netdev - Re: [Xen-devel] [3.15-rc3] Bisected: xen-netback mangles packets between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV

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Date:	Fri, 2 May 2014 18:28:18 +0200
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	<xen-devel@...ts.xen.org>, Ian Campbell <Ian.Campbell@...rix.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Xen-devel] [3.15-rc3] Bisected: xen-netback mangles packets between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy" series.


Friday, May 2, 2014, 5:26:33 PM, you wrote:

> On 02/05/14 16:21, Eric Dumazet wrote:
>> On Fri, 2014-05-02 at 15:47 +0100, Zoltan Kiss wrote:
>>
>>> Sorry, I was misleading and wrong. Can you try out this scenario with
>>> the attached patch?
>>
>> Guys, I already told you skb->truesize 'mismatch' could not explain
>> packet corruptions. This comes from an expert in this matter, you can
>> trust me.
>>
>> What could happens here is that TCP stack merges skbs (TCP coalescing)
> These packets shouldn't reach Dom0's TCP stack at all, 
> bridge/openvswitch grabs them before. And in the sending/receiving guest 
> these skbs don't have this flag.
> However generally it is possible that a guest talks directly to Dom0, in 
> which case your proposed fix could be valid.

I just tested Eric's patch alone .. and:

- It lasts longer .. the first upload goes OK (previously it would already bail out on 
  the first one)
- We still hit the "xenvif_handle_frag_list" path while uploading, but no "tx_frag_overflow" 
  occurred.

- But it bails out on the second upload .. with the message
  "_ssl.c:1415: error:140943FC:SSL routines:SSL3_READ_BYTES:sslv3 alert bad record mac"
- We also hit the "xenvif_handle_frag_list" path while uploading and this time we 
  also hit the "tx_frag_overflow" case.

--
Sander

>>
>> Problem is that SKBTX_DEV_ZEROCOPY addition did not take care of this.
>>
>> We have to forbid these merges from happening, because one skb has a
>> single destructor_arg.
>>
>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> index 1b62343f5837..85995a14aafc 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -3838,7 +3839,10 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>>               return true;
>>       }
>>
>> -     if (skb_has_frag_list(to) || skb_has_frag_list(from))
>> +     if (skb_has_frag_list(to) ||
>> +         skb_has_frag_list(from) ||
>> +         (skb_shinfo(to)->tx_flags & SKBTX_DEV_ZEROCOPY) ||
>> +         (skb_shinfo(from)->tx_flags & SKBTX_DEV_ZEROCOPY))
>>               return false;
>>
>>       if (skb_headlen(from) != 0) {
>>
>>
>>
>>
>>
>>



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