netdev - Re: [PATCH] ndisc: Ensure to reserve header space for encapsulation.

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Message-ID: <50DB4B3D.2050800@linux-ipv6.org>
Date:	Thu, 27 Dec 2012 04:08:45 +0900
From:	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org, davem@...emloft.net,
	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH] ndisc: Ensure to reserve header space for encapsulation.

Eric Dumazet wrote:
> On Tue, 2012-12-25 at 23:39 +0900, YOSHIFUJI Hideaki wrote:
>> We allocate sk_buff of MAX_HEADER + LL_RESERVED_SPACE(dev) + packet
>> length + dev->needed_tailroom, but reserved LL_RESERVED_SPACE(dev)
>> only.  This means that space for encapsulation was placed at the end
>> of buffer.  This does not make sense.
>>
>> Reserve the space correctly, like this:
>>
>> head                       data                   tail       end
>> +--------------------------------------------------------------+
>> +                |           |                      |          |
>> +--------------------------------------------------------------+
>> |<--MAX_HEADER-->|<-hlen---->|<---ipv6 packet------>|<--tlen-->|
>>                    =LL_
>>                     RESERVED_
>>                     SPACE(dev)
>>
>> Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
>> ---
:
> It looks like this MAX_HEADER reserve is not needed at all.
> 
> Space for encapsulation should already be in 
> int hlen = LL_RESERVED_SPACE(dev);
> 
> arp_create() for example doesnt add this MAX_HEADER 
> 
> If extra encapsulation is needed (at head or at tail), it really should
> be documented.

Current code does not make sense, at least.

Please refer to my previous posting: "[GIT PULL net-next 01/17]
ndisc: Fix size calculation for headers." around Dec/19.

My previous patch did remove MAX_HEADER from allocation.

--yoshfuji
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