[Qemu-devel] [PATCH 20/26] stream: pass new base image format to bdrv_change_backing_file

[Paolo Bonzini]

When an image is modified to point to the new backing file, the backing
file format is set to NULL, which means auto-probe.  This is wrong, in
fact it is a small security problem.

Reviewed-by: Kevin Wolf <kw...@redhat.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
 block/stream.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/block/stream.c b/block/stream.c
index 54f2b39..2b492d5 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -240,11 +240,14 @@ retry:
     }
 
     if (!block_job_is_cancelled(&s->common) && sector_num == end && ret == 0) {
-        const char *base_id = NULL;
+        const char *base_id = NULL, *base_fmt = NULL;
         if (base) {
             base_id = s->backing_file_id;
+            if (base->drv) {
+                base_fmt = base->drv->format_name;
+            }
         }
-        ret = bdrv_change_backing_file(bs, base_id, NULL);
+        ret = bdrv_change_backing_file(bs, base_id, base_fmt);
         close_unused_images(bs, base, base_id);
     }
 
-- 
1.7.9.3