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

[Thomas Huth <thuth@redhat.com>]

On 25/09/15 16:17, Markus Armbruster wrote:
> Thomas Huth <thuth@redhat.com> 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.

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?

 Thomas