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

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Date:	Fri, 07 Nov 2014 03:22:00 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zoltan Kiss <zoltan.kiss@...aro.org>
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>,
	Stefan Bader <stefan.bader@...onical.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 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)));

splice() code can generate such cases.


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