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

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Date:	Wed, 07 Aug 2013 10:36:30 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com, vanj@...gle.com,
	stephen@...workplumber.org, ycheng@...gle.com
Subject: Re: [PATCH] tcp: cubic: fix overflow error in bictcp_update()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 05 Aug 2013 17:10:15 -0700

> 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>

Applied and queued up for -stable.
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