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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.

Thursday, May 1, 2014, 5:46:01 PM, you wrote:

> On 01/05/14 14:59, Sander Eikelenboom wrote:
>> Thursday, May 1, 2014, 3:37:41 PM, you wrote:
>>> On 30/04/14 23:25, Sander Eikelenboom wrote:
>>>> Wednesday, April 30, 2014, 10:53:39 PM, you wrote:
>>>>> On 30/04/14 11:45, Sander Eikelenboom wrote:
>>>>>> Hi Zoltan,
>>>>>> Your series "TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy", 
>>>>>> merged into mainline with merge commit 
>>>>>> 4caeccb4de76440e433a15009636e77d003eb3d6,
>>>>>> seem to introduce a subtle bug on network traffic between 2 guests on a 
>>>>>> bridge on the same host.
>>>>>> I have one guest running apache as webdav server with SSL and another 
>>>>>> guest that is using that is uploading large files to that webdav server.
>>>>>> Small requests (some get's and propfind's) seem to work ok, but when the 
>>>>>> bulk uploading begins it fails with:
>>>>>> Attempt 1 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL 
>>>>>> routines:SSL3_READ_BYTES:sslv3 alert bad record mac
>>>>>> Attempt 2 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL 
>>>>>> routines:SSL3_READ_BYTES:sslv3 alert bad record mac
>>>>>> Attempt 3 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL 
>>>>>> routines:SSL3_READ_BYTES:sslv3 alert bad record mac
>>>>>> Attempt 4 failed. SSLError: [Errno 1] _ssl.c:1415: error:140943FC:SSL 
>>>>>> routines:SSL3_READ_BYTES:sslv3 alert bad record mac
>>>>>> So some how large (probably fragmented) packets can get mangled when 
>>>>>> from guest to guest on the same host.
>>>>>> I don't see this with clients that upload large files from external 
>>>>>> sources.
>>>>>> Probably if SSL wasn't complaining it would probably be unnoticed for 
>>>>>> longer and doing some silent corruption.
>>>>>> I first blamed openssl, since it started around all the latest openssl 
>>>>>> mayhem and updates, but it turns out it is all xen-netback related again.
>>>>>> Since these commits break bisectabillity:
>>>>>>        - 1bb332af4cd889e4b64dacbf4a793ceb3a70445d  (note in commit 
>>>>>> message && kernel panic)
>>>>>>        - 62bad3199a4c20505fc36c169deef20b25e17c5f  (kernel panic)
>>>>>> i stopped bisecting at this point.
>>>>>> The upside is .. it's 100% reproduceable :-)
>>>>> That's good :) Can you take tcpdump captures along the way (sending
>>>>> guest, dom0, receiving guest), and try to work out which packets are
>>>>> different, and where? Although taking captures in Dom0 might change your
>>>>> result, as it triggers the pages to be copied and unmapped before they
>>>>> reach their target.
>>>>> Thanks,
>>>>> Zoli
>>>> Hrrmm that sounds like a lot of data and a lot of work ..
>>> If you could make captures in the sending and receiving guest with
>>> tcpdump (take care of increasing snaplen so the whole packet is there,
>>> and filter to the SSH connection itself), and upload it somewhere for
>>> me, that would be enough for start. I will try to work out where the
>>> corruption happens.
>>> Also, do you have timestamps for the above mentioned log entries? I
>>> guess they appear on the receiving side.
>>> And some info about the componenets on the server, so I can work out
>>> where is that _ssl.c:1415, and which part of the packet it actually
>>> looks for.
>> They appear on the sending side (duplicity) .. the receiving side (apache +
>> mod_dav + ssl | gnu_tls) gives a "Could not get next bucket brigade (URI:"
> I will try to repro this case in house. What versions of these 
> components you used?

Both guests are debian wheezy.

The webdav server has:
ii  apache2-mpm-event                2.2.22-13+deb7u1           amd64        
Apache HTTP Server - event driven model
ii  apache2-utils                    2.2.22-13+deb7u1           amd64        uti
ii  apache2.2-bin                    2.2.22-13+deb7u1           amd64        Apa
ii  apache2.2-common                 2.2.22-13+deb7u1           amd64        Apa
ii  libapache2-mod-gnutls            0.5.10-1.1                 amd64        Apa

ii  libssl1.0.0:amd64                1.0.1e-2+deb7u7            amd64        SSL
ii  openssl                          1.0.1e-2+deb7u7            amd64        Sec

The guest with duplicity currently has a duplicity version from unstable 
recompiled for wheezy. But i previously also tried a downgrade to the standard 
wheezy version. It uses the webdav backend and a volumesize of 100MB.

Unfortunately it seems duplicity doesn't bail out at first instance, it seems 
only reports error after the so the full tcpdumps i got are also 100MB each.

Since the error seems to happen when it's going through 
"xenvif_handle_frag_list", i have added a bunch of ratelimited printk's.

Will run that for both the cases:
        skb->truesize -= skb->data_len;
        skb->truesize -= nskb->data_len;

Let's see what that does different and if that gives an insight in what is 

> Zoli

>>>> how ever .. could it be just a type and would the following make sense ?
>>>> diff --git a/drivers/net/xen-netback/netback.c 
>>>> b/drivers/net/xen-netback/netback.c
>>>> index 7666540..abeea10 100644
>>>> --- a/drivers/net/xen-netback/netback.c
>>>> +++ b/drivers/net/xen-netback/netback.c
>>>> @@ -1366,7 +1366,7 @@ static int xenvif_handle_frag_list(struct xenvif 
>>>> *vif, struct sk_buff *skb)
>>>>           xenvif_fill_frags(vif, nskb);
>>>>           /* Subtract frags size, we will correct it later */
>>>> -       skb->truesize -= skb->data_len;
>>>> +       skb->truesize -= nskb->data_len;
>>>>           skb->len += nskb->len;
>>>>           skb->data_len += nskb->len;
>>> Nope, that's correct there: after that skb->truesize will be the size of
>>> the struct plus the linear buffer itself. The code is just about the
>>> ditch the original fragments plus the skb on the frag_list. When the new
>>> pages are created, it will update it again.
>> Well i just went a head and tried this .. and the uploading does seem to 
>> work fine with this change
>> .. (that obviously doesn't say anything about correctness)
>>> Also, this code path runs only if the guest sends more slots we can
>>> handle (so we put the extra one to the frag_list until we can get rid of
>>> it). On Linux it can only happen with 3.2 or older guest kernels, and
>>> only occasionally. As you said, this is 100% reproducible, so I would
>>> doubt the problem is with this part of the code.
>> Well this assumption seems to be incorrect:
>>          - both dom0 and guest kernels are 3.15-rc3's.
>>          - but we do end up in this code path
>>> Zoli

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