[PATCH v2] block/stream: Drain subtree around graph change

[Hanna Reitz]

When the stream block job cuts out the nodes between top and base in
stream_prepare(), it does not drain the subtree manually; it fetches the
base node, and tries to insert it as the top node's backing node with
bdrv_set_backing_hd().  bdrv_set_backing_hd() however will drain, and so
the actual base node might change (because the base node is actually not
part of the stream job) before the old base node passed to
bdrv_set_backing_hd() is installed.

This has two implications:

First, the stream job does not keep a strong reference to the base node.
Therefore, if it is deleted in bdrv_set_backing_hd()'s drain (e.g.
because some other block job is drained to finish), we will get a
use-after-free.  We should keep a strong reference to that node.

Second, even with such a strong reference, the problem remains that the
base node might change before bdrv_set_backing_hd() actually runs and as
a result the wrong base node is installed.

Both effects can be seen in 030's TestParallelOps.test_overlapping_5()
case, which has five nodes, and simultaneously streams from the middle
node to the top node, and commits the middle node down to the base node.
As it is, this will sometimes crash, namely when we encounter the
above-described use-after-free.

Taking a strong reference to the base node, we no longer get a crash,
but the resuling block graph is less than ideal: The expected result is
obviously that all middle nodes are cut out and the base node is the
immediate backing child of the top node.  However, if stream_prepare()
takes a strong reference to its base node (the middle node), and then
the commit job finishes in bdrv_set_backing_hd(), supposedly dropping
that middle node, the stream job will just reinstall it again.

Therefore, we need to keep the whole subtree drained in
stream_prepare(), so that the graph modification it performs is
effectively atomic, i.e. that the base node it fetches is still the base
node when bdrv_set_backing_hd() sets it as the top node's backing node.

Verify this by asserting in said 030's test case that the base node is
always the top node's immediate backing child when both jobs are done.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
v2:
- Oops, the base can be NULL.  Would have noticed if I had ran all test
  cases from 030, and not just test_overlapping_5()...
  That means that keeping a strong reference to the base node must be
  conditional, based on whether there even is a base node or not.
  (I mean, technically we do not even need to keep a strong reference to
  that node, given that we are in a drained section, but I believe it is
  better style to do it anyway.)
---
 block/stream.c         | 15 ++++++++++++++-
 tests/qemu-iotests/030 |  5 +++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/block/stream.c b/block/stream.c
index 3acb59fe6a..694709bd25 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -64,7 +64,13 @@ static int stream_prepare(Job *job)
     bdrv_cor_filter_drop(s->cor_filter_bs);
     s->cor_filter_bs = NULL;
 
+    bdrv_subtree_drained_begin(s->above_base);
+
     base = bdrv_filter_or_cow_bs(s->above_base);
+    if (base) {
+        bdrv_ref(base);
+    }
+
     unfiltered_base = bdrv_skip_filters(base);
 
     if (bdrv_cow_child(unfiltered_bs)) {
@@ -75,14 +81,21 @@ static int stream_prepare(Job *job)
                 base_fmt = unfiltered_base->drv->format_name;
             }
         }
+
         bdrv_set_backing_hd(unfiltered_bs, base, &local_err);
         ret = bdrv_change_backing_file(unfiltered_bs, base_id, base_fmt, 
false);
         if (local_err) {
             error_report_err(local_err);
-            return -EPERM;
+            ret = -EPERM;
+            goto out;
         }
     }
 
+out:
+    if (base) {
+        bdrv_unref(base);
+    }
+    bdrv_subtree_drained_end(s->above_base);
     return ret;
 }
 
diff --git a/tests/qemu-iotests/030 b/tests/qemu-iotests/030
index 567bf1da67..14112835ed 100755
--- a/tests/qemu-iotests/030
+++ b/tests/qemu-iotests/030
@@ -436,6 +436,11 @@ class TestParallelOps(iotests.QMPTestCase):
         self.vm.run_job(job='node4', auto_dismiss=True)
         self.assert_no_active_block_jobs()
 
+        # Assert that node0 is now the backing node of node4
+        result = self.vm.qmp('query-named-block-nodes')
+        node4 = next(node for node in result['return'] if node['node-name'] == 
'node4')
+        self.assertEqual(node4['image']['backing-image']['filename'], 
self.imgs[0])
+
     # Test a block-stream and a block-commit job in parallel
     # Here the stream job is supposed to finish quickly in order to reproduce
     # the scenario that triggers the bug fixed in 3d5d319e1221 and 1a63a907507
-- 
2.35.1