Re: [PATCH] vhost-user: fix shared object return values

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Oct 16, 2024 at 11:06:06AM +0200, Albert Esteve wrote:
> VHOST_USER_BACKEND_SHARED_OBJECT_ADD and
> VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE state
> in the spec that they return 0 for successful
> operations, non-zero otherwise. However,
> implementation relies on the return types
> of the virtio-dmabuf library, with opposite
> semantics (true if everything is correct,
> false otherwise). Therefore, current implementaion
> violates the specification.
> 
> Revert the logic so that the implementation
> of the vhost-user handling methods matches
> the specification.
> 
> Fixes: 043e127a126bb3ceb5fc753deee27d261fd0c5ce
> Fixes: 160947666276c5b7f6bca4d746bcac2966635d79
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  hw/virtio/vhost-user.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 00561daa06..90917352a4 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1607,7 +1607,7 @@ vhost_user_backend_handle_shared_object_add(struct vhost_dev *dev,
>      QemuUUID uuid;
>  
>      memcpy(uuid.data, object->uuid, sizeof(object->uuid));
> -    return virtio_add_vhost_device(&uuid, dev);
> +    return !virtio_add_vhost_device(&uuid, dev);
>  }

This virtio_add_vhost_device() method returns a bool, but this
vhost_user_backend_handle_shared_object_add() method returns
an int, but fills that int with an inverted bool value. The
caller then assigns the return value to an int, but then
interprets the int as a bool, and assigns that bool result
to an u64.

This call chain is madness :-(

Change vhost_user_backend_handle_shared_object_add to return
a bool to reduce the madness IMHO.

>  
>  static int
> @@ -1623,16 +1623,16 @@ vhost_user_backend_handle_shared_object_remove(struct vhost_dev *dev,
>          struct vhost_dev *owner = virtio_lookup_vhost_device(&uuid);
>          if (dev != owner) {
>              /* Not allowed to remove non-owned entries */
> -            return 0;
> +            return -EPERM;
>          }
>          break;
>      }
>      default:
>          /* Not allowed to remove non-owned entries */
> -        return 0;
> +        return -EPERM;
>      }
>  
> -    return virtio_remove_resource(&uuid);
> +    return !virtio_remove_resource(&uuid);
>  }

These return values are inconsistent.

In some places you're returning a negative errno, but in this
last place you're returning true or false, by calling
virtio_remove_resource which is a 'bool' method & inverting it.

On top of this inconsistency, it has all the same madness mentioneed
above.

With regards,
Daniel
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