From mboxrd@z Thu Jan  1 00:00:00 1970
From: Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
Subject: Re: BPF code generation for radiotap
Date: Fri, 11 May 2007 16:49:56 -0700
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David Young wrote:
> Can I help someone with BPF know-how to write a radiotap code generator?
> It would be great if we could use BPF to filter out packets with, say,
> an RSSI below a certain threshold.

That will be possible only if the RSSI can be found without a loop, or 
if you're willing to have all the filtering done in userland.  The 
kernel BPF interpreter doesn't support backwards branches; the userland 
one does, to support chasing IPv6 protocol headers, but backward 
branches completely screw up the optimizer (so it gets turned off).