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

[Markus Armbruster]

Max Reitz <mreitz@redhat.com> writes:

> 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.

Yes.  And afterwards, using an initializer would be even simpler, I
guess.

>                                          (And can be done on top now
> anyway, especially given that Kevin probably wants to send a pull 
> request today.)

Yes, would be good to get this wrapped before the break.

> [...]
>
>> @@ -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>

Thanks!