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

[Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>]

On Fri, 2012-05-04 at 15:12 +0100, Ian Campbell wrote:
> 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?
> 

I'd like :(

BCC compiler used need some function which are not linked by BIOS.

Personally I would switch to something more clever like OpenWatcom but
mainly I wrote some small asm.

I think bochs code fix the issue providing a asm function used by
compiler but seems more hacky than my function.

Frediano

> > 
> > 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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