Subject: Re: [Xen-devel] BUG in xennet_make_frags with paged skb data

From: Stefan Bader <[email-masked]>
Date: Fri, 07 Nov 2014 13:15:59 +0100

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)));

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