netdev - Re: [PATCH] tcp: cubic: fix overflow error in bictcp

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Date:	Tue, 6 Aug 2013 11:12:51 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Van Jacobson <vanj@...gle.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: cubic: fix overflow error in bictcp_update()

On Mon, Aug 5, 2013 at 8:10 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> commit 17a6e9f1aa9 ("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@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Stephen Hemminger <stephen@...workplumber.org>

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Also a nice catch!

neal
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