linux-kernel - Re: [Xen-devel] BUG in xennet_make

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Date:	Fri, 07 Nov 2014 13:28:33 +0100
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Zoltan Kiss <zoltan.kiss@...aro.org>,
	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Jay Vosburgh <jay.vosburgh@...onical.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] BUG in xennet_make_frags with paged skb data

On 07.11.2014 13:21, Zoltan Kiss wrote:
> 
> 
> On 07/11/14 12:15, Stefan Bader wrote:
>> On 07.11.2014 12:22, Eric Dumazet wrote:
>>> On Fri, 2014-11-07 at 09:25 +0000, Zoltan Kiss wrote:
>>>
>>> Please do not top post.
>>>
>>>> Hi,
>>>>
>>>> AFAIK in this scenario your skb frag is wrong. The page pointer should
>>>> point to the original compound page (not a member of it), and offset
>>>> should be set accordingly.
>>>> For example, if your compound page is 16K (4 page), then the page
>>>> pointer should point to the first page, and if the data starts at the
>>>> 3rd page, then offset should be >8K
>>>
>>> This is not accurate.
>>>
>>> This BUG_ON() is wrong.
>>>
>>> It should instead be :
>>>
>>> BUG_ON(len + offset > PAGE_SIZE<<compound_order(compound_head(page)));
>>
>> would that not have to be
>>
>> BUG_ON((page-compound_head(page)*PAGE_SIZE)+offset+len >
>> PAGE_SIZE<<compound_order(compound_head(page)));
> 
> There should be a parentheses around "page-compound_head(page)".

*sigh* before submitting anything I'll have to make sure I run it past the
compiler at least. I never manager to type the beast without some error.

But you got the drift...

>>
>> since offset is adjusted to start from the tail page in that case.
>>>
>>> splice() code can generate such cases.
>>>
>>>
>>
>>



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