block: update in-memory backing file and format

These are needed to print "info block" output correctly.  QCOW2 does this
because it needs it to write the header, but QED does not, and common code
is the right place to do it.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/block.c b/block.c
index 068e41d9d9..dbd60e3f3e 100644
--- a/block.c
+++ b/block.c
@@ -1459,6 +1459,7 @@ int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt)
 {
     BlockDriver *drv = bs->drv;
+    int ret;
 
     /* Backing file format doesn't make sense without a backing file */
     if (backing_fmt && !backing_file) {
@@ -1466,10 +1467,16 @@ int bdrv_change_backing_file(BlockDriverState *bs,
     }
 
     if (drv->bdrv_change_backing_file != NULL) {
-        return drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
+        ret = drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
     } else {
-        return -ENOTSUP;
+        ret = -ENOTSUP;
+    }
+
+    if (ret == 0) {
+        pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
+        pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");
     }
+    return ret;
 }
 
 static int bdrv_check_byte_request(BlockDriverState *bs, int64_t offset,