netdev - Re: [Xen-devel] [PATCHv1 net-next] xen-netback: remove unconditional pull_skb

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Message-Id: <20141104.161704.1690311989900127361.davem@davemloft.net>
Date:	Tue, 04 Nov 2014 16:17:04 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	zoltan.kiss@...aro.org
Cc:	david.vrabel@...rix.com, Ian.Campbell@...rix.com,
	netdev@...r.kernel.org, malcolm.crossley@...rix.com,
	wei.liu2@...rix.com, xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] [PATCHv1 net-next] xen-netback: remove
 unconditional pull_skb_tail in guest Tx path

From: Zoltan Kiss <zoltan.kiss@...aro.org>
Date: Mon, 03 Nov 2014 18:23:03 +0000

> 
> 
> On 03/11/14 17:46, David Vrabel wrote:
>> On 03/11/14 17:39, Ian Campbell wrote:
>>> On Mon, 2014-11-03 at 17:23 +0000, David Vrabel wrote:
>>>> From: Malcolm Crossley <malcolm.crossley@...rix.com>
>>>>
>>>> Unconditionally pulling 128 bytes into the linear buffer is not
>>>> required. Netback has already grant copied up-to 128 bytes from the
>>>> first slot of a packet into the linear buffer. The first slot normally
>>>> contain all the IPv4/IPv6 and TCP/UDP headers.
>>>
>>> What about when it doesn't? It sounds as if we now won't pull up,
>>> which
>>> would be bad.
>>
>> The network stack will always pull any headers it needs to inspect
>> (the
>> frag may be a userspace page which has the same security issues as a
>> frag with a foreign page).
> I wouldn't bet my life on this, but indeed it should always happen.

I would bet my life on it.

Every protocol demux starts with pskb_may_pull() to pull frag data
into the linear area, if necessary, before looking at headers.
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