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Date: Fri, 29 Aug 2008 15:35:46 -0700
From: Andrew Thompson <thompsa@FreeBSD.org>
To: Luigi Rizzo <rizzo@iet.unipi.it>
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Cc: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net>,
	Sam Leffler <sam@freebsd.org>, net@freebsd.org, gnn@freebsd.org
Subject: Re: Small patch to multicast code...
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On Fri, Aug 29, 2008 at 06:41:45PM +0200, Luigi Rizzo wrote:
> On Fri, Aug 29, 2008 at 09:32:10AM -0700, Sam Leffler wrote:
> > Luigi Rizzo wrote:
> ...
> > >and to be more explicit - the result of m_pullup is that
> > >the number of bytes specified as m_pullup argument are in
> > >a private piece of memory -- the 'data' region within the mbuf -- so
> > >you can freely play with them without trouble.
> > >
> > >That is why i suggested to just increase the argument to m_pullup
> > >by the size of the udp header so one can overwrite the checksum
> > >within the mbuf without touching the shared part in the cluster
> > >(if any).
> > >  
> > 
> > Hmm, never considered the m_pullup guaranteed a private copy (but I see 
> > it in the code).  The original semantics were just that the data was 
> > contiguous.
> 
> funny, i thought the guarantee of a writable copy was also part
> of the original semantics :)

The bridge code does a deep copy of the packet for each interface it
broadcasts on due the firewall code modifying the headers. It sounds
like this should just be a copy+pullup instead.


Andrew