_load)
> +		return 0;
> +
> +	return speed - slave_load;
>   }
>   
>   static struct slave *tlb_get_least_loaded_slave(struct bonding *bond)
>   {
>   	struct slave *slave, *least_loaded;
>   	struct list_head *iter;
> -	long long max_gap;
> +	u64 max_gap = 0;
>   
>   	least_loaded = NULL;
> -	max_gap = LLONG_MIN;
>   
>   	/* Find the slave with the largest gap */
>   	bond_for_each_slave_rcu(bond, slave, iter) {
>   		if (bond_slave_can_tx(slave)) {
> -			long long gap = compute_gap(slave);
> +			u64 gap = compute_gap(slave);
>   
> -			if (max_gap < gap) {
> +			/* Make sure we have one available slave */
> +			if (max_gap <= gap) {
>   				least_loaded = slave;
>   				max_gap = gap;

I think Sashiko's review has a point here:
"Does clamping the gap to 0 completely break load balancing when all interfaces
are overloaded?
When all slaves are overloaded, compute_gap() returns 0 for all of them. Since
max_gap is initialized to 0, max_gap <= gap will evaluate to 0 <= 0, which is
true.
This means tlb_get_least_loaded_slave() will continually update least_loaded to
the current slave, ultimately routing all traffic to the last slave in the list
instead of distributing it across the least overloaded interfaces."

That is, compute_gap makes multiple different scenarios look the same:
  if speed is unknown           = 0
  if exactly equal capacity     = 0
  if overloaded by *any* amount = 0

So Sashiko's comment seems correct, it doesn't matter if a slave is overloaded
with 1 gbps or 100, they will look the same.

>   			}
> @@ -1354,8 +1366,14 @@ 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 +1557,27 @@ 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 +1621,13 @@ 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 -------- */
>