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

From: Hangbin Liu <redacted>

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 <redacted>
---
  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;

  			}

@@ -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 -------- */