From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jan Beulich" <JBeulich@suse.com>
Subject: Re: [Xen-devel] [PATCH] xen-netfront: pull on receive skb may
 need to happen earlier
Date: Wed, 10 Jul 2013 14:58:42 +0100
Message-ID: <51DD84B202000078000E3EF8@nat28.tlf.novell.com>
References: <51D57C1F.8070909@hunenet.nl>
 <20130704150137.GW7483@zion.uk.xensource.com>
 <51D6AED902000078000E2EA9@nat28.tlf.novell.com>
 <20130705145319.GB9050@zion.uk.xensource.com>
 <51DAA9B202000078000E3357@nat28.tlf.novell.com>
 <51DAE6CA02000078000E3566@nat28.tlf.novell.com>
 <20130708154814.GI8027@zion.uk.xensource.com>
 <51DBCF4F02000078000E3793@nat28.tlf.novell.com>
 <20130709165134.GG19798@zion.uk.xensource.com>
 <51DD221E02000078000E3BC6@nat28.tlf.novell.com>
 <20130710100416.GL19798@zion.uk.xensource.com>
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 <1373460644.5453.109.camel@hastur.hellion.org.uk>
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Cc: "Wei Liu" <wei.liu2@citrix.com>, <davem@davemloft.net>,
	"Dion Kant" <g.w.kant@hunenet.nl>, <xen-devel@lists.xen.org>,
	<netdev@vger.kernel.org>, <stable@vger.kernel.org>
To: "Ian Campbell" <ian.campbell@citrix.com>
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>>> On 10.07.13 at 14:50, Ian Campbell <ian.campbell@citrix.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@citrix.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