Re: [V2 PATCH 2/9] macvtap: zerocopy: fix truesize underestimation

From: Jason Wang
Date: Fri May 18 2012 - 06:10:23 EST


On 05/17/2012 11:28 PM, Shirley Ma wrote:
On Thu, 2012-05-17 at 10:59 +0800, Jason Wang wrote:
Didn't see how this affact skb->len. And for truesize, I think they
are
different, when the offset were not zero, the data in this vector
were
divided into two parts. First part is copied into skb directly, and
the
second were pinned from a whole userspace page by
get_user_pages_fast(),
so we need count the whole page to the socket limit to prevent evil
application.
What I meant that the code for skb->truesize has double added the first
offset if any left from that vector (partically copied into skb
directly, and then count pagesize which includes the offset (truesize +=
PAGE_SIZE)).

Yes, I get you mean. There's no difference between first frag and others: it's also possible for other frags that didn't occupy the whole page. Since we pin the whole user page, better to count the whole page size to prevent evil application.
Thanks
Shirley

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