Re: [Qemu-devel] [PATCH v3 6/7] qdev: Protect device-list-properties against broken devices

[Thomas Huth]

On 28/09/15 10:11, Markus Armbruster wrote:
> Thomas Huth <address@hidden> writes:
> 
>> On 25/09/15 16:17, Markus Armbruster wrote:
>>> Thomas Huth <address@hidden> writes:
>>>
>>>> On 24/09/15 20:57, Markus Armbruster wrote:
>>>>> Several devices don't survive object_unref(object_new(T)): they crash
>>>>> or hang during cleanup, or they leave dangling pointers behind.
>>>>>
>>>>> This breaks at least device-list-properties, because
>>>>> qmp_device_list_properties() needs to create a device to find its
>>>>> properties.  Broken in commit f4eb32b "qmp: show QOM properties in
>>>>> device-list-properties", v2.1.  Example reproducer:
>>>>>
>>>>>     $ qemu-system-aarch64 -nodefaults -display none -machine none
>>>>> -S -qmp stdio
>>>>>     {"QMP": {"version": {"qemu": {"micro": 50, "minor": 4,
>>>>> "major": 2}, "package": ""}, "capabilities": []}}
>>>>>     { "execute": "qmp_capabilities" }
>>>>>     {"return": {}}
>>>>>     { "execute": "device-list-properties", "arguments": {
>>>>> "typename": "pxa2xx-pcmcia" } }
>>>>>     qemu-system-aarch64: /home/armbru/work/qemu/memory.c:1307:
>>>>> memory_region_finalize: Assertion `((&mr->subregions)->tqh_first
>>>>> == ((void *)0))' failed.
>>>>>     Aborted (core dumped)
>>>>>     [Exit 134 (SIGABRT)]
>>>>>
>>>>> Unfortunately, I can't fix the problems in these devices right now.
>>>>> Instead, add DeviceClass member cannot_even_create_with_object_new_yet
>>>>> to mark them:
>> ...
>>>>>  static void pxa2xx_pcmcia_register_types(void)
>>>>> diff --git a/hw/ppc/spapr_rng.c b/hw/ppc/spapr_rng.c
>>>>> index ed43d5e..e1b115d 100644
>>>>> --- a/hw/ppc/spapr_rng.c
>>>>> +++ b/hw/ppc/spapr_rng.c
>>>>> @@ -169,6 +169,11 @@ static void spapr_rng_class_init(ObjectClass *oc, 
>>>>> void *data)
>>>>>      dc->realize = spapr_rng_realize;
>>>>>      set_bit(DEVICE_CATEGORY_MISC, dc->categories);
>>>>>      dc->props = spapr_rng_properties;
>>>>> +
>>>>> +    /*
>>>>> +     * Reason: crashes device-introspect-test for unknown reason.
>>>>> +     */
>>>>> +    dc->cannot_even_create_with_object_new_yet = true;
>>>>>  }
>>>>
>>>> Please don't do that! That breaks the help output from
>>>> "-device spapr-rng,?" which should help the user to see how to use this
>>>> device!
>>>
>>> Well, device-introspection-test makes qemu crash, with the backtrace
>>> pointing squarely to this device.  Stands to reason that device
>>> introspection could crash in normal usage, too.  Until the crash is
>>> debugged, we better disable introspection of this device.
>>>
>>> I quite agree that disabling introspection hurts users.  Just not as
>>> much as crashes :)
>>>
>>>> I tried to debug why this device breaks the test, but the test
>>>> environment is giving me a hard time ... how do you best hook a gdb into
>>>> that framework, so you can trace such problems?
>>>> Anyway, with some trial and error, I found out that it seems like the
>>>>
>>>>   object_resolve_path_type("", TYPE_SPAPR_RNG, NULL)
>>>>
>>>> in spapr_rng_instance_init() is causing the problems. Could it be that
>>>> object_resolve_path_type is not working with the test environment?
>>>
>>> I tried to figure out why this device breaks under this test, but
>>> couldn't, so I posted with the "for unknown reason" comment.
>>
>> I've debugged this now for a while (thanks for the tip with
>> MALLOC_PERTURB, by the way!) and it seems to me that the problem is in
>> the macio object than in spapr-rng - the latter is just the victim of
>> some memory corruption caused by the first one: The
>> object_resolve_path_type() crashes while trying to go through the macio
>> object.
>>
>> So could you please add the "dc->cannot_even_create_with_object_new_yet
>> = true;" to macio_class_init() instead? ... that seems to fix the crash
>> for me, too, and is likely the better place.
> 
> Hmm.
> 
> For most of the devices my patch marks, we have a pretty good idea on
> what's wrong with them.  spapr-rng is among the exceptions.  You believe
> it's actually "the macio object".  Which one?  "macio" is abstract...
> 
> You report introspecting "spapr-rng" crashes "while trying to go through
> the macio object".  I wonder how omitting introspection of macio objects
> (that's what marking them does to this test) could affect the object
> we're going through when we crash.

I have to correct myself: It's not going through the macio object, the
problem is actually the "macio[0]" property that is created during
memory_region_init() with object_property_add_child() ... the property
points to a free()d object when the crash happens.

>> Or maybe we could get this also fixed? The problem could be the
>> memory_region_init(&s->bar, NULL, "macio", 0x80000) in
>> macio_instance_init() ... is this ok here? Or does this rather have to
>> go to the realize() function instead?
> 
> Hmm, does creating and destroying a macio object leave the memory region
> behind?
> 
> Paolo, is calling memory_region_init() in an instance_init() method
> okay?

As Paolo mentioned, we likely need to pass an "owner" to
memory_region_init() or the macio memory region will get attached to
"/unattached" instead - and then leave a dangling link property behind
when the original macio object got destroyed.

By the way, there are some more spots like this in the code, e.g. in
pxa2xx_fir_instance_init() in hw/arm/pxa2xx.c ...

 Thomas