netdev - Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may need to happen earlier

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Date:	Wed, 10 Jul 2013 14:58:42 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ian Campbell" <ian.campbell@...rix.com>
Cc:	"Wei Liu" <wei.liu2@...rix.com>, <davem@...emloft.net>,
	"Dion Kant" <g.w.kant@...enet.nl>, <xen-devel@...ts.xen.org>,
	<netdev@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may
 need to happen earlier

>>> On 10.07.13 at 14:50, Ian Campbell <ian.campbell@...rix.com> wrote:
> On Wed, 2013-07-10 at 11:46 +0100, Jan Beulich wrote:
>> >>> On 10.07.13 at 12:04, Wei Liu <wei.liu2@...rix.com> wrote:
>> > Jan, looking at the commit log, the overrun issue in
>> > xennet_get_responses was not introduced by __pskb_pull_tail. The call to
>> > xennet_fill_frags has always been in the same place.
>> 
>> I'm convinced it was: Prior to that commit, if the first response slot
>> contained up to RX_COPY_THRESHOLD bytes, it got entirely
>> consumed into the linear portion of the SKB, leaving the number of
>> fragments available for filling at MAX_SKB_FRAGS. Said commit
>> dropped the early copying, leaving the fragment count at 1
>> unconditionally, and now accumulates all of the response slots into
>> fragments, only pulling after all of them got filled in. It neglected to
>> realize - due to the count now always being 1 at the beginning - that
>> this can lead to MAX_SKB_FRAGS + 1 frags getting filled, corrupting
>> memory.
> 
> That argument makes sense to me.
> 
> Is it possible to hit a scenario where we need to pull more than
> RX_COPY_THRESHOLD in order to fit all of the data in MAX_SKB_FRAGS ?

I'm not aware of any, but I'm no expert here in any way.

> Does this relate somehow to the patch Annie has sent out recently too?

I don't think so.

Jan

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