[Qemu-devel] [PATCH 2/3] qcow2: Fix write_zeroes with partially allocated backing file cluster

[Kevin Wolf <kwolf@redhat.com>]

In order to correctly check whether a given cluster is read as zero, we
don't only need to check whether bdrv_get_block_status_above() sets
BDRV_BLOCK_ZERO, but also if all sectors for the whole cluster have the
same status.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index a6012dc..c197ff3 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2412,7 +2412,7 @@ static bool is_zero_cluster(BlockDriverState *bs, int64_t start)
     BlockDriverState *file;
     int64_t res = bdrv_get_block_status_above(bs, NULL, start,
                                               s->cluster_sectors, &nr, &file);
-    return res >= 0 && (res & BDRV_BLOCK_ZERO);
+    return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == s->cluster_sectors;
 }
 
 static bool is_zero_cluster_top_locked(BlockDriverState *bs, int64_t start)
@@ -2423,6 +2423,7 @@ static bool is_zero_cluster_top_locked(BlockDriverState *bs, int64_t start)
     int ret;
 
     ret = qcow2_get_cluster_offset(bs, start << BDRV_SECTOR_BITS, &nr, &off);
+    assert(nr == s->cluster_sectors);
     return ret == QCOW2_CLUSTER_UNALLOCATED || ret == QCOW2_CLUSTER_ZERO;
 }
 
-- 
1.8.3.1