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

[Matt Turner]

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;
         }
     }
 
-- 
1.7.8.6