Re: [PATCH 4/6] hw/misc/xlnx-versal-trng: Fix missing ERRP_GUARD() in trng_prop_fault_event_set()

[Zhao Liu]

On Wed, Feb 21, 2024 at 12:47:33PM +0100, Markus Armbruster wrote:
> Date: Wed, 21 Feb 2024 12:47:33 +0100
> From: Markus Armbruster <armbru@redhat.com>
> Subject: Re: [PATCH 4/6] hw/misc/xlnx-versal-trng: Fix missing ERRP_GUARD()
>  in trng_prop_fault_event_set()
> 
> 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 trng_prop_fault_event_set() doesn't get the NULL errp
> > parameter as a "set" method of object property, it doesn't trigger the
> > dereference issue.
> >
> > To follow the requirement of errp, add missing ERRP_GUARD() in
> > trng_prop_fault_event_set().
> >
> > 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/misc/xlnx-versal-trng.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/hw/misc/xlnx-versal-trng.c b/hw/misc/xlnx-versal-trng.c
> > index b8111b8b6626..3579348a9d17 100644
> > --- a/hw/misc/xlnx-versal-trng.c
> > +++ b/hw/misc/xlnx-versal-trng.c
> > @@ -33,6 +33,7 @@
> >  #include "qemu/error-report.h"
> >  #include "qemu/guest-random.h"
> >  #include "qemu/timer.h"
> > +#include "qapi/error.h"
> >  #include "qapi/visitor.h"
> >  #include "migration/vmstate.h"
> >  #include "hw/qdev-properties.h"
> > @@ -641,6 +642,7 @@ static void trng_prop_fault_event_set(Object *obj, 
> > Visitor *v,
> >                                        const char *name, void *opaque,
> >                                        Error **errp)
> >  {
> > +    ERRP_GUARD();
> >      Property *prop = opaque;
> >      uint32_t *events = object_field_prop_ptr(obj, prop);
> 
>        visit_type_uint32(v, name, events, errp);
>        if (*errp) {
>            return;
>        }
> 
> Please do this instead:
> 
> diff --git a/hw/misc/xlnx-versal-trng.c b/hw/misc/xlnx-versal-trng.c
> index b8111b8b66..6495188dc7 100644
> --- a/hw/misc/xlnx-versal-trng.c
> +++ b/hw/misc/xlnx-versal-trng.c
> @@ -644,8 +644,7 @@ static void trng_prop_fault_event_set(Object *obj, 
> Visitor *v,
>      Property *prop = opaque;
>      uint32_t *events = object_field_prop_ptr(obj, prop);
>  
> -    visit_type_uint32(v, name, events, errp);
> -    if (*errp) {
> +    if (!visit_type_uint32(v, name, events, errp)) {
>          return;
>      }
>  

Thanks! I didn't think of that. Will do this.

Regards,
Zhao