On 01/23/2014 08:28 PM, Luiz Capitulino wrote:
> On Thu, 23 Jan 2014 17:33:33 +0200
> Stratos Psomadakis <pso...@grnet.gr> wrote:
>
>> On 01/23/2014 03:54 PM, Luiz Capitulino wrote:
>>> On Thu, 23 Jan 2014 08:44:02 -0500
>>> Luiz Capitulino <lcapitul...@redhat.com> wrote:
>>>
>>>> On Thu, 23 Jan 2014 19:23:51 +0800
>>>> Fam Zheng <f...@redhat.com> wrote:
>>>>
>>>>> Bcc:
>>>>> Subject: Re: [Qemu-devel] Possible bug in monitor code
>>>>> Reply-To:
>>>>> In-Reply-To: <52e0ec4b.7010...@grnet.gr>
>>>>>
>>>>> On Thu, 01/23 12:17, Stratos Psomadakis wrote:
>>>>>> On 01/23/2014 05:07 AM, Fam Zheng wrote:
>>>>>>> On Wed, 01/22 17:53, Stratos Psomadakis wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> we've encountered a weird issue regarding monitor (qmp and hmp)
>>>>>>>> behavior
>>>>>>>> with qemu-1.7 (and qemu-1.5). The following steps will reproduce the
>>>>>>>> issue:
>>>>>>>>
>>>>>>>> 1) Client A connects to qmp socket with socat
>>>>>>>> 2) Client A gets greeting message {"QMP": {"version": ..}
>>>>>>>> 3) Client A waits (select on the socket's fd)
>>>>>>>> 4) Client B tries to connect to the *same* qmp socket with socat
>>>> QMP/HMP can only handle a single client per connection, so this is
>>>> not supported. What you could do is to create multiple QMP/HMP instances
>>>> on the command-line, but this has never been fully tested so we don't
>>>> officially support this either (although it may work).
>>>>
>>>> Now, not gracefully failing on step 4 is a real bug here. I think the
>>>> best thing to do would be to close client's B connection. Patches are
>>>> welcome :)
>>> Thinking about this again, I think this should already be the case
>>> (as I don't believe chardev code is sitting on accept()). So maybe
>>> you triggered a race on chardev code or broke something there.
>> Yes, the chardev code doesn't accept any more connections until the
>> currently active connection is closed.
>>
>> However, when a new client tries to connect (while there's another qmp /
>> hmp connection active) the kernel, as long as there's room in the socket
>> backlog, will return to the client that the connection has been
>> successfully established and will queue the connection to be accepted
>> later, when qmp actually finishes with its active connection and
>> re-executes accept().
>>
>> The problem arises if the new client closes the connection before the
>> older one is finished. When qmp runs accept() again, it will get a
>> socket fd for a client who has now closed the connection. At this point,
>> the monitor control event handler will get triggered and try to write /
>> flush the greeting message to the client. The client however has closed
>> its socket so the write will fail with SIGPIPE / EPIPE. Neither the
>> qemu-char nor the monitor code seem to handle this situation.
>>
>> Btw, have you tried to reproduce it?
> Not yet, I may have some time tomorrow. How reproducible is it for you?We can trigger it (by following the steps described in the first mail) consistently. > Another question: have you tried to reproduce with an old qemu version > (say v1.0) to see if this bug always existed? If the bug was introduced > in some recent QEMU version you could try to bisect it. v1.1 is not affected. I checked the code and it seems the monitor code has been refactored since v1.1. > Maybe you could try to reproduce with a different subsystem so that we > can rule out or confirm monitor's involvement? Like -serial? It's actually a fault of the monitor_flush() function. As far as I can understand, monitor_flush() calls qemu_chr_fe_write() and doesn't handle all of the return codes / error cases properly (as I described in a previous mail). If you check the function, you'll see that the final case (where it set ups a watch / callback) always assumes an EAGAIN / EWOULDBLOCK error. If you can verify / confirm that this is the case and that the patch sent resolves the issue in a sane / correct way, I'll resubmit it properly (with git-format-patch, a git log msg etc). Thanks, Stratos > >> Thanks, >> Stratos >> >>>>>>>> 5) Client B does *NOT* get any greating message >>>>>>>> 6) Client B waits (select on the socket's fd) >>>>>>>> 7) Client B closes connection (kill socat) >>>>>>>> 8) Client A quits too >>>>>>>> 9) Client C connects to qmp socket >>>>>>>> 10) Client C gets *two* greeting messages!!! >>>>>>> Hi Stratos, thank you for debugging and reporting this. >>>>>>> >>>>>>> I tested this sequence but can't fully reproduce this. What I see is 5) >>>>>>> but no >>>>>>> 10). Client C acts normally. And your patch below doesn't solve it for >>>>>>> me. >>>>>> Hm, which qemu version (or repo branch / tag) did you use? We did a >>>>>> quick scan of the master branch code / commits, but we didn't find >>>>>> anything that might fix the issue. >>>>> I tried on qemu.git master, and also 1.7. I think it is a bug: in my >>>>> test, step >>>>> 5), B not getting any greeting message. >>>>> >>>>> But I get only one greeting message in step 10), which is a bit different >>>>> from >>>>> what you reported. >>>>> >>>>> And no difference with your patch applied. >>>>> >>>>> Cc'ing Luiz who maintains the monitor code and may have more ideas. >>>>> >>>>> Thanks, >>>>> >>>>> Fam >>>>> >>>>>>> To submit a patch, please follow instructions as described in >>>>>>> http://wiki.qemu.org/Contribute/SubmitAPatch >>>>>>> so it could be picked up by maintainers. Specifically, you need to >>>>>>> format your >>>>>>> patch email with "git format-patch" and add a "Signed-off-by:" line in >>>>>>> your >>>>>>> patch email. >>>>>> Ok. If any dev can confirm that this is a bug (and that the patch below >>>>>> is the correct way to fix it) I'll resubmit it properly. >>>>>> >>>>>> Thanks, >>>>>> Stratos >>>>>> >>>>>>> Thanks, >>>>>>> >>>>>>> Fam >>>>>>> >>>>>>>> After some investigation, we traced it down to the monitor_flush() >>>>>>>> function in monitor.c. Specifically, when a second client connects to >>>>>>>> the qmp (client B), while another one is already using it (client A), >>>>>>>> we >>>>>>>> get the following from stracing the second client (client B): >>>>>>>> >>>>>>>> connect(3, {sa_family=AF_FILE, path="foo.mon"}, 9) = 0 >>>>>>>> getsockname(3, {sa_family=AF_FILE, NULL}, [2]) = 0 >>>>>>>> select(4, [0 3], [1 3], [], NULL) = 2 (out [1 3]) >>>>>>>> select(4, [0 3], [], [], NULL >>>>>>>> >>>>>>>> So, the connect() syscall from client B succeeds, although client B >>>>>>>> connection has not yet been accepted by the qmp server (it's still in >>>>>>>> the backlog of the qmp listening socket). >>>>>>>> >>>>>>>> After killing client B and then client A, we see the following when >>>>>>>> stracing the qemu proc: >>>>>>>> >>>>>>>> 22363 accept4(6, {sa_family=AF_FILE, NULL}, [2], SOCK_CLOEXEC) = 9 >>>>>>>> 22363 fcntl(9, F_GETFL) = 0x2 (flags O_RDWR) >>>>>>>> 22363 fcntl(9, F_SETFL, O_RDWR|O_NONBLOCK) = 0 >>>>>>>> 22363 fstat(9, {st_mode=S_IFSOCK|0777, st_size=0, ...}) = 0 >>>>>>>> 22363 fcntl(9, F_GETFL) = 0x802 (flags >>>>>>>> O_RDWR|O_NONBLOCK) >>>>>>>> 22363 write(9, "{\"QMP\": {\"version\": {\"qemu\": {\"m"..., 127) = >>>>>>>> -1 EPIPE (Broken pipe) >>>>>>>> 22363 --- SIGPIPE (Broken pipe) @ 0 (0) --- >>>>>>>> >>>>>>>> The qmp server / qemu accepts the connection from client B (who has now >>>>>>>> closed the connection) and tries to write the greeting message to the >>>>>>>> socket fd. This results in write returning an error (EPIPE). >>>>>>>> >>>>>>>> The monitor_flush() function doesn't seem to handle this case (write >>>>>>>> error). Instead, it adds a watch / handler to retry the write >>>>>>>> operation. >>>>>>>> Thus, mon->outbuf is not cleaned up properly, which results in >>>>>>>> duplicate >>>>>>>> greeting messages for the next client to connect. >>>>>>>> >>>>>>>> The following seems to do the trick. >>>>>>>> >>>>>>>> diff --git a/monitor.c b/monitor.c >>>>>>>> index 845f608..5622f20 100644 >>>>>>>> --- a/monitor.c >>>>>>>> +++ b/monitor.c >>>>>>>> @@ -288,8 +288,8 @@ void monitor_flush(Monitor *mon) >>>>>>>> >>>>>>>> if (len && !mon->mux_out) { >>>>>>>> rc = qemu_chr_fe_write(mon->chr, (const uint8_t *) buf, len); >>>>>>>> - if (rc == len) { >>>>>>>> - /* all flushed */ >>>>>>>> + if ((rc < 0 && errno != EAGAIN) || (rc == len)) { >>>>>>>> + /* all flushed or error */ >>>>>>>> QDECREF(mon->outbuf); >>>>>>>> mon->outbuf = qstring_new(); >>>>>>>> return; >>>>>>>> >>>>>>>> Comments? >>>>>>>> >>>>>>>> Thanks, >>>>>>>> Stratos >>>>>>>> >>>>>>>> -- >>>>>>>> Stratos Psomadakis >>>>>>>> <pso...@grnet.gr> >>>>>>>> >>>>>> -- >>>>>> Stratos Psomadakis >>>>>> <pso...@grnet.gr> >>>>>> >>>>>> >> -- Stratos Psomadakis <pso...@grnet.gr>
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