[PATCH] vdpa: reject F_ENABLE_AFTER_DRIVER_OK if backend does not support it

["Eugenio Pérez" <eperezma@redhat.com>]

With the current code it is accepted as long as userland send it.

Although userland should not set a feature flag that has not been
offered to it with VHOST_GET_BACKEND_FEATURES, the current code will not
complain for it.

Since there is no specific reason for any parent to reject that backend
feature bit when it has been proposed, let's control it at vdpa frontend
level. Future patches may move this control to the parent driver.

Fixes: 967800d2d52e ("vdpa: accept VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK backend feature")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>

---
Sent with Fixes: tag pointing to git.kernel.org/pub/scm/linux/kernel/git/mst
commit. Please let me know if I should send a v3 of [1] instead.

[1] https://lore.kernel.org/lkml/20230609121244-mutt-send-email-mst@kernel.org/T/
---
 drivers/vhost/vdpa.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index e1abf29fed5b..a7e554352351 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -681,18 +681,21 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 {
 	struct vhost_vdpa *v = filep->private_data;
 	struct vhost_dev *d = &v->vdev;
+	const struct vdpa_config_ops *ops = v->vdpa->config;
 	void __user *argp = (void __user *)arg;
 	u64 __user *featurep = argp;
-	u64 features;
+	u64 features, parent_features = 0;
 	long r = 0;
 
 	if (cmd == VHOST_SET_BACKEND_FEATURES) {
 		if (copy_from_user(&features, featurep, sizeof(features)))
 			return -EFAULT;
+		if (ops->get_backend_features)
+			parent_features = ops->get_backend_features(v->vdpa);
 		if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
 				 BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
 				 BIT_ULL(VHOST_BACKEND_F_RESUME) |
-				 BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK)))
+				 parent_features))
 			return -EOPNOTSUPP;
 		if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
 		     !vhost_vdpa_can_suspend(v))
-- 
2.39.3