netdev - Re: [PATCH] Add sysctl to set the advertised TCP initial receive window.

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Date:	Wed, 09 Dec 2009 22:11:30 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Perches <joe@...ches.com>
CC:	chavey@...gle.com, davem@...emloft.net, netdev@...r.kernel.org,
	therbert@...gle.com
Subject: Re: [PATCH] Add sysctl to set the advertised TCP initial receive
 window.

Le 09/12/2009 21:55, Joe Perches a écrit :
> On Wed, 2009-12-09 at 21:33 +0100, Eric Dumazet wrote:
>> Le 09/12/2009 21:13, chavey@...gle.com a écrit :
>>> Add a sysctl, tcp_init_rcv_wnd, to set the TCP initial receive window
>>> size advertised by passive and active TCP connections.
>>> index f1813bc..7567edd 100644
>>> --- a/net/ipv4/tcp.c
>>> +++ b/net/ipv4/tcp.c
>>> @@ -2248,6 +2248,11 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>>>  		break;
>>>  #endif
>>>  
>>> +	case TCP_INIT_RCV_WND:
>>> +		val = min_t(int, val, TCP_INIT_RCV_WND_MAX);
>>> +		tp->rx_opt.init_rcv_wnd = val;
>>> +		break;
>>> +
>>
>> If user pass val = -1, you end with init_rcv_wnd = 255
>>
>> Is it what you want ?
>> Probably not :)
> 
> I believe min_t() is superfluous now because:
> +               .proc_handler   = proc_dointvec_minmax,
> +               .extra1         = &zero,
> +               .extra2         = &tcp_init_rcv_wnd_max
> 
> 

It is *not* superfluous, since sysctl_tcp_init_rcv_wnd is ignored if
user called setsockopt(TCP_INIT_RCV_WND) with a non null value :

+	if (tp->rx_opt.init_rcv_wnd == 0)
+		tp->rx_opt.init_rcv_wnd = sysctl_tcp_init_rcv_wnd;

So its probably better to not silently cap user provided val but report an error.

if (val < 0 || val > TCP_INIT_RCV_WND_MAX)
	err = -EINVAL
else
	tp->rx_opt.init_rcv_wnd = val;
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