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Date: Fri, 22 Aug 2008 09:27:25 -0700
From: Sam Leffler <sam@freebsd.org>
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To: Luigi Rizzo <rizzo@iet.unipi.it>
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Cc: gnn@freebsd.org, "Bruce M. Simpson" <bms@freebsd.org>, net@freebsd.org
Subject: Re: Small patch to multicast code...
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Luigi Rizzo wrote:
> 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.
>
>   

I agree that doing the deep copy is just papering over a bigger problem.

    Sam