block: add helper function to determine if a BDS is in a chain

This is a small helper function, to determine if 'base' is in the
chain of BlockDriverState 'top'.  It returns true if it is in the chain,
and false otherwise.

If either argument is NULL, it will also return false.

Reviewed-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

2 files changed, 12 insertions, 0 deletions

diff --git a/block.c b/block.c
index c111c297f7..f45e63c88b 100644
--- a/block.c
+++ b/block.c
@@ -3774,6 +3774,17 @@ BlockDriverState *bdrv_lookup_bs(const char *device,
     return NULL;
 }
 
+/* If 'base' is in the same chain as 'top', return true. Otherwise,
+ * return false.  If either argument is NULL, return false. */
+bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
+{
+    while (top && top != base) {
+        top = top->backing_hd;
+    }
+
+    return top != NULL;
+}
+
 BlockDriverState *bdrv_next(BlockDriverState *bs)
 {
     if (!bs) {
diff --git a/include/block/block.h b/include/block/block.h
index 7e92f549fb..29c9e50ddb 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -403,6 +403,7 @@ BlockDeviceInfoList *bdrv_named_nodes_list(void);
 BlockDriverState *bdrv_lookup_bs(const char *device,
                                  const char *node_name,
                                  Error **errp);
+bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base);
 BlockDriverState *bdrv_next(BlockDriverState *bs);
 void bdrv_iterate(void (*it)(void *opaque, BlockDriverState *bs),
                   void *opaque);