io_uring/uring_cmd: ensure that device supports IOPOLL

It's possible for a file type to support uring commands, but not
pollable ones. Hence before issuing one of those, we should check
that it is supported and error out upfront if it isn't.

Cc: [email protected]
Fixes: 5756a3a ("io_uring: add iopoll infrastructure for io_uring_cmd")
Link: axboe/liburing#816
Reviewed-by: Kanchan Joshi <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: axboe

io_uring/uring_cmd.c

@@ -108,7 +108,7 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
 	struct file *file = req->file;
 	int ret;
 
-	if (!req->file->f_op->uring_cmd)
+	if (!file->f_op->uring_cmd)
 		return -EOPNOTSUPP;
 
 	ret = security_uring_cmd(ioucmd);
@@ -120,6 +120,8 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags)
 	if (ctx->flags & IORING_SETUP_CQE32)
 		issue_flags |= IO_URING_F_CQE32;
 	if (ctx->flags & IORING_SETUP_IOPOLL) {
+		if (!file->f_op->uring_cmd_iopoll)
+			return -EOPNOTSUPP;
 		issue_flags |= IO_URING_F_IOPOLL;
 		req->iopoll_completed = 0;
 		WRITE_ONCE(ioucmd->cookie, NULL);