Re: [Qemu-block] [Qemu-devel] [PATCH for-4.1] scsi-cd: Fix inserting read-only media in empty drive

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> scsi-disks decides whether it has a read-only device by looking at
> whether the BlockBackend specified as drive=... is read-only. In the
> case of an anonymous BlockBackend (with a node name specified in
> drive=...), this is the read-only flag of the attached node. In the case
> of an empty anonymous BlockBackend, it's always read-write because
> nothing prevented it from being read-write.
>
> This is a problem because scsi-cd would take write permissions on the
> anonymous BlockBackend of an empty drive created without a drive=...
> option. Using blockdev-insert-medium with a read-only node fails then
> with the error message "Block node is read-only".
>
> Fix scsi_realize() so that scsi-cd devices always take read-only
> permissions on their BlockBackend instead.
>
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1733920
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  hw/scsi/scsi-disk.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> index 8e95e3e38d..af3e622dc5 100644
> --- a/hw/scsi/scsi-disk.c
> +++ b/hw/scsi/scsi-disk.c
> @@ -2318,6 +2318,7 @@ static void 
> scsi_disk_unit_attention_reported(SCSIDevice *dev)
>  static void scsi_realize(SCSIDevice *dev, Error **errp)
>  {
>      SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, dev);
> +    bool read_only;
>  
>      if (!s->qdev.conf.blk) {
>          error_setg(errp, "drive property not set");
> @@ -2351,8 +2352,13 @@ static void scsi_realize(SCSIDevice *dev, Error **errp)
>              return;
>          }
>      }
> -    if (!blkconf_apply_backend_options(&dev->conf,
> -                                       blk_is_read_only(s->qdev.conf.blk),
> +
> +    read_only = blk_is_read_only(s->qdev.conf.blk);
> +    if (dev->type == TYPE_ROM) {
> +        read_only = true;
> +    }
> +
> +    if (!blkconf_apply_backend_options(&dev->conf, read_only,
>                                         dev->type == TYPE_DISK, errp)) {
>          return;
>      }

For what it's worth, we have code similar to the one after this patch in

* ide_dev_initfn()

* xen_block_realize()  (I guess)

We have code similar to the one before this patch in

* floppy_drive_realize()

  I figure we avoid the problem by recomputing read-only on media
  change, in fd_change_cb().  Funny: looks like a medium's
  read-only-ness lingers after unload until the next medium is loaded.

* nvme_realize()

* virtio_blk_device_realize()

* scsi_generic_realize()

* usb_msd_storage_realize()

Are these all okay?  Should they work more like floppy?  If not, what
makes floppy special?