[Qemu-devel] [PATCH v2] iscsi: Refuse to open as writable if the LUN is write protected

[Fam Zheng]

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>
---
v2: Improve error message.
    Fall back to a warning if mode sense failed.
    Check unmarshal return value.
---
 block/iscsi.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/block/iscsi.c b/block/iscsi.c
index 233f462..dcacbca 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -1219,6 +1219,44 @@ static void iscsi_attach_aio_context(BlockDriverState 
*bs,
               qemu_clock_get_ms(QEMU_CLOCK_REALTIME) + NOP_INTERVAL);
 }
 
+static bool iscsi_is_write_protected(IscsiLun *iscsilun)
+{
+    struct scsi_task *task;
+    struct scsi_mode_sense *ms = NULL;
+
+    task = iscsi_modesense6_sync(iscsilun->iscsi, iscsilun->lun,
+            1, SCSI_MODESENSE_PC_CURRENT,
+            0x3F,
+            0, 255);
+
+    if (task == NULL) {
+        error_report("Failed to send MODE_SENSE6 command: %s",
+                     iscsi_get_error(iscsilun->iscsi));
+        goto out;
+    }
+
+    if (task->status != SCSI_STATUS_GOOD) {
+        error_report("MODE_SENSE6 failed: %s",
+                     iscsi_get_error(iscsilun->iscsi));
+        goto out;
+    }
+    ms = scsi_datain_unmarshall(task);
+    if (!ms) {
+        error_report("MODE_SENSE6 failed: %s",
+                     iscsi_get_error(iscsilun->iscsi));
+        goto out;
+    }
+out:
+    if (task) {
+        scsi_free_scsi_task(task);
+    }
+    if (!ms) {
+        error_report("Assuming write enabled");
+        return false;
+    }
+    return ms->device_specific_parameter & 0x80;
+}
+
 /*
  * We support iscsi url's on the form
  * iscsi://[<username>%<password>@]<host>[:<port>]/<targetname>/<lun>
@@ -1339,6 +1377,14 @@ 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)
+            && iscsi_is_write_protected(iscsilun)) {
+        error_setg(errp, "Cannot open a write protected LUN as read-write");
+        ret = -EPERM;
+        goto out;
+    }
+
     iscsi_readcapacity_sync(iscsilun, &local_err);
     if (local_err != NULL) {
         error_propagate(errp, local_err);
-- 
1.9.3