[PULL,02/12] block: Fail gracefully when blockdev-snapshot creates loops

diff --git a/block.c b/block.c
index 45f653a88b..580cb77a70 100644
--- a/block.c
+++ b/block.c
@@ -84,6 +84,9 @@  static BlockDriverState *bdrv_open_inherit(const char *filename,
                                            BdrvChildRole child_role,
                                            Error **errp);
 
+static bool bdrv_recurse_has_child(BlockDriverState *bs,
+                                   BlockDriverState *child);
+
 static void bdrv_replace_child_noperm(BdrvChild *child,
                                       BlockDriverState *new_bs);
 static void bdrv_remove_file_or_backing_child(BlockDriverState *bs,
@@ -2673,6 +2676,7 @@  static void bdrv_replace_child_noperm(BdrvChild *child,
     int drain_saldo;
 
     assert(!child->frozen);
+    assert(old_bs != new_bs);
 
     if (old_bs && new_bs) {
         assert(bdrv_get_aio_context(old_bs) == bdrv_get_aio_context(new_bs));
@@ -2892,6 +2896,12 @@  static int bdrv_attach_child_noperm(BlockDriverState *parent_bs,
 
     assert(parent_bs->drv);
 
+    if (bdrv_recurse_has_child(child_bs, parent_bs)) {
+        error_setg(errp, "Making '%s' a %s child of '%s' would create a cycle",
+                   child_bs->node_name, child_name, parent_bs->node_name);
+        return -EINVAL;
+    }
+
     bdrv_get_cumulative_perm(parent_bs, &perm, &shared_perm);
     bdrv_child_perm(parent_bs, child_bs, NULL, child_role, NULL,
                     perm, shared_perm, &perm, &shared_perm);
diff --git a/tests/qemu-iotests/085 b/tests/qemu-iotests/085
index d557522943..de74262a26 100755
--- a/tests/qemu-iotests/085
+++ b/tests/qemu-iotests/085
@@ -103,11 +103,18 @@  do_blockdev_add()
 }
 
 # ${1}: unique identifier for the snapshot filename
-add_snapshot_image()
+create_snapshot_image()
 {
     base_image="${TEST_DIR}/$((${1}-1))-${snapshot_virt0}"
     snapshot_file="${TEST_DIR}/${1}-${snapshot_virt0}"
     TEST_IMG=$snapshot_file _make_test_img -u -b "${base_image}" -F $IMGFMT "$size"
+}
+
+# ${1}: unique identifier for the snapshot filename
+add_snapshot_image()
+{
+    snapshot_file="${TEST_DIR}/${1}-${snapshot_virt0}"
+    create_snapshot_image "$1"
     do_blockdev_add "$1" "'backing': null, " "${snapshot_file}"
 }
 
@@ -230,6 +237,28 @@  _make_test_img -b "${TEST_IMG}.base" -F $IMGFMT "$size"
 do_blockdev_add ${SNAPSHOTS} "" "${TEST_IMG}"
 blockdev_snapshot ${SNAPSHOTS} error
 
+echo
+echo === Invalid command - creating loops ===
+echo
+
+SNAPSHOTS=$((${SNAPSHOTS}+1))
+add_snapshot_image ${SNAPSHOTS}
+
+_send_qemu_cmd $h "{ 'execute': 'blockdev-snapshot',
+                     'arguments': { 'node':'snap_${SNAPSHOTS}',
+                                    'overlay':'snap_${SNAPSHOTS}' }
+                   }" "error"
+
+SNAPSHOTS=$((${SNAPSHOTS}+1))
+create_snapshot_image ${SNAPSHOTS}
+do_blockdev_add ${SNAPSHOTS} "'backing': 'snap_$((${SNAPSHOTS}-1))', " \
+    "${TEST_DIR}/${SNAPSHOTS}-${snapshot_virt0}"
+
+_send_qemu_cmd $h "{ 'execute': 'blockdev-snapshot',
+                     'arguments': { 'node':'snap_${SNAPSHOTS}',
+                                    'overlay':'snap_$((${SNAPSHOTS}-1))' }
+                   }" "error"
+
 echo
 echo === Invalid command - The node does not exist ===
 echo
diff --git a/tests/qemu-iotests/085.out b/tests/qemu-iotests/085.out
index 1d4c565b6d..b543b992ff 100644
--- a/tests/qemu-iotests/085.out
+++ b/tests/qemu-iotests/085.out
@@ -217,15 +217,42 @@  Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/
                                      'overlay':'snap_13' } }
 {"error": {"class": "GenericError", "desc": "The overlay already has a backing image"}}
 
+=== Invalid command - creating loops ===
+
+Formatting 'TEST_DIR/14-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/13-snapshot-v0.IMGFMT backing_fmt=IMGFMT
+{ 'execute': 'blockdev-add', 'arguments':
+           { 'driver': 'IMGFMT', 'node-name': 'snap_14', 'backing': null,
+             'file':
+             { 'driver': 'file', 'filename': 'TEST_DIR/14-snapshot-v0.IMGFMT',
+               'node-name': 'file_14' } } }
+{"return": {}}
+{ 'execute': 'blockdev-snapshot',
+                     'arguments': { 'node':'snap_14',
+                                    'overlay':'snap_14' }
+                   }
+{"error": {"class": "GenericError", "desc": "Making 'snap_14' a backing child of 'snap_14' would create a cycle"}}
+Formatting 'TEST_DIR/15-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/14-snapshot-v0.IMGFMT backing_fmt=IMGFMT
+{ 'execute': 'blockdev-add', 'arguments':
+           { 'driver': 'IMGFMT', 'node-name': 'snap_15', 'backing': 'snap_14',
+             'file':
+             { 'driver': 'file', 'filename': 'TEST_DIR/15-snapshot-v0.IMGFMT',
+               'node-name': 'file_15' } } }
+{"return": {}}
+{ 'execute': 'blockdev-snapshot',
+                     'arguments': { 'node':'snap_15',
+                                    'overlay':'snap_14' }
+                   }
+{"error": {"class": "GenericError", "desc": "Making 'snap_15' a backing child of 'snap_14' would create a cycle"}}
+
 === Invalid command - The node does not exist ===
 
 { 'execute': 'blockdev-snapshot',
                       'arguments': { 'node': 'virtio0',
-                                     'overlay':'snap_14' } }
-{"error": {"class": "GenericError", "desc": "Cannot find device='snap_14' nor node-name='snap_14'"}}
+                                     'overlay':'snap_16' } }
+{"error": {"class": "GenericError", "desc": "Cannot find device='snap_16' nor node-name='snap_16'"}}
 { 'execute': 'blockdev-snapshot',
                      'arguments': { 'node':'nodevice',
-                                    'overlay':'snap_13' }
+                                    'overlay':'snap_15' }
                    }
 {"error": {"class": "GenericError", "desc": "Cannot find device='nodevice' nor node-name='nodevice'"}}
 *** done