Re: [Qemu-block] [Qemu-devel] [RFC PATCH 28/56] block: Widen dirty bitmap granularity to uint64_t for safety

[John Snow]

On 08/07/2017 10:45 AM, Markus Armbruster wrote:
> Block dirty bitmaps represent granularity in bytes as uint32_t.  It
> must be a power of two and a multiple of BDRV_SECTOR_SIZE.
> 
> The trouble with uint32_t is computations like this one in
> mirror_do_read():
> 
>     uint64_t max_bytes;
> 
>     max_bytes = s->granularity * s->max_iov;
> 
> The operands of * are uint32_t and int, so the product is computed in
> uint32_t (assuming 32 bit int), then zero-extended to uint64_t.
> 
> Since granularity is generally combined with 64 bit file offsets, it's
> best to make it 64 bits, too.  Less opportunity to screw up.
> 
> Signed-off-by: Markus Armbruster <address@hidden>

[bweeooop]

> --- a/block/dirty-bitmap.c
> +++ b/block/dirty-bitmap.c

[buuuuweeeep]

> @@ -506,16 +506,11 @@ uint32_t 
> bdrv_get_default_bitmap_granularity(BlockDriverState *bs)
>      return granularity;
>  }
>  
> -uint32_t bdrv_dirty_bitmap_granularity(const BdrvDirtyBitmap *bitmap)
> +uint64_t bdrv_dirty_bitmap_granularity(const BdrvDirtyBitmap *bitmap)
>  {
>      return BDRV_SECTOR_SIZE << hbitmap_granularity(bitmap->bitmap);
>  }
>  
> -uint32_t bdrv_dirty_bitmap_meta_granularity(BdrvDirtyBitmap *bitmap)
> -{
> -    return BDRV_SECTOR_SIZE << hbitmap_granularity(bitmap->meta);
> -}

Why? Unused? Not cool enough to mention?