[Qemu-devel] [PATCH 1/2] qcow2: Undo leaked allocations in co_writev

[Max Reitz]

If the write request spans more than one L2 table,
qcow2_alloc_cluster_offset cannot handle the required allocations
atomically. This results in leaks if it allocated new clusters in any
but the last L2 table touched and an error occurs in qcow2_co_writev
before having established the L2 link. These non-atomic allocations
were, however, indeed successful and are therefore given to the caller
in the L2Meta list.

If an error occurs in qcow2_co_writev and the L2Meta list is unwound,
all its remaining entries are clusters whose L2 links were not yet
established. Thus, all allocations in that list should be undone.

Signed-off-by: Max Reitz <address@hidden>
---
 block/qcow2.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index b2489fb..6bedd5d 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1017,6 +1017,13 @@ fail:
     while (l2meta != NULL) {
         QCowL2Meta *next;
 
+        /* Undo all leaked allocations */
+        if (l2meta->nb_clusters != 0) {
+            qcow2_free_clusters(bs, l2meta->alloc_offset,
+                                l2meta->nb_clusters << s->cluster_bits,
+                                QCOW2_DISCARD_ALWAYS);
+        }
+
         if (l2meta->nb_clusters != 0) {
             QLIST_REMOVE(l2meta, next_in_flight);
         }
-- 
1.8.3.1