From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Backing may be zero after failed bdrv_attach_child in
bdrv_set_backing_hd, which leads to SIGSEGV.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20170928120300.58164-1-vsementsov@virtuozzo.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/mirror.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 459b80f8f3..3b6f0c5772 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1058,6 +1058,11 @@ static int coroutine_fn bdrv_mirror_top_pdiscard(BlockDriverState *bs,
 
 static void bdrv_mirror_top_refresh_filename(BlockDriverState *bs, QDict *opts)
 {
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_attach_child in
+         * bdrv_set_backing_hd */
+        return;
+    }
     bdrv_refresh_filename(bs->backing->bs);
     pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
             bs->backing->bs->filename);
-- 
2.13.6