Re: [Qemu-devel] Re: [PATCH RESEND 2/2] vnc: Fix heap corruption

[Stefan Weil <weil@mail.berlios.de>]

Am 02.03.2011 19:47, schrieb Peter Maydell:
> On 2 March 2011 18:36, Stefan Weil <weil@mail.berlios.de> wrote:
>> No. I dont't think that the third parameter of bitmap_clear is
>> ok like that. See my patch for the correct value.
>
> Wen's patch:
>
> + const size_t width = ds_get_width(vd->ds) / 16;
> [...]
> -    bitmap_set(width_mask, 0, (ds_get_width(vd->ds) / 16));
> -    bitmap_clear(width_mask, (ds_get_width(vd->ds) / 16),
> -                 VNC_DIRTY_WORDS * BITS_PER_LONG);
> +    bitmap_set(width_mask, 0, width);
> +    bitmap_clear(width_mask, width, VNC_DIRTY_WORDS * BITS_PER_LONG - 
> width);
>
> Your patch:
>
> bitmap_clear(width_mask, (ds_get_width(vd->ds) / 16),
> - VNC_DIRTY_WORDS * BITS_PER_LONG);
> + (VNC_MAX_WIDTH - ds_get_width(vd->ds)) / 16);
>
> Since ui/vnc.h has:
>
> #define VNC_DIRTY_WORDS (VNC_MAX_WIDTH / (16 * BITS_PER_LONG))
>
> the third parameter to bitmap_clear is the same value in
> both cases, isn't it? Or is this a rounding bug?
>
> -- PMM

Because of rounding effects, both values can be different.

The part missing in my patch is correct handling of another
rounding effect:

VNC_DIRTY_WORDS is exact for 32 bit long values (and the
"old" code which used uint32_t until some weeks ago), where
VNC_DIRTY_WORDS = 2560/16/32 = 5.

For 64 bit values, VNC_DIRTY_WORDS = 2560/16/64 = 2 (rounded)!

Stefan W.