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SubjectRe: [Xen-devel] BUG in xennet_make_frags with paged skb data
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.
>
>


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