fb: reorder Bresenham error correction to avoid overshoot.

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
Signed-off-by: Simon Schubert <2@0x2c.or>
Tested-by: Mitch Davis <mjd+freedesktop.org@afork.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

1 files changed, 9 insertions, 9 deletions

diff --git a/fb/fbseg.c b/fb/fbseg.c
index 0e4e0a9f7..1848387f6 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;
         }
     }