netdev - [PATCH] tcp_cubic: fix the range of delayed

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Date:	Wed, 30 Apr 2014 17:34:09 +0800
From:	Liu Yu <allanliuyu@...il.com>
To:	eric.dumazet@...il.com, ncardwell@...gle.com,
	shemminger@...tta.com, "David S. Miller" <davem@...emloft.net>
CC:	netdev@...r.kernel.org
Subject: [PATCH]  tcp_cubic: fix the range of delayed_ack

From: Liu Yu <allanyuliu@...cent.com>

commit b9f47a3aaeab (tcp_cubic: limit delayed_ack ratio to prevent
divide error) try to prevent divide error, but there is still a little
chance that delayed_ack can reach zero. In case the param cnt get
negative value, then ratio+cnt would overflow and may happen to be zero.
As a result, min(ratio, ACK_RATIO_LIMIT) will calculate to be zero.

In some old kernels, such as 2.6.32, there is a bug that would
pass negative param, which then ultimately leads to this divide error.

commit 5b35e1e6e9c (tcp: fix tcp_trim_head() to adjust segment count
with skb MSS) fixed the negative param issue. However, 
it's safe that we fix the range of delayed_ack as well,
to make sure we do not hit a divide by zero.

CC: Stephen Hemminger <shemminger@...tta.com>
Signed-off-by: Liu Yu <allanyuliu@...cent.com>
---
 net/ipv4/tcp_cubic.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 8bf2245..b4f1b29 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -409,7 +409,7 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 		ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT;
 		ratio += cnt;

-		ca->delayed_ack = min(ratio, ACK_RATIO_LIMIT);
+		ca->delayed_ack = clamp(ratio, 1U, ACK_RATIO_LIMIT);
 	}

 	/* Some calls are for duplicates without timetamps */
-- 
1.7.1

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