[Qemu-devel] [PATCH v2] qcow2: Zero-initialise first cluster for new images

[Kevin Wolf <kwolf@redhat.com>]

Strictly speaking, this is only required for has_zero_init() == false,
but it's easy enough to just do a cluster-aligned write that is padded
with zeros after the header.

This fixes that after 'qemu-img create' header extensions are attempted
to be parsed that are really just random leftover data.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/qcow2.c | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 6e5d98d..a205f31 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1471,7 +1471,7 @@ static int qcow2_create2(const char *filename, int64_t total_size,
      * size for any qcow2 image.
      */
     BlockDriverState* bs;
-    QCowHeader header;
+    QCowHeader *header;
     uint8_t* refcount_table;
     Error *local_err = NULL;
     int ret;
@@ -1489,30 +1489,34 @@ static int qcow2_create2(const char *filename, int64_t total_size,
     }
 
     /* Write the header */
-    memset(&header, 0, sizeof(header));
-    header.magic = cpu_to_be32(QCOW_MAGIC);
-    header.version = cpu_to_be32(version);
-    header.cluster_bits = cpu_to_be32(cluster_bits);
-    header.size = cpu_to_be64(0);
-    header.l1_table_offset = cpu_to_be64(0);
-    header.l1_size = cpu_to_be32(0);
-    header.refcount_table_offset = cpu_to_be64(cluster_size);
-    header.refcount_table_clusters = cpu_to_be32(1);
-    header.refcount_order = cpu_to_be32(3 + REFCOUNT_SHIFT);
-    header.header_length = cpu_to_be32(sizeof(header));
+    QEMU_BUILD_BUG_ON((1 << MIN_CLUSTER_BITS) < sizeof(*header));
+    header = g_malloc0(cluster_size);
+    *header = (QCowHeader) {
+        .magic                      = cpu_to_be32(QCOW_MAGIC),
+        .version                    = cpu_to_be32(version),
+        .cluster_bits               = cpu_to_be32(cluster_bits),
+        .size                       = cpu_to_be64(0),
+        .l1_table_offset            = cpu_to_be64(0),
+        .l1_size                    = cpu_to_be32(0),
+        .refcount_table_offset      = cpu_to_be64(cluster_size),
+        .refcount_table_clusters    = cpu_to_be32(1),
+        .refcount_order             = cpu_to_be32(3 + REFCOUNT_SHIFT),
+        .header_length              = cpu_to_be32(sizeof(*header)),
+    };
 
     if (flags & BLOCK_FLAG_ENCRYPT) {
-        header.crypt_method = cpu_to_be32(QCOW_CRYPT_AES);
+        header->crypt_method = cpu_to_be32(QCOW_CRYPT_AES);
     } else {
-        header.crypt_method = cpu_to_be32(QCOW_CRYPT_NONE);
+        header->crypt_method = cpu_to_be32(QCOW_CRYPT_NONE);
     }
 
     if (flags & BLOCK_FLAG_LAZY_REFCOUNTS) {
-        header.compatible_features |=
+        header->compatible_features |=
             cpu_to_be64(QCOW2_COMPAT_LAZY_REFCOUNTS);
     }
 
-    ret = bdrv_pwrite(bs, 0, &header, sizeof(header));
+    ret = bdrv_pwrite(bs, 0, header, cluster_size);
+    g_free(header);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not write qcow2 header");
         goto out;
-- 
1.8.1.4