[Qemu-devel] [PATCH] mirror: prevent 'top' mode mirroring when no backing file specified on the destination

[sochin jiang <sochin.jiang@huawei.com>]

 Mirroring using 'top' mode without backing file specified on the target can be success,
 but end with a disaster.

 For example:
   Migration can be success in this situation while the virtual machine on the destination
 is no longer usable because of backing lost.

 Remind the user earlier and return error in case of misoperation.

Signed-off-by: sochin jiang <sochin.jiang@huawei.com>
---
 block/mirror.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 301ba92..3476696 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1038,6 +1038,12 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
         error_setg(errp, "Sync mode 'incremental' not supported");
         return;
     }
+    if (mode == MIRROR_SYNC_MODE_TOP && !backing_bs(target))
+    {
+        error_setg(errp, "Target Backing required using Sync mode 'top'");
+        return;
+    }
+
     is_none_mode = mode == MIRROR_SYNC_MODE_NONE;
     base = mode == MIRROR_SYNC_MODE_TOP ? backing_bs(bs) : NULL;
     mirror_start_job(job_id, bs, BLOCK_JOB_DEFAULT, target, replaces,
-- 
1.8.3.1