netdev - Re: [PATCH net-next v2 8/9] net: filter: rework/optimize internal BPF interpreter's instruction set

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Message-ID: <53321E3B.8080709@linux.intel.com>
Date:	Tue, 25 Mar 2014 17:24:27 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	David Miller <davem@...emloft.net>, dborkman@...hat.com
CC:	ast@...mgrid.com, netdev@...r.kernel.org, hagen@...u.net,
	keescook@...omium.org, pmoore@...hat.com, mingo@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 8/9] net: filter: rework/optimize internal
 BPF interpreter's instruction set

On 03/25/2014 05:17 PM, David Miller wrote:
> From: Daniel Borkmann <dborkman@...hat.com>
> Date: Tue, 25 Mar 2014 13:10:56 +0100
> 
>>   - Conditional jt/jf targets replaced with jt/fall-through,
>>     and forward/backward jumps now possible as well
> 
> Ummm... does anyone even remember why backwards jumps were not allowed
> in the original BPF?
> 
> It's to guarentee forward progress in the filter.
> 
> Allowing branching backward would allow endless loops.
> 
> Please remove this feature.
> 

If you allow loops, it greatly increases the expressibility of the
language, but we would really need another control to limit CPU usage.

	-hpa

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