Re: [PATCH] tests/qemu_iotests: Minimize usage of used ports

[Lukáš Doktor]

Dne 06. 02. 20 v 17:59 Max Reitz napsal(a):
> On 06.02.20 17:48, Eric Blake wrote:
>> On 2/6/20 10:37 AM, Max Reitz wrote:
>>
>>>>
>>>> thank you and I am sorry for not digging deep enough. This week my CI
>>>> failed with:
>>>>
>>>> 01:24:06 DEBUG| [stdout] +ERROR: test_inet (__main__.QemuNBD)
>>>> 01:24:06 DEBUG| [stdout]
>>>> +----------------------------------------------------------------------
>>>> 01:24:06 DEBUG| [stdout] +Traceback (most recent call last):
>>>> 01:24:06 DEBUG| [stdout] +  File "147", line 85, in setUp
>>>> 01:24:06 DEBUG| [stdout] +    self.vm.launch()
>>>> 01:24:06 DEBUG| [stdout] +  File
>>>> "/home/jenkins/ppc64le/qemu-master/tests/qemu-iotests/../../python/qemu/machine.py",
>>>> line 302, in launch
>>>> 01:24:06 DEBUG| [stdout] +    self._launch()
>>>> 01:24:06 DEBUG| [stdout] +  File
>>>> "/home/jenkins/ppc64le/qemu-master/tests/qemu-iotests/../../python/qemu/machine.py",
>>>> line 319, in _launch
>>>> 01:24:06 DEBUG| [stdout] +    self._pre_launch()
>>>> 01:24:06 DEBUG| [stdout] +  File
>>>> "/home/jenkins/ppc64le/qemu-master/tests/qemu-iotests/../../python/qemu/qtest.py",
>>>> line 106, in _pre_launch
>>>> 01:24:06 DEBUG| [stdout] +    super(QEMUQtestMachine,
>>>> self)._pre_launch()
>>>> 01:24:06 DEBUG| [stdout] +  File
>>>> "/home/jenkins/ppc64le/qemu-master/tests/qemu-iotests/../../python/qemu/machine.py",
>>>> line 270, in _pre_launch
>>>> 01:24:06 DEBUG| [stdout] +    self._qmp =
>>>> qmp.QEMUMonitorProtocol(self._vm_monitor, server=True)
>>>> 01:24:06 DEBUG| [stdout] +  File
>>>> "/home/jenkins/ppc64le/qemu-master/tests/qemu-iotests/../../python/qemu/qmp.py",
>>>> line 60, in __init__
>>>> 01:24:06 DEBUG| [stdout] +    self.__sock.bind(self.__address)
>>>> 01:24:06 DEBUG| [stdout] +OSError: [Errno 98] Address already in use
>>
>> Was this test 147?  If so, see:
>> https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg01469.html
>>
>> because that failure matches what I was seeing.
>>
>>>>
>>>> I made the mistake of reproducing this on my home system using the
>>>> qemu revision that I had and assuming it's caused by a used port. So
>>>> I limited the port range and used nc to occupy the port. It sort-of
>>>> reproduced but instead of Address already in use it hanged until I
>>>> kill the nc process. Then it failed with:
>>>>
>>>> +Traceback (most recent call last):
>>>> +  File "147", line 124, in test_inet
>>>> +    flatten_sock_addr(address))
>>>> +  File "147", line 59, in client_test
>>>> +    self.assert_qmp(result, 'return', {})
>>>> +  File
>>>> "/home/medic/Work/Projekty/qemu/tests/qemu-iotests/iotests.py", line
>>>> 821, in assert_qmp
>>>> +    result = self.dictpath(d, path)
>>>> +  File
>>>> "/home/medic/Work/Projekty/qemu/tests/qemu-iotests/iotests.py", line
>>>> 797, in dictpath
>>>> +    self.fail('failed path traversal for "%s" in "%s"' % (path,
>>>> str(d)))
>>>> +AssertionError: failed path traversal for "return" in "{'error':
>>>> {'class': 'GenericError', 'desc': 'Failed to read initial magic:
>>>> Unexpected end-of-file before all bytes were read'}}"
>>>>
>>
>> That's a secondary failure, I assume if the initial bug is fixed we are
>> less likely to hit the secondary one; but the secondary one may still be
>> worth fixing.
>>
>>>> After a brief study I thought qemu is not doing the job well enough
>>>> and wanted to add a protection. Anyway after a more thorough overview
>>>> I came to a different conclusion and that is that we are facing the
>>>> same issue as with incoming migration about a year ago. What happened
>>>> is that I started "nc -l localhost 32789" which results in:
>>>>
>>>> COMMAND   PID  USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
>>>> nc      26758 medic    3u  IPv6 9579487      0t0  TCP localhost:32789
>>>> (LISTEN)
>>>>
>>>> Then we start the server by "_try_server_up" where qemu-nbd detects
>>>> the port is occupied on IPv6 but available on IPv4, so it claims it:
>>>> COMMAND   PID  USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
>>>> nc        26758 medic    3u  IPv6 9579487      0t0  TCP
>>>> localhost:32789 (LISTEN)
>>>> qemu-nbd  26927 medic    4u  IPv4 9591857      0t0  TCP
>>>> localhost:32789 (LISTEN)
>>>>
>>>> and reports success. Then we try to connect but the hotplugged VM
>>>> first attempts to connect on the IPv6 address and hangs for infinity.
>>>>
>>>> Now is this an expected behavior? If so then we need the
>>>> find_free_address (but preferably directly in _try_server_up just
>>>> before starting the qemu-nbd) to leave as little time-frame for
>>>> collision as possible. Otherwise the test is alright and qemu-nbd
>>>> needs a fix to bail out in case some address is already used (IIRC
>>>> this is what incoming migration does).
>>>
>>> Ah, OK.
>>>
>>> Well, expected behavior...  It’s a shame, that’s what it is.
>>
>> In libnbd, we recently improved the testsuite by switching over to
>> systemd-style fd passing: instead of asking qemu-nbd to open a random
>> port (and hoping it is available), we instead pre-open the port (where
>> failure is under our control) and then pass in that fd with environment
>> variables to qemu-nbd, which in turn guarantees that qemu-nbd won't hit
>> failures in trying to use the port.  Maybe we should utilize that more
>> in qemu's own testsuite.
> 
> I suppose you’re welcome, but it sounds a bit like overkill for now. :-)
> 

Still you need to test all the variants...

>> Also, I need to revisit my proposed patches for letting qemu-nbd support
>> TLS over Unix sockets, as that's another way to avoid TCP contention
>> (right now, qemu has an anachronistic prohibition against the
>> combination of TLS and Unix sockets).
> 
> 147 specifically wants to test inet, though (among other things).
> Support for TLS over Unix sockets won’t change that.  (I suppose it will
> reduce the number of inet NBD sockets in other tests, though.)
> 
>>>> My second mistake was testing this on the old code-base and rebasing
>>>> it only before sending the patch (without testing after the rebase).
>>>> If I were to test it first, I would have found out that the real
>>>> reproducer is simply running the test as the commit
>>>> 8dff69b9415b4287e900358744b732195e1ab2e2 broke it.
>>>>
>>>>
>>>> So basically there are 2 actions:
>>>>
>>>> 1. fix the test as on my system it fails in 100% of cases, bisect
>>>> says the first bad commit is
>>>> 8dff69b9415b4287e900358744b732195e1ab2e2. Would anyone have time in
>>>> digging into this? I already spent way too much time on this and
>>>> don't really know what that commit is trying to do.
>>>
>>> Yep, I’ve sent a patch:
>>>
>>> https://lists.nongnu.org/archive/html/qemu-block/2020-02/msg00294.html
>>

Thank you :-)

>> Ah, so we did notice the same problem.
>>
>>>
>>>> 2. decide on the behavior when IPv4/6 is already in use (bail-out or
>>>> start).
>>>> 2a. In case it should bail-out than the test is correct and there is
>>>> no need for my patch. On the other hand qemu-nbd has to be fixed
>>>
>>> I don’t think it makes much sense to let qemu’s NBD server ensure that
>>> it claims both IPv4 and IPv6 in case the user specifies a
>>> non-descriptive hostname.
>>>
>>>> 2b. Otherwise I can send a v2 that will check the port in the
>>>> _try_server_up just before starting qemu-nbd to minimize the risk of
>>>> using a utilized port (or should you decide it's not worth checking,
>>>> I can simply forget about this)
>>>
>>> Hm.  It wouldn’t be fully reliable, but, well...  The risk would be
>>> minimal.
>>>
>>> OTOH, would it work if we just did a %s/localhost/127.0.0.1/ in the
>>> test?  We have specific cases for IPv6, so I think it makes sense to
>>> force IPv4 in all other cases.
>>
>> Except then it will fail on machines configured for IPv6-only.
> 
> So we’ll just have to test whether IPv4 works, just like we already do
> for IPv6, no?
> 

Sure, using ::1 for IPv6 and 127.0.0.1 for IPv4 cases would work. The question 
is whether the behavior is really expected. In migration it was considered 
dangerous, because you can have 2 VMs starting the migration at the same time. 
They both might attempt to get the same port, one would receive IPv4 the other 
IPv6. Then depending on the order of start migrate you might end-up attempting 
to migrate the other VMs instead of the intended ones.

The same can happen here, you might start 2 nbd exports at the same time, get 
the same port without any failures and then depending on luck get the right or 
wrong export when attempting to connect. So I think bailing out in case we fail 
to get all interfaces would be the most appropriate (note the same situation is 
for 0.0.0.0 where you might end-up serving multiple different disks on 
different interfaces). Anyway I leave it to you. If you decide you don't want 
to fail than using ::1/127.0.0.1 sounds like a good idea. Otherwise it'd make 
sense to create a test that especially uses ::1 and then localhost to make sure 
it bails-out.

Regards,
Lukáš

> Max
>

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