block/blkio: do not use open flags in qemu_open()

qemu_open() in blkio_virtio_blk_common_open() is used to open the
character device (e.g. /dev/vhost-vdpa-0 or /dev/vfio/vfio) or in
the future eventually the unix socket.

In all these cases we cannot open the path in read-only mode,
when the `read-only` option of blockdev is on, because the exchange
of IOCTL commands for example will fail.

In order to open the device read-only, we have to use the `read-only`
property of the libblkio driver as we already do in blkio_file_open().

Fixes: cad2ccc ("block/blkio: use qemu_open() to support fd passing for virtio-blk")
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2225439
Reported-by: Qing Wang <qinwang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20230726074807.14041-1-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/blkio.c

@@ -686,15 +686,18 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
      * layer through the "/dev/fdset/N" special path.
      */
     if (fd_supported) {
-        int open_flags;
-
-        if (flags & BDRV_O_RDWR) {
-            open_flags = O_RDWR;
-        } else {
-            open_flags = O_RDONLY;
-        }
-
-        fd = qemu_open(path, open_flags, errp);
+        /*
+         * `path` can contain the path of a character device
+         * (e.g. /dev/vhost-vdpa-0 or /dev/vfio/vfio) or a unix socket.
+         *
+         * So, we should always open it with O_RDWR flag, also if BDRV_O_RDWR
+         * is not set in the open flags, because the exchange of IOCTL commands
+         * for example will fail.
+         *
+         * In order to open the device read-only, we are using the `read-only`
+         * property of the libblkio driver in blkio_file_open().
+         */
+        fd = qemu_open(path, O_RDWR, errp);
         if (fd < 0) {
             return -EINVAL;
         }