[Qemu-devel] [PATCH] scsi-generic: let guests recognize readonly=on on passthrough devices

[Qemu-devel] [PATCH] scsi-generic: let guests recognize readonly=on on passthrough devices

[Paolo Bonzini]

Passed-through SCSI devices can be opened with the readonly=on option.
When this happens, Linux filters away write commands so that the guest
cannot overwrite the contents of the device.

However, the guest does not know that the device is read-only, and
accepts writes.  The writes only fail later when the page cache is
flushed.

This patch modifies scsi-generic to modify the MODE SENSE data and
set the read-only bit in the device-specific parameters, so that
the guest OS treats the disk as write protected.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/scsi/scsi-generic.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c
index 1b6350b..a4626f7 100644
--- a/hw/scsi/scsi-generic.c
+++ b/hw/scsi/scsi-generic.c
@@ -210,6 +210,20 @@ static void scsi_read_complete(void * opaque, int ret)
     }
     blk_set_guest_block_size(s->conf.blk, s->blocksize);
 
+    /* Patch MODE SENSE device specific parameters if the BDS is opened
+     * readonly.
+     */
+    if ((s->type == TYPE_DISK || s->type == TYPE_TAPE) &&
+        blk_is_read_only(s->conf.blk) &&
+        (r->req.cmd.buf[0] == MODE_SENSE ||
+         r->req.cmd.buf[0] == MODE_SENSE_10) &&
+        (r->req.cmd.buf[1] & 0x8) == 0) {
+        if (r->req.cmd.buf[0] == MODE_SENSE) {
+            r->buf[2] |= 0x80;
+        } else  {
+            r->buf[3] |= 0x80;
+        }
+    }
     scsi_req_data(&r->req, len);
     scsi_req_unref(&r->req);
 }
-- 
2.5.0

Re: [Qemu-devel] [PATCH] scsi-generic: let guests recognize readonly=on on passthrough devices

[Fam Zheng]

On Wed, 09/16 17:29, Paolo Bonzini wrote:
> Passed-through SCSI devices can be opened with the readonly=on option.
> When this happens, Linux filters away write commands so that the guest
> cannot overwrite the contents of the device.
> 
> However, the guest does not know that the device is read-only, and
> accepts writes.  The writes only fail later when the page cache is
> flushed.
> 
> This patch modifies scsi-generic to modify the MODE SENSE data and
> set the read-only bit in the device-specific parameters, so that
> the guest OS treats the disk as write protected.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/scsi/scsi-generic.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c
> index 1b6350b..a4626f7 100644
> --- a/hw/scsi/scsi-generic.c
> +++ b/hw/scsi/scsi-generic.c
> @@ -210,6 +210,20 @@ static void scsi_read_complete(void * opaque, int ret)
>      }
>      blk_set_guest_block_size(s->conf.blk, s->blocksize);
>  
> +    /* Patch MODE SENSE device specific parameters if the BDS is opened
> +     * readonly.
> +     */
> +    if ((s->type == TYPE_DISK || s->type == TYPE_TAPE) &&
> +        blk_is_read_only(s->conf.blk) &&
> +        (r->req.cmd.buf[0] == MODE_SENSE ||
> +         r->req.cmd.buf[0] == MODE_SENSE_10) &&
> +        (r->req.cmd.buf[1] & 0x8) == 0) {
> +        if (r->req.cmd.buf[0] == MODE_SENSE) {
> +            r->buf[2] |= 0x80;
> +        } else  {
> +            r->buf[3] |= 0x80;
> +        }
> +    }
>      scsi_req_data(&r->req, len);
>      scsi_req_unref(&r->req);
>  }
> -- 
> 2.5.0
> 
> 

Reviewed-by: Fam Zheng <famz@redhat.com>