Re: [Xen-devel] [PATCH 3/7] vgabios: Fix size computation overflow

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-05-04 at 13:36 +0100, Frediano Ziglio wrote:
> Remove an overflow computing width x height x bit which does
> not fit into a 16 bits. I wrote a routine to multiple these value
> and get the size required for framebuffer in segment unit (64k).

Couldn't this be done in C using a suitably wide temporary variable?

> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
> ---
>  tools/firmware/vgabios/vbe.c |   30 ++++++++++++++++++++++++++++--
>  1 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/firmware/vgabios/vbe.c b/tools/firmware/vgabios/vbe.c
> index 3d42216..35d9866 100644
> --- a/tools/firmware/vgabios/vbe.c
> +++ b/tools/firmware/vgabios/vbe.c
> @@ -742,6 +742,29 @@ no_vbe_flag:
>    jmp  _display_string
>  ASM_END  
>  
> +ASM_START
> +_size64:
> +  push bp
> +  mov  bp, sp
> +  push dx
> +
> +; multiply bbp by yres first as results fit in 16bits
> +; then multiply by xres
> +  mov  ax, 8[bp]
> +  mul  word 6[bp]
> +  mul  word 4[bp]
> +; divide by 2^19 ceiling result
> +  add  ax, #0xffff
> +  adc  dx, #7
> +  mov  ax, dx
> +  shr  ax, #3
> +
> +  pop  dx
> +  pop  bp
> +  ret
> +ASM_END
> +
> +
>  /** Function 00h - Return VBE Controller Information
>   * 
>   * Input:
> @@ -846,9 +869,12 @@ Bit16u *AX;Bit16u ES;Bit16u DI;
>                  
>          do
>          {
> +                Bit16u size_64k = size64(cur_info->info.XResolution, 
> cur_info->info.YResolution, cur_info->info.BitsPerPixel);
> +                Bit16u max_bpp = dispi_get_max_bpp();
> +
>                  if ((cur_info->info.XResolution <= dispi_get_max_xres()) &&
> -                    (cur_info->info.BitsPerPixel <= dispi_get_max_bpp()) &&
> -                    (cur_info->info.XResolution * cur_info->info.XResolution 
> * cur_info->info.BitsPerPixel <= vbe_info_block.TotalMemory << 19 )) {
> +                    (cur_info->info.BitsPerPixel <= max_bpp) &&
> +                    (size_64k <= vbe_info_block.TotalMemory)) {
>  #ifdef DEBUG
>                    printf("VBE found mode %x => %x\n", 
> cur_info->mode,cur_mode);
>                    cur_mode++;



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