Re: [PATCH 2/6] hw/display/macfb: Fix missing ERRP_GUARD() in macfb_nubus_realize()

[Zhao Liu]

On Wed, Feb 21, 2024 at 12:32:43PM +0100, Markus Armbruster wrote:
> Date: Wed, 21 Feb 2024 12:32:43 +0100
> From: Markus Armbruster <armbru@redhat.com>
> Subject: Re: [PATCH 2/6] hw/display/macfb: Fix missing ERRP_GUARD() in
>  macfb_nubus_realize()
> 
> Zhao Liu <zhao1.liu@linux.intel.com> writes:
> 
> > From: Zhao Liu <zhao1.liu@intel.com>
> >
> > As the comment in qapi/error, dereferencing @errp requires
> > ERRP_GUARD():
> >
> > * = Why, when and how to use ERRP_GUARD() =
> > *
> > * Without ERRP_GUARD(), use of the @errp parameter is restricted:
> > * - It must not be dereferenced, because it may be null.
> > * - It should not be passed to error_prepend() or
> > *   error_append_hint(), because that doesn't work with &error_fatal.
> > * ERRP_GUARD() lifts these restrictions.
> > *
> > * To use ERRP_GUARD(), add it right at the beginning of the function.
> > * @errp can then be used without worrying about the argument being
> > * NULL or &error_fatal.
> > *
> > * Using it when it's not needed is safe, but please avoid cluttering
> > * the source with useless code.
> >
> > Currently, since macfb_nubus_realize() - as a DeviceClass.realize()
> > method - doesn't get the NULL errp parameter, it doesn't trigger the
> > dereference issue.
> >
> > To follow the requirement of errp, add missing ERRP_GUARD() in
> > macfb_nubus_realize().
> >
> > Suggested-by: Markus Armbruster <armbru@redhat.com>
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > ---
> > Suggested by credit:
> >  Markus: Referred his explanation about ERRP_GUARD().
> > ---
> >  hw/display/macfb.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/hw/display/macfb.c b/hw/display/macfb.c
> > index 418e99c8e18e..1ace341a0ff4 100644
> > --- a/hw/display/macfb.c
> > +++ b/hw/display/macfb.c
> > @@ -714,6 +714,7 @@ static void macfb_nubus_set_irq(void *opaque, int n, 
> > int level)
> >  
> >  static void macfb_nubus_realize(DeviceState *dev, Error **errp)
> >  {
> > +    ERRP_GUARD();
> >      NubusDevice *nd = NUBUS_DEVICE(dev);
> >      MacfbNubusState *s = NUBUS_MACFB(dev);
> >      MacfbNubusDeviceClass *ndc = NUBUS_MACFB_GET_CLASS(dev);
> 
> The dereference is
> 
>        ndc->parent_realize(dev, errp);
>        if (*errp) {
>            return;
>        }
> 
> We check *errp, because neither the callback returns void.
> 
> Reviewed-by: Markus Armbruster <armbru@redhat.com>

Thanks! Will add more description in commit message as you suggested.

Regards,
Zhao