tcp_cubic: limit delayed_ack ratio to prevent divide error

TCP Cubic keeps a metric that estimates the amount of delayed
acknowledgements to use in adjusting the window. If an abnormally
large number of packets are acknowledged at once, then the update
could wrap and reach zero. This kind of ACK could only
happen when there was a large window and huge number of
ACK's were lost.

This patch limits the value of delayed ack ratio. The choice of 32
is just a conservative value since normally it should be range of
1 to 4 packets.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 34340c9c95fab4f3c9aa108fbed4dd7f69028cd9..f376b05cca818fd9496fe0cb8540a051e3e7e50a 100644 (file)
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -93,6 +93,7 @@ struct bictcp {
        u32     ack_cnt;        /* number of acks */
        u32     tcp_cwnd;       /* estimated tcp cwnd */
 #define ACK_RATIO_SHIFT        4
+#define ACK_RATIO_LIMIT (32u << ACK_RATIO_SHIFT)
        u16     delayed_ack;    /* estimate the ratio of Packets/ACKs << 4 */
        u8      sample_cnt;     /* number of samples to decide curr_rtt */
        u8      found;          /* the exit point is found? */
@@ -398,8 +399,12 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
        u32 delay;
 
        if (icsk->icsk_ca_state == TCP_CA_Open) {
-               cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
-               ca->delayed_ack += cnt;
+               u32 ratio = ca->delayed_ack;
+
+               ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT;
+               ratio += cnt;
+
+               ca->delayed_ack = min(ratio, ACK_RATIO_LIMIT);
        }
 
        /* Some calls are for duplicates without timetamps */