netdev - RE: [PATCH net] tcp: Fix integer-overflows in TCP vegas

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Date:	Wed, 23 Jul 2014 08:38:30 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Christoph Paasch' <christoph.paasch@...ouvain.be>,
	David Miller <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Doug Leith <doug.leith@...m.ie>
Subject: RE: [PATCH net] tcp: Fix integer-overflows in TCP vegas

From: Of Christoph Paasch
> In vegas we do a multiplication of the cwnd and the rtt. This
> may overflow and thus their result is stored in a u64. However, we first
> need to cast the cwnd so that actually 64-bit arithmetic is done.
> 
> Cc: Doug Leith <doug.leith@...m.ie>
> Fixes: 8d3a564da34e (tcp: tcp_vegas cong avoid fix)
> Signed-off-by: Christoph Paasch <christoph.paasch@...ouvain.be>
> ---
>  net/ipv4/tcp_vegas.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
> index 9a5e05f27f4f..6a4bdea2a0fb 100644
> --- a/net/ipv4/tcp_vegas.c
> +++ b/net/ipv4/tcp_vegas.c
> @@ -218,7 +218,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
>  			 * This is:
>  			 *     (actual rate in segments) * baseRTT
>  			 */
> -			target_cwnd = tp->snd_cwnd * vegas->baseRTT / rtt;
> +			target_cwnd = (u64)tp->snd_cwnd * vegas->baseRTT / rtt;

Won't that add a reference to the 64bit divide function?
If snd_cwnd is small then maybe:
	target_cwnd = (256u * vegas->baseRTT) / rtt * tp->snd_cwnd / 256u;
If large I think low bits are always zero so:
	target_cwnd = (256u * vegas->baseRTT) / rtt * (tp->snd_cwnd / 256u);
Possibly with a different power of 2...

	David



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