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

[Fam Zheng <famz@redhat.com>]

On 2013年11月26日 18:48, Kevin Wolf wrote:
> 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>
> ---
>   block/qcow2.c | 37 +++++++++++++++++++++----------------
>   1 file changed, 21 insertions(+), 16 deletions(-)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 6e5d98d..7c18587 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 +1489,35 @@ 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_malloc(cluster_size);
> +    memset(header, 0, cluster_size);

Could just be:

     header = g_malloc0(cluster_size);

But either way,

Reviewed-by: Fam Zheng <famz@redhat.com>

> +    *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;
>