Re: [PATCH 3/9] block/vpc: Don't abuse the footer buffer for dynamic header

[Max Reitz]

On 17.12.20 17:19, Markus Armbruster wrote:
> create_dynamic_disk() takes a buffer holding the footer as first
> argument.  It writes out the footer (512 bytes), then reuses the
> buffer to initialize and write out the dynamic header (1024 bytes).
>
> Works, because the caller passes a buffer that is large enough for
> both purposes.  I hate that.
>
> Use a separate buffer for the dynamic header, and adjust the caller's
> buffer.
>
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>   block/vpc.c | 22 ++++++++++++----------
>   1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/block/vpc.c b/block/vpc.c
> index d18ecc3da1..34186640ee 100644
> --- a/block/vpc.c
> +++ b/block/vpc.c
> @@ -822,8 +822,9 @@ static int calculate_geometry(int64_t total_sectors, 
> uint16_t *cyls,
>   static int create_dynamic_disk(BlockBackend *blk, uint8_t *buf,
>                                  int64_t total_sectors)
>   {
> +    uint8_t dyndisk_header_buf[1024];
>       VHDDynDiskHeader *dyndisk_header =
> -        (VHDDynDiskHeader *) buf;
> +        (VHDDynDiskHeader *)dyndisk_header_buf;
>       uint8_t bat_sector[512];
>       size_t block_size, num_bat_entries;
>       int i;
> @@ -858,7 +859,7 @@ static int create_dynamic_disk(BlockBackend *blk, uint8_t 
> *buf,
>       }
>   
>       /* Prepare the Dynamic Disk Header */
> -    memset(buf, 0, 1024);
> +    memset(dyndisk_header_buf, 0, 1024);
>   
>       memcpy(dyndisk_header->magic, "cxsparse", 8);

I’d prefer something like

*dyndisk_header = (VHDDynDiskHeader){
    ...
};

but that isn’t possible before patch 5.  (And can be done on top now 
anyway, especially given that Kevin probably wants to send a pull 
request today.)

[...]

> @@ -972,8 +974,8 @@ static int coroutine_fn vpc_co_create(BlockdevCreateOptions 
> *opts,
>       BlockBackend *blk = NULL;
>       BlockDriverState *bs = NULL;
>   
> -    uint8_t buf[1024];
> -    VHDFooter *footer = (VHDFooter *) buf;
> +    uint8_t footer_buf[HEADER_SIZE];
> +    VHDFooter *footer = (VHDFooter *)footer_buf;
>       uint16_t cyls = 0;
>       uint8_t heads = 0;
>       uint8_t secs_per_cyl = 0;
> @@ -1036,7 +1038,7 @@ static int coroutine_fn 
> vpc_co_create(BlockdevCreateOptions *opts,
>       }
>   
>       /* Prepare the Hard Disk Footer */
> -    memset(buf, 0, 1024);
> +    memset(footer_buf, 0, HEADER_SIZE);
>   
>       memcpy(footer->creator, "conectix", 8);

Same here, except here it’s patch 7.

Reviewed-by: Max Reitz <mreitz@redhat.com>