Re: [Qemu-devel] [PATCH v3 1/6] vmdk: Use g_random_int to generate CID

[Don Koch]

On Wed, 3 Dec 2014 18:28:36 +0800
Fam Zheng <address@hidden> wrote:

> This replaces two "time(NULL)" invocations with "g_random_int()".
> According to VMDK spec, CID "is a random 32‐bit value updated the first
> time the content of the virtual disk is modified after the virtual disk
> is opened". Using "seconds since epoch" is just a "lame way" to generate
> it, and not completely safe because of the low precision.
> 
> Suggested-by: Markus Armbruster <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> Reviewed-by: Markus Armbruster <address@hidden>
> ---

Reviewed-by: Don Koch <address@hidden>

>  block/vmdk.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 2cbfd3e..ebb4b70 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -28,6 +28,7 @@
>  #include "qemu/module.h"
>  #include "migration/migration.h"
>  #include <zlib.h>
> +#include <glib.h>
>  
>  #define VMDK3_MAGIC (('C' << 24) | ('O' << 16) | ('W' << 8) | 'D')
>  #define VMDK4_MAGIC (('K' << 24) | ('D' << 16) | ('M' << 8) | 'V')
> @@ -1538,7 +1539,7 @@ static int vmdk_write(BlockDriverState *bs, int64_t 
> sector_num,
>          /* update CID on the first write every time the virtual disk is
>           * opened */
>          if (!s->cid_updated) {
> -            ret = vmdk_write_cid(bs, time(NULL));
> +            ret = vmdk_write_cid(bs, g_random_int());
>              if (ret < 0) {
>                  return ret;
>              }
> @@ -1922,7 +1923,7 @@ static int vmdk_create(const char *filename, QemuOpts 
> *opts, Error **errp)
>      }
>      /* generate descriptor file */
>      desc = g_strdup_printf(desc_template,
> -                           (uint32_t)time(NULL),
> +                           g_random_int(),
>                             parent_cid,
>                             fmt,
>                             parent_desc_line,
> -- 
> 1.9.3
> 
>