[cairo] [PATCH] stroker: Check for scaling overflow in computing half line widths
Bryce Harrington
bryce at osg.samsung.com
Fri May 5 00:57:27 UTC 2017
On Fri, Mar 24, 2017 at 12:36:41PM +0000, Chris Wilson wrote:
> Given a combination of a large scaling matrix and a large line, we can
> easily generate a half line width that is unrepresentable in our 24.8
> fixed-point. This leads to spurious errors later, such as generating
> negative height boxes, and so asking pixman to fill to infinity. To
> avoid this, we can check for overflow in calculating the half line with,
> though we still lack adequate range checking on the final stroke path.
>
> References: https://bugs.webkit.org/show_bug.cgi?id=16793
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: magomez at igalia.com
The discussion thread seems to have died out on this.
I went ahead and verified the testcase Miguel Gomez provided does indeed
crash without this patch and doesn't crash with it applied. So in the
interest of seeing the fix move ahead I've pushed this patch to trunk:
Acked-by: Bryce Harrington <bryce at osg.samsung.com>
Tested-by: Bryce Harrington <bryce at osg.samsung.com>
To ssh://git.cairographics.org/git/cairo
a3cc46d..91b2500 master -> master
If there are further refinements in mind, that can be tackled as
follow-on work.
Bryce
> ---
> src/cairo-fixed-private.h | 13 +++++++++++++
> src/cairo-path-stroke-boxes.c | 25 ++++++++++++++++++++-----
> src/cairo-path-stroke-polygon.c | 1 +
> src/cairo-path-stroke-traps.c | 1 +
> src/cairo-path-stroke.c | 1 +
> 5 files changed, 36 insertions(+), 5 deletions(-)
>
> diff --git a/src/cairo-fixed-private.h b/src/cairo-fixed-private.h
> index 9ff8f7503..8ee895b92 100644
> --- a/src/cairo-fixed-private.h
> +++ b/src/cairo-fixed-private.h
> @@ -53,6 +53,9 @@
> #define CAIRO_FIXED_ONE_DOUBLE ((double)(1 << CAIRO_FIXED_FRAC_BITS))
> #define CAIRO_FIXED_EPSILON ((cairo_fixed_t)(1))
>
> +#define CAIRO_FIXED_MAX (~0u >> (CAIRO_FIXED_FRAC_BITS + 1))
> +#define CAIRO_FIXED_MIN (-(int)CAIRO_FIXED_MAX)
> +
> #define CAIRO_FIXED_ERROR_DOUBLE (1. / (2 * CAIRO_FIXED_ONE_DOUBLE))
>
> #define CAIRO_FIXED_FRAC_MASK ((cairo_fixed_t)(((cairo_fixed_unsigned_t)(-1)) >> (CAIRO_FIXED_BITS - CAIRO_FIXED_FRAC_BITS)))
> @@ -123,6 +126,16 @@ _cairo_fixed_from_double (double d)
> #endif
> }
>
> +static inline cairo_bool_t
> +_cairo_fixed_from_double_safe (cairo_fixed_t *f, double d)
> +{
> + if (unlikely (d < CAIRO_FIXED_MIN || d > CAIRO_FIXED_MAX))
> + return FALSE;
> +
> + *f = _cairo_fixed_from_double (d);
> + return TRUE;
> +}
> +
> #else
> # error Please define a magic number for your fixed point type!
> # error See cairo-fixed-private.h for details.
> diff --git a/src/cairo-path-stroke-boxes.c b/src/cairo-path-stroke-boxes.c
> index 6511d2383..c77b4a2cc 100644
> --- a/src/cairo-path-stroke-boxes.c
> +++ b/src/cairo-path-stroke-boxes.c
> @@ -98,6 +98,8 @@ _cairo_rectilinear_stroker_init (cairo_rectilinear_stroker_t *stroker,
> cairo_antialias_t antialias,
> cairo_boxes_t *boxes)
> {
> + double half_line_width;
> +
> /* This special-case rectilinear stroker only supports
> * miter-joined lines (not curves) and a translation-only matrix
> * (though it could probably be extended to support a matrix with
> @@ -127,15 +129,22 @@ _cairo_rectilinear_stroker_init (cairo_rectilinear_stroker_t *stroker,
> if (! _cairo_matrix_is_scale (ctm))
> return FALSE;
>
> + half_line_width = stroke_style->line_width / 2.0;
> +
> + if (! _cairo_fixed_from_double_safe (&stroker->half_line_x,
> + fabs(ctm->xx) * half_line_width))
> + return FALSE;
> + assert (stroker->half_line_x > 0);
> +
> + if (! _cairo_fixed_from_double_safe (&stroker->half_line_y,
> + fabs(ctm->yy) * half_line_width))
> + return FALSE;
> + assert (stroker->half_line_y > 0);
> +
> stroker->stroke_style = stroke_style;
> stroker->ctm = ctm;
> stroker->antialias = antialias;
>
> - stroker->half_line_x =
> - _cairo_fixed_from_double (fabs(ctm->xx) * stroke_style->line_width / 2.0);
> - stroker->half_line_y =
> - _cairo_fixed_from_double (fabs(ctm->yy) * stroke_style->line_width / 2.0);
> -
> stroker->open_sub_path = FALSE;
> stroker->segments = stroker->segments_embedded;
> stroker->segments_size = ARRAY_LENGTH (stroker->segments_embedded);
> @@ -287,6 +296,8 @@ _cairo_rectilinear_stroker_emit_segments (cairo_rectilinear_stroker_t *stroker)
> box.p1.x = b->x;
> box.p2.x = a->x;
> }
> + assert (box.p1.x < box.p2.x);
> +
> if (a->y < b->y) {
> box.p1.y = a->y;
> box.p2.y = b->y;
> @@ -294,6 +305,7 @@ _cairo_rectilinear_stroker_emit_segments (cairo_rectilinear_stroker_t *stroker)
> box.p1.y = b->y;
> box.p2.y = a->y;
> }
> + assert (box.p1.y < box.p2.y);
>
> status = _cairo_boxes_add (stroker->boxes, stroker->antialias, &box);
> if (unlikely (status))
> @@ -404,6 +416,8 @@ _cairo_rectilinear_stroker_emit_segments_dashed (cairo_rectilinear_stroker_t *st
> box.p1.x = b->x;
> box.p2.x = a->x;
> }
> + assert (box.p1.x < box.p2.x);
> +
> if (a->y < b->y) {
> box.p1.y = a->y;
> box.p2.y = b->y;
> @@ -411,6 +425,7 @@ _cairo_rectilinear_stroker_emit_segments_dashed (cairo_rectilinear_stroker_t *st
> box.p1.y = b->y;
> box.p2.y = a->y;
> }
> + assert (box.p1.y < box.p2.y);
>
> status = _cairo_boxes_add (stroker->boxes, stroker->antialias, &box);
> if (unlikely (status))
> diff --git a/src/cairo-path-stroke-polygon.c b/src/cairo-path-stroke-polygon.c
> index f9f70642f..1cbd3f8e7 100644
> --- a/src/cairo-path-stroke-polygon.c
> +++ b/src/cairo-path-stroke-polygon.c
> @@ -1280,6 +1280,7 @@ _cairo_path_fixed_stroke_to_polygon (const cairo_path_fixed_t *path,
> for (i = 1; i < polygon->num_limits; i++)
> _cairo_box_add_box (&stroker.bounds, &polygon->limits[i]);
>
> + /* XXX check overflow */
> _cairo_stroke_style_max_distance_from_path (style, path, ctm, &dx, &dy);
> fdx = _cairo_fixed_from_double (dx);
> fdy = _cairo_fixed_from_double (dy);
> diff --git a/src/cairo-path-stroke-traps.c b/src/cairo-path-stroke-traps.c
> index e183d1024..9eb60dda2 100644
> --- a/src/cairo-path-stroke-traps.c
> +++ b/src/cairo-path-stroke-traps.c
> @@ -150,6 +150,7 @@ stroker_init (struct stroker *stroker,
> _cairo_stroke_style_max_line_distance_from_path (stroker->style, path,
> stroker->ctm, &dx, &dy);
>
> + /* XXX check overflow */
> fdx = _cairo_fixed_from_double (dx);
> fdy = _cairo_fixed_from_double (dy);
>
> diff --git a/src/cairo-path-stroke.c b/src/cairo-path-stroke.c
> index a4e17fd8b..b8adf8107 100644
> --- a/src/cairo-path-stroke.c
> +++ b/src/cairo-path-stroke.c
> @@ -110,6 +110,7 @@ _cairo_stroker_limit (cairo_stroker_t *stroker,
> _cairo_stroke_style_max_distance_from_path (&stroker->style, path,
> stroker->ctm, &dx, &dy);
>
> + /* XXX report overflow! */
> fdx = _cairo_fixed_from_double (dx);
> fdy = _cairo_fixed_from_double (dy);
>
> --
> 2.11.0
>
> --
> cairo mailing list
> cairo at cairographics.org
> https://lists.cairographics.org/mailman/listinfo/cairo
More information about the cairo
mailing list