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tcp: cubic: fix overflow error in bictcp_update() 

commit 17a6e9f1aa ("tcp_cubic: fix clock dependency") added an
overflow error in bictcp_update() in following code :

/* change the unit from HZ to bictcp_HZ */
t = ((tcp_time_stamp + msecs_to_jiffies(ca->delay_min>>3) -
      ca->epoch_start) << BICTCP_HZ) / HZ;

Because msecs_to_jiffies() being unsigned long, compiler does
implicit type promotion.

We really want to constrain (tcp_time_stamp - ca->epoch_start)
to a signed 32bit value, or else 't' has unexpected high values.

This bugs triggers an increase of retransmit rates ~24 days after
boot [1], as the high order bit of tcp_time_stamp flips.

[1] for hosts with HZ=1000

Big thanks to Van Jacobson for spotting this problem.

Diagnosed-by: Van Jacobson <vanj@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2013-08-05 17:10:15 -07:00 committed by David S. Miller
parent 248ba8ec05
commit 2ed0edf909
1 changed files with 6 additions and 4 deletions
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10
net/ipv4/tcp_cubic.c
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@ -206,8 +206,8 @@ static u32 cubic_root(u64 a)
*/
static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
{
u64 offs;
u32 delta, t, bic_target, max_cnt;
u32 delta, bic_target, max_cnt;
u64 offs, t;
ca->ack_cnt++; /* count the number of ACKs */
@ -250,9 +250,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
* if the cwnd < 1 million packets !!!
*/
t = (s32)(tcp_time_stamp - ca->epoch_start);
t += msecs_to_jiffies(ca->delay_min >> 3);
/* change the unit from HZ to bictcp_HZ */
t = ((tcp_time_stamp + msecs_to_jiffies(ca->delay_min>>3)
- ca->epoch_start) << BICTCP_HZ) / HZ;
t <<= BICTCP_HZ;
do_div(t, HZ);
if (t < ca->bic_K) /* t - K */
offs = ca->bic_K - t;