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Date: Fri, 22 Aug 2008 10:30:20 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: "Bruce M. Simpson" <bms@freebsd.org>
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Cc: gnn@freebsd.org, net@freebsd.org
Subject: Re: Small patch to multicast code...
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On Fri, Aug 22, 2008 at 03:27:11AM +0100, Bruce M. Simpson wrote:
> gnn@freebsd.org wrote:
> >>The only thing i can think of is that it's the UDP checksum,
> >>residing beyond hlen, which is overwritten somewhere in the
> >>call to if_simloop -- in which case perhaps a better fix is
> >>to m_pullup() the udp header as well ?
> >>    
> >
> >It is the checksum that gets trashed, yes.
> >...
> >The m_*() routines actually have reasonable comments, it just seems
> >the wrong one was used here.
> >  
> 
> Actually, m_copy() has been legacy for some time now -- see comments.

The API is undocumented, but in this specific function the reason
for using one rather than the other is undocumented. Especially,
if you use m_dup() then the call to m_pullup should be
unnecessary.

That's why I suggested to explain clearly what is wrong in the original
code, so even if now we only apply a temporary bandaid (for lack of time,
etc.) hopefully later this can be reverted to something more efficient
such as pulling up the UDP header as well.

	cheers
	luigi