netdev - Re: [tcpdump-workers] Modular arithmetic

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Message-Id: <2B555EBC-A130-476B-9E29-DB75E668904E@alum.mit.edu>
Date:	Mon, 10 Sep 2012 10:50:30 -0700
From:	Guy Harris <guy@...m.mit.edu>
To:	"David Laight" <David.Laight@...LAB.COM>
Cc:	"Eric Dumazet" <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	Jay Schulist <jschlst@...ba.org>,
	Andi Kleen <ak@...ux.intel.com>,
	tcpdump-workers@...ts.tcpdump.org
Subject: Re: [tcpdump-workers] Modular arithmetic

On Sep 10, 2012, at 3:41 AM, "David Laight" <David.Laight@...LAB.COM> wrote:

>>> What about the other OS - eg all the BSDs?
>>> I had a vague idea that BPF was supposed to be reasonable portable.
>> 
>> Yes, does it mean BPF is frozen ?
>> 
>> Or is BSD so hard to update these days ?
> 
> Not really - but it some other places that need updating in order
> to make this useful for cross-platform tools (like tcpdump).
> 
> The 'real fun (tm)' happens when NetBSD tries to run Linux binaries
> that include the Linux libpcap.

Additional fun happens when a Linux system with a kernel that doesn't have the mod instruction tries to run Linux binaries that include a Linux libpcap that generates code using the mod instruction; this is not a BSD-vs.-Linux issue, it's a "kernel that lacks the mod instruction vs. libpcap that generates code that includes the mod instruction" issue.

BSD/OS X, Linux, Solaris, and the WinPcap driver need, if they adopt new BPF instructions, to have a mechanism by which libpcap (or anything else using BPF filtering) can inquire about the capabilities of the OS BPF interpreter; libpcap would use that to determine what code to generate if generating code for the in-kernel BPF interpreter.

(Please reply to tcpdump-workers@...ts.tcpdump.org as well as to netdev@...r.kernel.org, so that people not on both of those lists can follow the discussion.  Messages from non-members of tcpdump-workers to tcpdump-workers shouldn't bounce, but they do need to be approved by a moderator; Michael, if you want to at least temporarily turn the flood on for my e-mail address, so I can help moderate, go ahead.)--
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