Re: [Qemu-devel] [PATCH] do boundary check based on absolute value

["René Rebe" <rene@exactcode.de>]

Hi,

Glauber Costa wrote:
> For backward operations, dstpitch and srcpitch can
> be negative. This leads BLTUNSAFE macro into an
> overflow, and as a result, it avoids performing
> operations that are perfectly valid.
>
> The visible effect that led to that patch was the gnome-panel
> bar in Fedora10. Before this patch, you could see garbage
> clobbering a big portion of the bar.
>
> After this patch, this garbage is gone.
>   
Confirmed to fix corruption with simple window mangers like
blackbox/fluxbox when moving windows out of the screen
bounds, which had on my TODO for review.
> Signed-off-by: Glauber Costa <glommer@redhat.com>
> ---
>  hw/cirrus_vga.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/hw/cirrus_vga.c b/hw/cirrus_vga.c
> index e0cf458..5690719 100644
> --- a/hw/cirrus_vga.c
> +++ b/hw/cirrus_vga.c
> @@ -221,15 +221,17 @@
>  #define CIRRUS_HOOK_NOT_HANDLED 0
>  #define CIRRUS_HOOK_HANDLED 1
>  
> +#define ABS(a) ((signed)(a) > 0 ? a : -a)
> +
>  #define BLTUNSAFE(s) \
>      ( \
>          ( /* check dst is within bounds */ \
> -            (s)->cirrus_blt_height * (s)->cirrus_blt_dstpitch \
> +            (s)->cirrus_blt_height * ABS((s)->cirrus_blt_dstpitch) \
>                  + ((s)->cirrus_blt_dstaddr & (s)->cirrus_addr_mask) > \
>                      (s)->vram_size \
>          ) || \
>          ( /* check src is within bounds */ \
> -            (s)->cirrus_blt_height * (s)->cirrus_blt_srcpitch \
> +            (s)->cirrus_blt_height * ABS((s)->cirrus_blt_srcpitch) \
>                  + ((s)->cirrus_blt_srcaddr & (s)->cirrus_addr_mask) > \
>                      (s)->vram_size \
>          ) \
>   

-- 
  René Rebe - ExactCODE GmbH - Europe, Germany, Berlin
  http://exactcode.de | http://t2-project.org | http://rene.rebe.name