netdev - Re: RFC: New BGF 'LOOP' instruction

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Message-Id: <20100803.001809.25133218.davem@davemloft.net>
Date:	Tue, 03 Aug 2010 00:18:09 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	leonerd@...nerd.org.uk
Cc:	netdev@...r.kernel.org
Subject: Re: RFC: New BGF 'LOOP' instruction

From: Paul LeoNerd Evans <leonerd@...nerd.org.uk>
Date: Tue, 3 Aug 2010 08:04:27 +0100

> On Mon, Aug 02, 2010 at 10:13:41PM -0700, David Miller wrote:
>> > Any comments on this, while I proceed? Barring any major complaints,
>> > I'll have a hack at some code and present a patch in due course...
>> 
>> We're not adding loop instructions, it's just asking for trouble
>> since any user can attach BPF filters to a socket and it's just
>> way too easy to make a loop endless.
>> 
>> There's a reason no loop primitives were added to the original
>> BPF specification, perhaps you should take a look at what their
>> reasoning was.
> 
> Yes. I am very aware of that.
> 
> Please read carefully my suggestion. These loops cannot be made endless
> - they will be bounded by, at most, the number of bytes in the packet
> buffer. The loop is required to increment X at least 1 at every
> iteration, and will not allow it to continue past the end of the packet.
> This puts a strict bound on the runtime of the loop.

That makes the looping construct largely useless, which I mentioned in
my second reply to this thread.

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