Re: [Qemu-devel] [PATCH 2/2] block: Avoid second open for format probing

[Stefan Hajnoczi]

On Tue, Nov 13, 2012 at 03:14:55PM +0100, Kevin Wolf wrote:
> @@ -691,12 +685,15 @@ static int bdrv_open_common(BlockDriverState *bs, const 
> char *filename,
>  
>      /* Open the image, either directly or using a protocol */
>      if (drv->bdrv_file_open) {
> +        if (file != NULL) {
> +            bdrv_swap(file, bs);
> +            bdrv_delete(file);
> +        }
>          ret = drv->bdrv_file_open(bs, filename, open_flags);
>      } else {
[...]
>      /* Open the image */
> -    ret = bdrv_open_common(bs, filename, flags, drv);
> +    ret = bdrv_open_common(bs, file, filename, flags, drv);
>      if (ret < 0) {
>          goto unlink_and_fail;
>      }
> @@ -894,6 +895,9 @@ int bdrv_open(BlockDriverState *bs, const char *filename, 
> int flags,
>      return 0;
>  
>  unlink_and_fail:
> +    if (file != NULL) {
> +        bdrv_delete(file);
> +    }

Not sure I understand this code path.

We have a protocol (the driver implements .bdrv_file_open()) so we swap
file and bs, then delete old bs.  Then we call .bdrv_file_open() on the
already open file BDS.

Is it okay to call .bdrv_file_open() on an already open BDS?

Now if .bdrv_file_open() fails we will bdrv_delete() the already deleted
file BDS.  This is a double-free.

Stefan