[libvirt] [vbox-dev] Assert with libvirt + xen hvm

[Cole Robinson]

Reviving old thread, this came up again via a Fedora bug report:

https://bugzilla.redhat.com/show_bug.cgi?id=1278847
https://retrace.fedoraproject.org/faf/reports/597209/

I took a cursory look at libxl's sigchld handling... it's intense to say the
least, but there's some driver options that tweak the handling. Maybe there's
a simple fix.

jfehlig, any suggestions here? The summary is that virtualbox loads a stub
SIGCHLD handler of its own, which causes libxl to assert in
sigchld_installhandler_core

Thanks,
Cole

On 01/26/2015 08:17 AM, Klaus Espenlaub wrote:
> Hi all,
> 
> On 26.01.2015 10:52, Michal Privoznik wrote:
>> [CC'ing vbox-dev list]
>>
>> On 23.01.2015 21:42, CloudPatch Staff wrote:
>>> After some debugging we found what was causing of the assert. In our
>>> configuration we have
>>> two kernels to boot, one is a pv-linux for Xen dom0 and another just a
>>> normal linux kernel.
>>> We have libvirt built with both Xen and vbox support. When running with
>>> Xen, the libxl
>>> driver is used so it ends calling libxenlight who doesn't want any
>>> SIGCHLD handler set.
>>> Normally this is the case but, since we have vbox support in libvirt the
>>> vbox driver loads
>>> some of the vbox libs and one of them sets a SIGCHLD handler. When
>>> libxenlight checks
>>> if there is any handler for SIGCHLD it finds that one and fails.
>>>
>>> Here is the backtrace from when vbox is setting the handler:
>>>
>>> (gdb) bt
>>> #0  0x00007ffff71d8250 in sigaction () from /lib64/libpthread.so.0
>>> #1  0x00007fffedbf3716 in ?? () from /usr/lib64/virtualbox/VBoxRT.so
>>> #2  0x00007fffedbf3960 in ?? () from /usr/lib64/virtualbox/VBoxRT.so
>>> #3  0x00007fffedeea485 in VBoxGetCAPIFunctions () from
>>> /usr/lib64/virtualbox/VBoxXPCOMC.so
> [...]
> 
> That's
> https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Main/cbinding/VBoxCAPI.cpp#L718
> - initializing the runtime our code uses which abstracts platform specifics.
> 
> The rationale why the runtime installs a dummy signal handler is that if
> SIGCHLD is set to be ignored (that's the default) then POSIX compliant
> waitpid() won't work. It's mentioned in the wait(2) man page on my linux
> system, see the notes about the ECHILD error code.
> 
> It's really excessive that some other code prevents using the full
> functionality. VirtualBox checks if the SIGCHLD handler is already set. If the
> signal isn't ignored it happily continues without touching SIGCHLD. So if
> libvirt wants to establish a sane default it would also work.
> 
> To state the obvious: running an API client with crippled waitpid() can lead
> to extremely strange behavior. XPCOM creates worker threads on demand and
> expects to be able to wait on their termination.
> 
>>> After building libvirt without vbox support the assert disappeared.
>>
>> So what are you saying is that vbox interferes with libxenlight? While I
>> see why vbox library wants SIGCHLD handler, maybe they became fault
>> tolerant meanwhile. Or?
> 
> Hope the explanations help resolving the conflict. It's not that we want to
> sabotage anyone else.
> 
> Klaus
>>
>>>
>>>
>>> On Fri, Jan 23, 2015 at 5:14 AM, Michal Privoznik <mprivozn at redhat.com
>>> <mailto:mprivozn at redhat.com>> wrote:
>>>
>>>      On 22.01.2015 17:49, CloudPatch Staff wrote:
>>>      > We're hitting an assert whenever we try to create an HVM instance under
>>>      > Xen via libvirtd.
>>>      >
>>>      > System is running on Gentoo, package information as follows:
>>>      >
>>>      > app-emulation/xen-4.5.0 USE="api debug flask hvm pam pygrub python qemu
>>>      > screen"
>>>      > app-emulation/xen-tools-4.5.0 USE="api debug flask hvm pam pygrub
>>> python
>>>      > qemu screen"
>>>      > app-emulation/libvirt-1.2.11-r2:0/1.2.11 USE="caps libvirtd lvm macvtap
>>>      > nls qemu udev vepa virtualbox xen"
>>>      >
>>>      > The following commands are run in parallel:
>>>      >
>>>      > vmmachine ~ # libvirtd --listen
>>>      > 2015-01-22 16:33:13.596+0000: 2620: info : libvirt version: 1.2.11
>>>      > 2015-01-22 16:33:13.596+0000: 2620: error : udevGetDMIData:1607 :
>>> Failed
>>>      > to get udev device for syspath '/sys/devices/virtual/dmi/id' or
>>>      > '/sys/class/dmi/id'
>>>      > libvirtd: libxl_fork.c:350: sigchld_installhandler_core: Assertion
>>>      > `((void)"application must negotiate with libxl about SIGCHLD",
>>>      > !(sigchld_saved_action.sa_flags & 4) &&
>>>      > (sigchld_saved_action.__sigaction_handler.sa_handler ==
>>>      > ((__sighandler_t) 0) ||
>>>      > sigchld_saved_action.__sigaction_handler.sa_handler ==
>>> ((__sighandler_t)
>>>      > 1)))' failed.
>>>      > Aborted
>>>
>>>      Interesting. Can you attach a debugger so we can see stacktrace?
>>>
>>>      Michal
>>>
>>>
>>
>> Michal
>>
> 
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