Subject: Re: [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.

From: Steven Haigh <[email-masked]>
Date: Wed, 07 May 2014 03:07:53 +1000

On a related note, I'm seeing this on an Arch Linux DomU and kernel
3.14.2. Its running aria2c bit torrent client.

The Dom0 is EL6 with Xen 4.2.4 / 3.14.2 kernel.

I also lodged this on the Arch bug tracker:
	https://bugs.archlinux.org/task/40244

In a nutshell, I see:
[ 7432.398096] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7434.270870] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7434.782199] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7435.184794] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7437.530677] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7437.973905] xen_netfront: xennet: skb rides the rocket: 19 slots
[ 7438.383947] xen_netfront: xennet: skb rides the rocket: 19 slots

On 05/05/14 20:19, Sander Eikelenboom wrote:
> 
> Hi Zoltan,
> 
> This weekend i tried some more things, the summary:
> 
> 
> 1) It's a PITA to isolate your patches that went into 3.15 (to rule out any other changes) and apply 
>    them to 3.14.2, which is tested and worked ok. Could you put up a git tree 
>    somewhere and rebase your patch series on 3.14.2 for testing ?
> 
> 2) Does the test suite you are using also has tests verifying that the content of packets isn't altered ?
> 
> 3) It's possible to simplify the test case to a apache webdav server and a 
>    simple curl put, this simplifies testing and puts ssl and duplicity out of the equation.
> 
> 4) There seem to be (at least) two, from the eye of it, separate issues with 
>    netback / netfront.
>    a) Assumption that  "An upstream guest shouldn't be able to send 18 slots" is 
>       false, which probably triggers the netback tx_frag_overflow case.
>    b) Corruption of packet content when:
>           - sending packets between guests on the same routed network bridge,
>           - sending packets between host (dom) and guest goes ok.
>    c) Both a and b are regressions from 3.14(.2), although at least a) seems just 
>       uncovering a latent bug revealed by changed semantics.
> 
> 5) Test outcome
> 
> --
> Sander
> 
> Ad 1) git tree somewhere and rebase your patch series on 3.14.2:
>     This is of course unless you are able to trigger this yourself and debug it with the simplified testcase described in (3).
> 
> Ad 3) simplify the test case:
>     My current setup:
>     - working: host kernel 3.14.2 and guest kernels all 3.15-rc4 on Debian wheezy
>     - not working: host and guest kernels all 3.15-rc4 on Debian wheezy (.config attached)
>     - not working: host and guest kernels all 3.15-rc4 + Eric's patch on Debian wheezy (.config attached)
>  
>     - guests are on a routed bridge (normal linux kernel bridge which is routed 
>       with eth0 and eth1.
>     - receiving guest has apache 2.2 running with mod_dav.
> 
>     - test:
>           - create a 100mb testfile with a pattern (used perl script is attached)
>           - Use curl in dom0 or in the sending guest to send the testfile:
>             curl --upload-file testfile.bin http://webdav-guest/storagelocation/
>           - check the md5sum of testfile.bin on both sender and receiver
> 
> Ad 4a) Assumption that  "An upstream guest shouldn't be able to send 18 slots":
>     - xen-netfront does this slot check in "xennet_start_xmit":
>         slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
>                 xennet_count_skb_frag_slots(skb);
>         if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
>                 net_alert_ratelimited(
>                         "xennet: skb rides the rocket: %d slots\n", slots);
>                 goto drop;
>         }
> 
>     - The "MAX_SKB_FRAGS + 1" was changed due to: http://www.gossamer-threads.com/lists/xen/devel/266980,
>       but it doesn't seem to be the proper solution.
>     - So your assumption doesn't hold, MAX_SKB_FRAGS==17, so 18 slots can come through.
>     - On 3.15-rc4 i now started to see this warning getting triggered and packets dropped, i don't see this on 3.14.2:
>       [  118.526583] xen_netfront: xennet: skb rides the rocket: 19 slots | skb_shinfo(skb)->nr_frags: 3, len: 186, offset: 4070, skb->len: 62330, skb->data_len: 62144, skb->truesize: 63424, np->tx.sring->rsp_prod: 21434, np->tx.rsp_cons: 21434  DIV_ROUND_UP(offset + len, PAGE_SIZE): 2 
>     - So probably some change in semantics makes this thing popup again.
>     - What i don't understand is why in:
>       xen-netfront this slots check is done when the skb is already dequeued (so dropping is the only thing left to do),
>       while in xen-netback it is done before the packet is dequeued (which now seems to work correct since the fixup of Paul to 3.14)
> 
>     - so your assumption isn't true, but it seems netfront needs to be fixed for that.
> 
>     - A lot of the (slot) checking logic and frag handling seems to be about the same in xen-netfront and xen-netback, although they seem to have diverted
>       somewhat, wouldn't it make sense to put a lot of the generic helper functions in a xen-netcommon.c and share them ?
> 
> Ad 4b) Corruption of packet content:
>     -  The dom0 case doesn't use zerocopy (tx_zerocopy_success: 0 &&  tx_frag_overflow: 0)
>     -  I'm getting less convinced it's (directly) coupled to (4a) and the tx_frag_overflow case, although they can occur at about the same time, it
>        doesn't necesarrily, the testfile is also corrupt when there is no tx_frag_overflow reported for both vifs:
>            ethtool -S vif2.0 (sender)
>            NIC statistics:
>            rx_gso_checksum_fixup: 0
>            tx_zerocopy_sent: 25705
>            tx_zerocopy_success: 25538
>            tx_zerocopy_fail: 167
>            tx_frag_overflow: 0
>            
>            ethtool -S vif1.0 (receiver)
>            NIC statistics:
>            rx_gso_checksum_fixup: 0
>            tx_zerocopy_sent: 246916
>            tx_zerocopy_success: 1
>            tx_zerocopy_fail: 246915
>            tx_frag_overflow: 0
> 
> 
> Ad 5) The test described in (3) results into (repeated 5 times each) these md5sums for testfile.bin::
>     - generated file: fe599e44789799bae5b6db3df9a34e2d
>     
>     - dom0 3.14.2        - dom0 to guest:  fe599e44789799bae5b6db3df9a34e2d
>     - dom0 3.14.2        - guest to guest: fe599e44789799bae5b6db3df9a34e2d
>     
>     - dom0 3.15-rc4      - dom0 to guest:  fe599e44789799bae5b6db3df9a34e2d
>     - dom0 3.15-rc4      - guest to guest: 2f51d9baad6f7b2c99aa51e14878a55a fb7df5de7d08b6ad24aa9166949de8c9 0c0afc145f4fed9231e4f1ab6243d02f ef83ace3aafd7e57b8b2fbe324d38995 ffab10c9906381415e5697d2c0e05da3
>     
>     - dom0 3.15-rc4+eric - dom0 to guest:  fe599e44789799bae5b6db3df9a34e2d
>     - dom0 3.15-rc4+eric - guest to guest: eb8f48c5613478bb0a69a6115570c713 66fc191b4a04ccddd8b926bc2f57c2b9 99891e0397ca119b0cfaea80b0c6b1f0 0899ab428d102791345c67fa4b608b36 4cc2e3badabc465630d8002004fc0fa3
> 
>    - That's no good for the guest to guest case .. so inspect the received testfile.bin:
>      - length is exactly the same .. good
>      - beginning and ending magic strings are there .. good
>      - the md5sums differ every time .. no good
>      - diff the files to see what is different (one diff from the hexdumps is attached):
>          - although the byte counting strings should be unique, in the received testfile.bin they are not, for example:
>                grep -i -n 76234752 testfile2.hex
>                4764674:048b4010  20 20 20 20 20 20 20 20  37 36 32 33 34 37 35 32  |        76234752|
>                4764930:048b5010  20 20 20 20 20 20 20 20  37 36 32 33 34 37 35 32  |        76234752|
> 
>     - So what do we have so far:
>         - it look likes all packet metadata is correct, so no warnings or errors from the network stack.
>         - only the actual payload gets mangled (otherwise i would have expected warnings from the network stack)
>         - it seems to only get mangled when it is travelling "xen-netfront -> xen-netback -> linux netw. bridge -> xen-netback -> xen-netfront".
>         - it seems NOT to get mangled when it is travelling "xen-netback -> xen-netfront" only.
>         - it's not random corruption, it seems data from older/other frags/packets is used instead of the right data.
>         - and a simple test case ... so i hope you can reproduce.
> 
> --
> Sander
> 
> 
> 
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-- 
Steven Haigh

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