linux-kernel - [PATCH 3.16 185/306] net/mlx4_en: Resolve dividing by zero in 32-bit system

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Date:   Wed, 15 Feb 2017 22:41:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Tariq Toukan" <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Eugenia Emantayev" <eugenia@...lanox.com>
Subject: [PATCH 3.16 185/306] net/mlx4_en: Resolve dividing by zero in
 32-bit system

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eugenia Emantayev <eugenia@...lanox.com>

commit 4850cf4581578216468b7b3c3d06cc5abb0a697d upstream.

When doing roundup_pow_of_two for large enough number with
bit 31, an overflow will occur and a value equal to 1 will
be returned. In this case 1 will be subtracted from the return
value and division by zero will be reached.

Fixes: 31c128b66e5b ("net/mlx4_en: Choose time-stamping shift value according to HW frequency")
Signed-off-by: Eugenia Emantayev <eugenia@...lanox.com>
Signed-off-by: Tariq Toukan <tariqt@...lanox.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/net/ethernet/mellanox/mlx4/en_clock.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/drivers/net/ethernet/mellanox/mlx4/en_clock.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_clock.c
@@ -294,8 +294,11 @@ static u32 freq_to_shift(u16 freq)
 {
 	u32 freq_khz = freq * 1000;
 	u64 max_val_cycles = freq_khz * 1000 * MLX4_EN_WRAP_AROUND_SEC;
+	u64 tmp_rounded =
+		roundup_pow_of_two(max_val_cycles) > max_val_cycles ?
+		roundup_pow_of_two(max_val_cycles) - 1 : UINT_MAX;
 	u64 max_val_cycles_rounded = is_power_of_2(max_val_cycles + 1) ?
-		max_val_cycles : roundup_pow_of_two(max_val_cycles) - 1;
+		max_val_cycles : tmp_rounded;
 	/* calculate max possible multiplier in order to fit in 64bit */
 	u64 max_mul = div_u64(0xffffffffffffffffULL, max_val_cycles_rounded);