From b1e1af394d9dac425eaac7ab6b77cb5225eaf26c Mon Sep 17 00:00:00 2001 From: Hanna Reitz Date: Thu, 24 Mar 2022 15:09:07 +0100 Subject: block/stream: Drain subtree around graph change 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 Message-Id: <20220324140907.17192-1-hreitz@redhat.com> Reviewed-by: Eric Blake Acked-by: Vladimir Sementsov-Ogievskiy --- block/stream.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) (limited to 'block') 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; } -- cgit v1.2.3