[Qemu-devel] [PATCH v2 06/11] qcow2: Fix .bdrv_has_zero_init()

[Max Reitz]

If a qcow2 file is preallocated, it can no longer guarantee that it
initially appears as filled with zeroes.

So implement .bdrv_has_zero_init() by checking whether the file is
preallocated; if so, forward the call to the underlying storage node,
except for when it is encrypted: Encrypted preallocated images always
return effectively random data, so .bdrv_has_zero_init() must always
return 0 for them.

.bdrv_has_zero_init_truncate() can remain bdrv_has_zero_init_1(),
because it presupposes PREALLOC_MODE_OFF.

Reported-by: Stefano Garzarella <address@hidden>
Signed-off-by: Max Reitz <address@hidden>
---
 block/qcow2.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 5c40f54d64..b4e73aa443 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4631,6 +4631,33 @@ static ImageInfoSpecific 
*qcow2_get_specific_info(BlockDriverState *bs,
     return spec_info;
 }
 
+static int qcow2_has_zero_init(BlockDriverState *bs)
+{
+    BDRVQcow2State *s = bs->opaque;
+    bool preallocated;
+
+    if (qemu_in_coroutine()) {
+        qemu_co_mutex_lock(&s->lock);
+    }
+    /*
+     * Check preallocation status: Preallocated images have all L2
+     * tables allocated, nonpreallocated images have none.  It is
+     * therefore enough to check the first one.
+     */
+    preallocated = s->l1_size > 0 && s->l1_table[0] != 0;
+    if (qemu_in_coroutine()) {
+        qemu_co_mutex_unlock(&s->lock);
+    }
+
+    if (!preallocated) {
+        return 1;
+    } else if (bs->encrypted) {
+        return 0;
+    } else {
+        return bdrv_has_zero_init(s->data_file->bs);
+    }
+}
+
 static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
                               int64_t pos)
 {
@@ -5186,7 +5213,7 @@ BlockDriver bdrv_qcow2 = {
     .bdrv_child_perm      = bdrv_format_default_perms,
     .bdrv_co_create_opts  = qcow2_co_create_opts,
     .bdrv_co_create       = qcow2_co_create,
-    .bdrv_has_zero_init = bdrv_has_zero_init_1,
+    .bdrv_has_zero_init   = qcow2_has_zero_init,
     .bdrv_has_zero_init_truncate = bdrv_has_zero_init_1,
     .bdrv_co_block_status = qcow2_co_block_status,
 
-- 
2.21.0