RDMA/uverbs: Support external FD uobjects

Add support for uobjects that wrap externally allocated file
descriptors (FDs).

In this mode, the FD number still follows the standard uverbs allocation
flow, but the file pointer is allocated externally and has its own fops
and private data.

As a result, alloc_begin_fd_uobject() must handle cases where
fd_type->fops is NULL, and both alloc_commit_fd_uobject() and
alloc_abort_fd_uobject() must account for whether filp->private_data
exists, since it is populated outside the standard uverbs flow.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/20260201-dmabuf-export-v3-1-da238b614fe3@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>

Author: yishaih

drivers/infiniband/core/rdma_core.c

@@ -465,7 +465,7 @@ alloc_begin_fd_uobject(const struct uverbs_api_object *obj,
 
 	fd_type =
 		container_of(obj->type_attrs, struct uverbs_obj_fd_type, type);
-	if (WARN_ON(fd_type->fops->release != &uverbs_uobject_fd_release &&
+	if (WARN_ON(fd_type->fops && fd_type->fops->release != &uverbs_uobject_fd_release &&
 		    fd_type->fops->release != &uverbs_async_event_release)) {
 		ret = ERR_PTR(-EINVAL);
 		goto err_fd;
@@ -477,14 +477,16 @@ alloc_begin_fd_uobject(const struct uverbs_api_object *obj,
 		goto err_fd;
 	}
 
-	/* Note that uverbs_uobject_fd_release() is called during abort */
-	filp = anon_inode_getfile(fd_type->name, fd_type->fops, NULL,
-				  fd_type->flags);
-	if (IS_ERR(filp)) {
-		ret = ERR_CAST(filp);
-		goto err_getfile;
+	if (fd_type->fops) {
+		/* Note that uverbs_uobject_fd_release() is called during abort */
+		filp = anon_inode_getfile(fd_type->name, fd_type->fops, NULL,
+					  fd_type->flags);
+		if (IS_ERR(filp)) {
+			ret = ERR_CAST(filp);
+			goto err_getfile;
+		}
+		uobj->object = filp;
 	}
-	uobj->object = filp;
 
 	uobj->id = new_fd;
 	return uobj;
@@ -561,7 +563,9 @@ static void alloc_abort_fd_uobject(struct ib_uobject *uobj)
 {
 	struct file *filp = uobj->object;
 
-	fput(filp);
+	if (filp)
+		fput(filp);
+
 	put_unused_fd(uobj->id);
 }
 
@@ -628,11 +632,14 @@ static void alloc_commit_fd_uobject(struct ib_uobject *uobj)
 	/* This shouldn't be used anymore. Use the file object instead */
 	uobj->id = 0;
 
-	/*
-	 * NOTE: Once we install the file we loose ownership of our kref on
-	 * uobj. It will be put by uverbs_uobject_fd_release()
-	 */
-	filp->private_data = uobj;
+	if (!filp->private_data) {
+		/*
+		 * NOTE: Once we install the file we loose ownership of our kref on
+		 * uobj. It will be put by uverbs_uobject_fd_release()
+		 */
+		filp->private_data = uobj;
+	}
+
 	fd_install(fd, filp);
 }