Re: [Qemu-devel] [PATCH v3 2/3] qdev: add cleanup logic in device_set_realized() to avoid resource leak

[Peter Crosthwaite]

On Tue, Sep 2, 2014 at 12:13 PM, Gonglei (Arei) <address@hidden> wrote:
>> From: address@hidden
>> [mailto:address@hidden On Behalf Of Peter Crosthwaite
>> Sent: Tuesday, September 02, 2014 6:51 AM
>> >> On Mon, Sep 1, 2014 at 3:50 PM,  <address@hidden> wrote:
>> >> > From: Gonglei <address@hidden>
>> >> >
>> >> > At present, this function doesn't have partial cleanup implemented,
>> >> > which will cause resource leak in some scenarios.
>> >> >
>> >> > Example:
>> >> >
>> >> > 1. Assuming that "dc->realize(dev, &local_err)" execute successful
>> >> >    and local_err == NULL;
>> >> > 2. Executing device hotplug in hotplug_handler_plug(), but failed
>> >> >   (It is prone to occur). Then local_err != NULL;
>> >> > 3. error_propagate(errp, local_err) and return. But the resources
>> >> >  which been allocated in dc->realize() will be leaked.
>> >> >  Simple backtrace:
>> >> >   dc->realize()
>> >> >    |->device_realize
>> >> >             |->pci_qdev_init()
>> >> >                 |->do_pci_register_device()
>> >> >                 |->etc.
>> >> >
>> >> > Adding fuller cleanup logic which assure that function can
>> >> > goto appropriate error label as local_err population is
>> >> > detected as each relevant point.
>> >> >
>> >> > Signed-off-by: Gonglei <address@hidden>
>> >> > ---
>> >> >  hw/core/qdev.c | 73
>> >> ++++++++++++++++++++++++++++++++++++++++++----------------
>> >> >  1 file changed, 53 insertions(+), 20 deletions(-)
>> >> >
>> >> > diff --git a/hw/core/qdev.c b/hw/core/qdev.c
>> >> > index 4a1ac5b..2a82cc1 100644
>> >> > --- a/hw/core/qdev.c
>> >> > +++ b/hw/core/qdev.c
>> >> > @@ -834,12 +834,14 @@ static void device_set_realized(Object *obj,
>> bool
>> >> value, Error **errp)
>> >> >              dc->realize(dev, &local_err);
>> >> >          }
>> >> >
>> >> > -        if (dev->parent_bus && dev->parent_bus->hotplug_handler &&
>> >> > -            local_err == NULL) {
>> >> > +        if (local_err != NULL) {
>> >> > +            goto fail;
>> >> > +        }
>> >> > +
>> >> > +        if (dev->parent_bus && dev->parent_bus->hotplug_handler) {
>> >> >
>> hotplug_handler_plug(dev->parent_bus->hotplug_handler,
>> >> >                                   dev, &local_err);
>> >> > -        } else if (local_err == NULL &&
>> >> > -                   object_dynamic_cast(qdev_get_machine(),
>> >> TYPE_MACHINE)) {
>> >> > +        } else if (object_dynamic_cast(qdev_get_machine(),
>> >> TYPE_MACHINE)) {
>> >> >              HotplugHandler *hotplug_ctrl;
>> >> >              MachineState *machine =
>> MACHINE(qdev_get_machine());
>> >> >              MachineClass *mc = MACHINE_GET_CLASS(machine);
>> >> > @@ -852,21 +854,25 @@ static void device_set_realized(Object *obj,
>> bool
>> >> value, Error **errp)
>> >> >              }
>> >> >          }
>> >> >
>> >> > -        if (qdev_get_vmsd(dev) && local_err == NULL) {
>> >> > +        if (local_err != NULL) {
>> >> > +            goto post_realize_fail;
>> >> > +        }
>> >> > +
>> >> > +        if (qdev_get_vmsd(dev)) {
>> >> >              vmstate_register_with_alias_id(dev, -1,
>> >> qdev_get_vmsd(dev), dev,
>> >> >
>> >> dev->instance_id_alias,
>> >> >
>> >> dev->alias_required_for_version);
>> >> >          }
>> >> > -        if (local_err == NULL) {
>> >> > -            QLIST_FOREACH(bus, &dev->child_bus, sibling) {
>> >> > -                object_property_set_bool(OBJECT(bus), true,
>> "realized",
>> >> > -                                         &local_err);
>> >> > -                if (local_err != NULL) {
>> >> > -                    break;
>> >> > -                }
>> >> > +
>> >> > +        QLIST_FOREACH(bus, &dev->child_bus, sibling) {
>> >> > +            object_property_set_bool(OBJECT(bus), true, "realized",
>> >> > +                                     &local_err);
>> >> > +            if (local_err != NULL) {
>> >> > +                goto set_realized_fail;
>> >>
>> >> should this label be child_realize_fail for clarity? The object is
>> >> already realized by now so this name unqualified is a little
>> >> misleading.
>> >>
>> >> >              }
>> >> >          }
>> >> > -        if (dev->hotplugged && local_err == NULL) {
>> >> > +
>> >> > +        if (dev->hotplugged) {
>> >> >              device_reset(dev);
>> >> >          }
>> >> >          dev->pending_deleted_event = false;
>> >> > @@ -875,24 +881,51 @@ static void device_set_realized(Object *obj,
>> bool
>> >> value, Error **errp)
>> >> >              object_property_set_bool(OBJECT(bus), false, "realized",
>> >> >                                       &local_err);
>> >> >              if (local_err != NULL) {
>> >> > -                break;
>> >> > +                goto set_unrealized_fail;
>> >> >              }
>> >> >          }
>> >> > -        if (qdev_get_vmsd(dev) && local_err == NULL) {
>> >> > +        if (qdev_get_vmsd(dev)) {
>> >> >              vmstate_unregister(dev, qdev_get_vmsd(dev), dev);
>> >> >          }
>> >> > -        if (dc->unrealize && local_err == NULL) {
>> >> > +        if (dc->unrealize) {
>> >> >              dc->unrealize(dev, &local_err);
>> >> >          }
>> >> >          dev->pending_deleted_event = true;
>> >> > -    }
>> >> >
>> >> > -    if (local_err != NULL) {
>> >> > -        error_propagate(errp, local_err);
>> >> > -        return;
>> >> > +        if (local_err != NULL) {
>> >> > +            goto fail;
>> >> > +        }
>> >> >      }
>> >> >
>> >> >      dev->realized = value;
>> >> > +    return;
>> >> > +
>> >> > +set_unrealized_fail:
>> >>
>> >> child_unrealize_fail
>> >>
>> >> > +    QLIST_FOREACH(bus, &dev->child_bus, sibling) {
>> >> > +        object_property_set_bool(OBJECT(bus), true, "realized",
>> >>
>> >> "true" doesn't look right to me. It means if the recursive unrealize
>> >> botches it will re-realize all children? The idea behind my comment
>> >> last spin was to forcefully unrealize all children regardless of
>> >> early-iteration errors. I think set_realized_fail and set
>> >> unrealized_fail should be the same logic. You should keep going with
>> >> your cleanup on error rather than undo the cleanup already done.
>> >>
>> > I knew your mean, but forgot to explain in pervious email, sorry for this.
>> > When one child unrealized failed, the interface will return error for user,
>> > so user think the device existed yet in QEMU, but not hotplugged.
>> > If we keep going with cleanup, the state saved by QEMU is not same with
>> user's
>> > thought.
>>
>> This is also broken in original code right?
>>
> Yes. Any child unrealize failed, would return error immediately.
>
>> So there's an argument to say that this case would be a fatal error,
>> i.e. the hotplug interface should be &error_abort ing on the unrealize
>> path. I don't think Qdev has a chance of being able to guarantee that
>> a botched recursive unrealize has no side-effects so hotplug framework
>> has no ability for sane recovery in these cases.
>>
> OK. But actually I more agree with using NULL instead of
> using '&error_abort', we can chose to ignore this kind of error, but not 
> abort.
>

Yes, use NULL here, but my idea is that hotplug framework may use
&error_abort if unrealization fail doesn't make sense in that context.
That's out of scope of this series so don't need to worry about it.

Regards,
Peter

> What's your opinion? Thanks. :)
>
> Best regards,
> -Gonglei