Re: [PATCH v2] hw/virtio/vdpa-dev: Check returned value instead of dereferencing @errp

[Michael S. Tsirkin]

On Wed, Jul 17, 2024 at 12:26:15AM +0800, Zhao Liu 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: Jason Wang <jasowang@redhat.com>
> Acked-by: Eugenio Pérez <eperezma@redhat.com>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> ---
> v2:
>  * Added a/b from Eugenio.
>  * Deleted unnecessary ERRP_GUARD(). (Eugenio)
> ---
>  hw/virtio/vdpa-dev.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/virtio/vdpa-dev.c b/hw/virtio/vdpa-dev.c
> index 64b96b226c39..8a1e16fce3de 100644
> --- a/hw/virtio/vdpa-dev.c
> +++ b/hw/virtio/vdpa-dev.c
> @@ -63,19 +63,19 @@ static void vhost_vdpa_device_realize(DeviceState *dev, 
> Error **errp)
>      }
>  
>      v->vhostfd = qemu_open(v->vhostdev, O_RDWR, errp);
> -    if (*errp) {
> +    if (v->vhostfd < 0) {
>          return;
>      }
>  
>      v->vdev_id = vhost_vdpa_device_get_u32(v->vhostfd,
>                                             VHOST_VDPA_GET_DEVICE_ID, errp);
> -    if (*errp) {
> +    if (v->vdev_id < 0) {
>          goto out;
>      }

vdev_id is unsigned, no idea how is this supposed to work.

>  
>      max_queue_size = vhost_vdpa_device_get_u32(v->vhostfd,
>                                                 VHOST_VDPA_GET_VRING_NUM, 
> errp);
> -    if (*errp) {
> +    if (max_queue_size < 0) {
>          goto out;
>      }
>  
max_queue_size is unsigned, too.

> @@ -89,7 +89,7 @@ static void vhost_vdpa_device_realize(DeviceState *dev, 
> Error **errp)
>  
>      v->num_queues = vhost_vdpa_device_get_u32(v->vhostfd,
>                                                VHOST_VDPA_GET_VQS_COUNT, 
> errp);
> -    if (*errp) {
> +    if (v->num_queues < 0) {
>          goto out;
>      }
>  

num_queues is unsigned, too.

> @@ -127,7 +127,7 @@ static void vhost_vdpa_device_realize(DeviceState *dev, 
> Error **errp)
>      v->config_size = vhost_vdpa_device_get_u32(v->vhostfd,
>                                                 VHOST_VDPA_GET_CONFIG_SIZE,
>                                                 errp);
> -    if (*errp) {
> +    if (v->config_size < 0) {
>          goto vhost_cleanup;
>      }
>  
> -- 
> 2.34.1