Commit 2ed0edf9 authored Aug 05, 2013 by Eric Dumazet Committed by David S. Miller Aug 07, 2013

tcp: cubic: fix overflow error in bictcp_update()



commit 17a6e9f1 ("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>

parent 248ba8ec

net/ipv4/tcp_cubic.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -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;

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