netdev - Re: [PATCH] tcp: restrict net.ipv4.tcp_adv_min

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Date:	Sun, 14 Nov 2010 20:49:43 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	davem@...emloft.net, shemminger@...ux-foundation.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: restrict net.ipv4.tcp_adv_min_scale (#20312)

Le dimanche 14 novembre 2010 à 17:18 +0200, Alexey Dobriyan a écrit :
> tcp_win_from_space() does the following:
> 
> 	if (sysctl_tcp_adv_win_scale <= 0)
> 		return space >> (-sysctl_tcp_adv_win_scale);
> 	else
> 		return space - (space >> sysctl_tcp_adv_win_scale);
> 
> "space" is int.
> 
> As per C99 6.5.7 (3) shifting int for 32 or more bits is
> undefined behaviour.
> 
> Indeed, if sysctl_tcp_adv_win_scale is exactly 32, space >> 32 equals
> space and function returns 0;
> 
> Which means we busyloop in tcp_fixup_rcvbuf().
> 
> Restrict net.ipv4.tcp_adv_win_scale to [-31, 31].
> 
> Fix https://bugzilla.kernel.org/show_bug.cgi?id=20312
> 
> Steps to reproduce:
> 
> 	echo 32 >/proc/sys/net/ipv4/tcp_adv_win_scale
> 	wget www.kernel.org
> 	[softlockup]
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
> 

Please fix your patch title, you speak of adv_min_scale ...

> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -26,6 +26,8 @@ static int zero;
>  static int tcp_retr1_max = 255;
>  static int ip_local_port_range_min[] = { 1, 1 };
>  static int ip_local_port_range_max[] = { 65535, 65535 };
> +static int _minus_31 = -31;
> +static int _31 = 31;

Please use normal symbols, not starting by underscore.

>  
>  /* Update system visible IP port range */
>  static void set_local_port_range(int range[2])
> @@ -426,7 +428,9 @@ static struct ctl_table ipv4_table[] = {
>  		.data		= &sysctl_tcp_adv_win_scale,
>  		.maxlen		= sizeof(int),
>  		.mode		= 0644,
> -		.proc_handler	= proc_dointvec
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &_minus_31,
> +		.extra2		= &_31,
>  	},
>  	{
>  		.procname	= "tcp_tw_reuse",

Please change Documentation/networking/ip-sysctl.txt to reflect possible
values of adv_win_scale

Thanks


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