Re: [Qemu-devel] [PATCH] block: Close backing file early in bdrv_img_create

[Kevin Wolf <kwolf@redhat.com>]

Am 27.11.2013 um 21:05 hat Max Reitz geschrieben:
> Leaving the backing file open although it is not needed anymore can
> cause problems if it is opened through a block driver which allows
> exclusive access only and if the create function of the block driver
> used for the top image (the one being created) tries to close and reopen
> the image file (which will include opening the backing file a second
> time).
> 
> In particular, this will happen with a backing file opened through
> qemu-nbd and using qcow2 as the top image file format (which reopens the
> image to flush it to disk).
> 
> In addition, the BlockDriverState in bdrv_img_create() is used for the
> backing file only; it should therefore be made local to the respective
> block.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 382ea71..e5a8a4c 100644
> --- a/block.c
> +++ b/block.c
> @@ -4504,7 +4504,6 @@ void bdrv_img_create(const char *filename, const char *fmt,
>  {
>      QEMUOptionParameter *param = NULL, *create_options = NULL;
>      QEMUOptionParameter *backing_fmt, *backing_file, *size;
> -    BlockDriverState *bs = NULL;
>      BlockDriver *drv, *proto_drv;
>      BlockDriver *backing_drv = NULL;
>      Error *local_err = NULL;
> @@ -4583,6 +4582,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
>      size = get_option_parameter(param, BLOCK_OPT_SIZE);
>      if (size && size->value.n == -1) {
>          if (backing_file && backing_file->value.s) {
> +            BlockDriverState *bs;
>              uint64_t size;
>              char buf[32];
>              int back_flags;

More context:

            /* backing files always opened read-only */
            back_flags =
                flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);

            bs = bdrv_new("");

            ret = bdrv_open(bs, backing_file->value.s, NULL, back_flags,
                            backing_drv, &local_err);
            if (ret < 0) {
                error_setg_errno(errp, -ret, "Could not open '%s': %s",
                                 backing_file->value.s,
                                 error_get_pretty(local_err));
                error_free(local_err);
                local_err = NULL;
                goto out;
            }
            bdrv_get_geometry(bs, &size);
            size *= 512;

> @@ -4608,6 +4608,8 @@ void bdrv_img_create(const char *filename, const char *fmt,
>  
>              snprintf(buf, sizeof(buf), "%" PRId64, size);
>              set_option_parameter(param, BLOCK_OPT_SIZE, buf);
> +
> +            bdrv_unref(bs);
>          } else {
>              error_setg(errp, "Image creation needs a size parameter");
>              goto out;

bs is now leaked if bdrv_open() fails.

Kevin