[PATCH] fb: reorder Bresenham error correction to avoid overshoot.

[Simon Schubert]

Looking good!

Signed-off-by: Simon Schubert <2 at 0x2c.or>

thanks,
   simon

On 08/12/2012 06:40 PM, Matt Turner wrote:
> From: Simon Schubert <2 at 0x2c.org>
>
> When fbBresSolid draws a line, it can happen that after the last
> pixel, the Bresenham error term overflows, and fbBresSolid paints
> another pixel before adjusting the error term.
>
> However, if this happens on the last pixel (len=0), this extra pixel
> might overshoot the boundary, and, in rare cases, lead to a segfault.
>
> Fix this issue by adjusting for the Bresenham error term before
> drawing the main pixel, not after.
>
> Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=24274
> Cc: Simon Schubert <2 at 0x2c.org>
> Cc: Mitch Davis <mjd+freedesktop.org at afork.com>
> Signed-off-by: Matt Turner <mattst88 at gmail.com>
> ---
> [mattst88] Rebased after indent.
>
> Simon, can I get your Signed-off-by?
>
> Mitch, can I get a Tested-by tag?
>
>   fb/fbseg.c |   18 +++++++++---------
>   1 files changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/fb/fbseg.c b/fb/fbseg.c
> index 0e4e0a9..1848387 100644
> --- a/fb/fbseg.c
> +++ b/fb/fbseg.c
> @@ -65,6 +65,12 @@ fbBresSolid(DrawablePtr pDrawable,
>       if (axis == X_AXIS) {
>           bits = 0;
>           while (len--) {
> +            if (e >= 0) {
> +                WRITE(dst, FbDoMaskRRop (READ(dst), and, xor, bits));
> +                bits = 0;
> +                dst += dstStride;
> +                e += e3;
> +            }
>               bits |= mask;
>               mask = fbBresShiftMask(mask, signdx, dstBpp);
>               if (!mask) {
> @@ -74,21 +80,12 @@ fbBresSolid(DrawablePtr pDrawable,
>                   mask = mask0;
>               }
>               e += e1;
> -            if (e >= 0) {
> -                WRITE(dst, FbDoMaskRRop(READ(dst), and, xor, bits));
> -                bits = 0;
> -                dst += dstStride;
> -                e += e3;
> -            }
>           }
>           if (bits)
>               WRITE(dst, FbDoMaskRRop(READ(dst), and, xor, bits));
>       }
>       else {
>           while (len--) {
> -            WRITE(dst, FbDoMaskRRop(READ(dst), and, xor, mask));
> -            dst += dstStride;
> -            e += e1;
>               if (e >= 0) {
>                   e += e3;
>                   mask = fbBresShiftMask(mask, signdx, dstBpp);
> @@ -97,6 +94,9 @@ fbBresSolid(DrawablePtr pDrawable,
>                       mask = mask0;
>                   }
>               }
> +            WRITE(dst, FbDoMaskRRop(READ(dst), and, xor, mask));
> +            dst += dstStride;
> +            e += e1;
>           }
>       }
>