Re: [Qemu-devel] [PATCH] iscsi: Refuse to open as writable if the LUN is write protected

[Peter Lieven]

On 29.10.2014 19:28, Paolo Bonzini wrote:

On 10/29/2014 05:18 PM, Peter Lieven wrote:

Am 29.10.2014 um 14:31 schrieb Paolo Bonzini:

On 10/29/2014 02:13 PM, Fam Zheng wrote:

Before, when a write protected iSCSI target is attached as scsi-disk
with BDRV_O_RDWR, we report it as writable, while in fact all writes
will fail.

One way to improve this is to report write protect flag as true to
guest, but a even better way is to refuse using a write protected LUN to
guest.

Target write protect flag is checked with a mode sense query.

Signed-off-by: Fam Zheng <address@hidden>
---
 block/iscsi.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/block/iscsi.c b/block/iscsi.c
index 233f462..c154928 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -1339,6 +1339,36 @@ static int iscsi_open(BlockDriverState *bs, QDict *options, int flags,
     scsi_free_scsi_task(task);
     task = NULL;
 
+    /* Check the write protect flag of the LUN if we want to write */
+    if (flags & BDRV_O_RDWR) {
+        struct scsi_mode_sense *ms;
+
+        task = iscsi_modesense6_sync(iscsilun->iscsi, iscsilun->lun,
+                                      1, SCSI_MODESENSE_PC_CURRENT,
+                                      0x3F,
+                                      0, 255);
+
+        if (task == NULL) {
+            error_setg(errp, "Failed to send MODE_SENSE10 command: %s\n",

This is MODE SENSE(6).  Fixed and applied.

Paolo

+                       iscsi_get_error(iscsilun->iscsi));
+            ret = -EINVAL;
+            goto out;
+        }
+
+        if (task->status != SCSI_STATUS_GOOD) {
+            error_setg(errp, "MODE_SENSE10 failed: %s\n",
+                       iscsi_get_error(iscsi));
+            ret = -EINVAL;
+            goto out;
+        }
+        ms = scsi_datain_unmarshall(task);

scsi_datain_unmarshall may fail. You need to check for NULL here.

Thanks for the remark, I fixed this too.

Paolo

I am not 100% happy with this patch as it may break support for some targets.

It seems that MODESENSE is somewhat tricky with some targets. See function  sd_read_write_protect_flag
in drivers/scsi/sd.c of the Linux kernel.

I would not fail if this modesense fails, but just assume write enabled. Maybe drop a warning.

In the command itself:
 0x3F => SCSI_MODEPAGE_RETURN_ALL_PAGES

Can you please post the resulting patch somewhere for review.

Thanks,
Peter