Re: [PATCH] hw/virtio/vdpa-dev: Check returned value instead of dereferencing @errp

[Zhao Liu]

On Mon, Jul 15, 2024 at 11:01:08PM +0200, Eugenio Perez Martin wrote:
> Date: Mon, 15 Jul 2024 23:01:08 +0200
> From: Eugenio Perez Martin <eperezma@redhat.com>
> Subject: Re: [PATCH] hw/virtio/vdpa-dev: Check returned value instead of
>  dereferencing @errp
> 
> On Mon, Jul 15, 2024 at 11:45 AM Zhao Liu <zhao1.liu@intel.com> wrote:
> >
> > 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.
> > ...
> > * 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.
> >
> > Though vhost_vdpa_device_realize() is called at DeviceClass.realize()
> > context and won't get NULL @errp, it's still better to follow the
> > requirement to add the ERRP_GUARD().
> >
> > But qemu_open() and vhost_vdpa_device_get_u32()'s return values can
> > distinguish between successful and unsuccessful calls, so check the
> > return values directly without dereferencing @errp, which eliminates
> > the need of ERRP_GUARD().
> >
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: "Eugenio Pérez" <eperezma@redhat.com>
> > Cc: Jason Wang <jasowang@redhat.com>
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > ---
> >  hw/virtio/vdpa-dev.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/hw/virtio/vdpa-dev.c b/hw/virtio/vdpa-dev.c
> > index 64b96b226c39..7b439efdc1d3 100644
> > --- a/hw/virtio/vdpa-dev.c
> > +++ b/hw/virtio/vdpa-dev.c
> > @@ -50,6 +50,7 @@ vhost_vdpa_device_get_u32(int fd, unsigned long int cmd, 
> > Error **errp)
> >
> >  static void vhost_vdpa_device_realize(DeviceState *dev, Error **errp)
> >  {
> > +    ERRP_GUARD();
> 
> Good catch, thank you! But removing the err dereferencing eliminates
> the need for ERRP_GUARD(), doesn't it?
>

Thanks Eugenio! You're right and I forgot to delete it. I'll post a new
version.