[PATCH rdma-next] lib/dim: Prevent overflow in calculation of ratio statistics

[Leon Romanovsky <leon@kernel.org>]

From: Leon Romanovsky <leonro@mellanox.com>

Multiply by 100 can potentially overflow cpms value and will produce
incorrect wrong ratio statistics. Update code to use built-in division
macro, so it will fix the following UBSAN warning.

 [ 1040.120129] ================================================================================
 [ 1040.127124] UBSAN: Undefined behaviour in lib/dim/dim.c:78:23
 [ 1040.130118] signed integer overflow:
 [ 1040.131643] 134718714 * 100 cannot be represented in type 'int'
 [ 1040.134374] CPU: 0 PID: 22846 Comm: iperf3 Not tainted 5.2.0-rc6-for-upstream-dbg-2019-06-29_03-18-13-29 #1
 [ 1040.139068] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
 [ 1040.144469] Call Trace:
 [ 1040.145897]  <IRQ>
 [ 1040.147366]  dump_stack+0x9a/0xeb
 [ 1040.149061]  ubsan_epilogue+0x9/0x7c
 [ 1040.150462]  handle_overflow+0x16d/0x198
 [ 1040.151911]  ? __ubsan_handle_negate_overflow+0x15c/0x15c
 [ 1040.153679]  ? sk_free+0x15/0x30
 [ 1040.155011]  ? kvm_clock_read+0x14/0x30
 [ 1040.156433]  ? kvm_sched_clock_read+0x5/0x10
 [ 1040.157952]  ? sched_clock+0x5/0x10
 [ 1040.159318]  ? sched_clock_cpu+0x18/0x260
 [ 1040.160801]  dim_calc_stats+0x4a1/0x4c0
 [ 1040.162274]  net_dim+0x147/0x920
 [ 1040.163592]  ? net_dim_stats_compare+0x330/0x330
 [ 1040.165283]  mlx5e_napi_poll+0x410/0x1030 [mlx5_core]
 [ 1040.166876]  ? lock_stats+0xd41/0x1740
 [ 1040.168266]  ? mlx5e_trigger_irq+0x550/0x550 [mlx5_core]
 [ 1040.169918]  ? __module_text_address+0x13/0x140
 [ 1040.171409]  ? lock_stats+0xd41/0x1740
 [ 1040.172757]  ? net_rx_action+0x262/0xda0
 [ 1040.174156]  net_rx_action+0x421/0xda0
 [ 1040.175519]  ? napi_complete_done+0x370/0x370
 [ 1040.176979]  ? kvm_clock_read+0x14/0x30
 [ 1040.178316]  ? kvm_sched_clock_read+0x5/0x10
 [ 1040.179690]  ? sched_clock+0x5/0x10
 [ 1040.180920]  ? sched_clock_cpu+0x18/0x260
 [ 1040.182286]  __do_softirq+0x287/0xb4e
 [ 1040.183581]  ? irqtime_account_irq+0x1d5/0x3b0
 [ 1040.184998]  irq_exit+0x17d/0x1d0
 [ 1040.186212]  do_IRQ+0x129/0x220
 [ 1040.187412]  common_interrupt+0xf/0xf
 [ 1040.188673]  </IRQ>
 [ 1040.189685] RIP: 0033:0x7f092c41a07a
 [ 1040.190884] Code: 45 31 f6 e9 8a 00 00 00 0f 1f 84 00 00 00 00 00 48
89 df ff 93 88 01 00 00 85 c0 0f 88 c7 00 00 00 48 98 48 01 85 88 02 00
00 <48> 8b 85 c8 02 00 00 48 83 85 90 02 00 00 01 48 83 78 10 00 74 0b
 [ 1040.195584] RSP: 002b:00007fffbebe7870 EFLAGS: 00000206 ORIG_RAX: ffffffffffffffd7
 [ 1040.197933] RAX: 0000000000020000 RBX: 0000000000e239b0 RCX: 000000000006b280
 [ 1040.199740] RDX: 0000000000020000 RSI: 00007f092c805000 RDI: 0000000000000007
 [ 1040.201525] RBP: 0000000000e21260 R08: 0000000000000000 R09: 00007fffbebfb0a0
 [ 1040.203237] R10: 0000000000000380 R11: 0000000000000246 R12: 00007fffbebe7950
 [ 1040.204944] R13: 0000000000000007 R14: 0000000000000001 R15: 00007fffbebe7870
 [ 1040.206686] ================================================================================

Fixes: 398c2b05bbee ("linux/dim: Add completions count to dim_sample")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
---
 lib/dim/dim.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/dim/dim.c b/lib/dim/dim.c
index 439d641ec796..38045d6d0538 100644
--- a/lib/dim/dim.c
+++ b/lib/dim/dim.c
@@ -74,8 +74,8 @@ void dim_calc_stats(struct dim_sample *start, struct dim_sample *end,
 					delta_us);
 	curr_stats->cpms = DIV_ROUND_UP(ncomps * USEC_PER_MSEC, delta_us);
 	if (curr_stats->epms != 0)
-		curr_stats->cpe_ratio =
-				(curr_stats->cpms * 100) / curr_stats->epms;
+		curr_stats->cpe_ratio = DIV_ROUND_DOWN_ULL(
+			curr_stats->cpms * 100, curr_stats->epms);
 	else
 		curr_stats->cpe_ratio = 0;

--
2.20.1