[gdk-pixbuf] png: Fix some integer overflows

[Matthias Clasen <matthiasc src gnome org>]

commit 8714ab407c54d5989d15a78eb15550c2d52d95b8
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Aug 24 14:13:37 2015 -0400

    png: Fix some integer overflows
    
    The png loader was not careful enough in some places. Width * height
    can overflow an integer.
    
    This should fix http://bugzilla.gnome.org/734556.

 gdk-pixbuf/io-png.c |   15 ++++++++-------
 1 files changed, 8 insertions(+), 7 deletions(-)
---
diff --git a/gdk-pixbuf/io-png.c b/gdk-pixbuf/io-png.c
index 3336b1e..5690875 100644
--- a/gdk-pixbuf/io-png.c
+++ b/gdk-pixbuf/io-png.c
@@ -267,6 +267,7 @@ gdk_pixbuf__png_image_load (FILE *f, GError **error)
         gchar *density_str;
         guint32 retval;
         gint compression_type;
+        gpointer ptr;
 
 #ifdef PNG_USER_MEM_SUPPORTED
        png_ptr = png_create_read_struct_2 (PNG_LIBPNG_VER_STRING,
@@ -326,8 +327,8 @@ gdk_pixbuf__png_image_load (FILE *f, GError **error)
 
        rows = g_new (png_bytep, h);
 
-       for (i = 0; i < h; i++)
-               rows[i] = pixbuf->pixels + i * pixbuf->rowstride;
+        for (i = 0, ptr = pixbuf->pixels; i < h; i++, ptr += pixbuf->rowstride)
+               rows[i] = ptr;
 
        png_read_image (png_ptr, rows);
         png_read_end (png_ptr, info_ptr);
@@ -745,6 +746,7 @@ png_row_callback   (png_structp png_read_ptr,
 {
         LoadContext* lc;
         guchar* old_row = NULL;
+        gsize rowstride;
 
         lc = png_get_progressive_ptr(png_read_ptr);
 
@@ -770,8 +772,9 @@ png_row_callback   (png_structp png_read_ptr,
         lc->max_row_seen_in_chunk = MAX(lc->max_row_seen_in_chunk, ((gint)row_num));
         lc->last_row_seen_in_chunk = row_num;
         lc->last_pass_seen_in_chunk = pass_num;
-        
-        old_row = lc->pixbuf->pixels + (row_num * lc->pixbuf->rowstride);
+
+        rowstride = lc->pixbuf->rowstride;
+        old_row = lc->pixbuf->pixels + (row_num * rowstride);
 
         png_progressive_combine_row(lc->png_read_ptr, old_row, new_row);
 }
@@ -1123,11 +1126,9 @@ static gboolean real_save_png (GdkPixbuf        *pixbuf,
        png_set_shift (png_ptr, &sig_bit);
        png_set_packing (png_ptr);
 
-       ptr = pixels;
-       for (y = 0; y < h; y++) {
+       for (y = 0, ptr = pixels; y < h; y++, ptr += rowstride) {
                row_ptr = (png_bytep)ptr;
                png_write_rows (png_ptr, &row_ptr, 1);
-               ptr += rowstride;
        }
 
        png_write_end (png_ptr, info_ptr);