[PATCH v3 2/3] gdiplus: Avoid copying GpImage's busy flag in select_frame_wic().

[Jinoh Kang]

From: Jinoh Kang

The 'busy' field in GpImage is used as an atomic variable.  The C11
standard (§5.1.2.4, paragraph 25) states that two conflicting actions to
a memory location shall be both atomic operations, or otherwise properly
synchronized; otherwise, it constitutes a data race.

However, select_frame_wic() performs a non-atomic access to the 'busy'
field on a GpImage that is potentially accessible by other threads.
This happens when select_frame_wic() copies new_image to the old image
object.  Although it does attempt to preserve the value of the 'busy'
field by setting new_image->busy = image->busy first, thereby
effectively assigning an identical value to the field, it is unclear
that this does not actually constitute a theoretical, if not practical,
data race.  This also prevents replacing the busy flag with a mutex or
other synchronization primitives.

Therefore, skip the 'busy' field when copying fields from the new image
to the original image object.
---
 dlls/gdiplus/image.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/dlls/gdiplus/image.c b/dlls/gdiplus/image.c
index 93d2a366e82..629599ddf2c 100644
--- a/dlls/gdiplus/image.c
+++ b/dlls/gdiplus/image.c
@@ -3799,6 +3799,7 @@ static GpStatus decode_image_wic(IStream *stream, REFGUID container,

static GpStatus select_frame_wic(GpImage *image, UINT active_frame)
 {
+    size_t obj_size, body_offset;
     GpImage *new_image;
     GpStatus status;

@@ -3806,15 +3807,24 @@ static GpStatus select_frame_wic(GpImage *image, UINT active_frame)
     if(status != Ok)
         return status;

-    new_image->busy = image->busy;
+    if (image->type == ImageTypeBitmap)
+        obj_size = sizeof(GpBitmap);
+    else if (image->type == ImageTypeMetafile)
+        obj_size = sizeof(GpMetafile);
+    else
+    {
+        ERR("unknown image type: %d\n", image->type);
+        GdipDisposeImage(new_image);
+        return GenericError;
+    }
+
     memcpy(&new_image->format, &image->format, sizeof(GUID));
     new_image->encoder = image->encoder;
     image->encoder = NULL;
     free_image_data(image);
-    if (image->type == ImageTypeBitmap)
-        *(GpBitmap *)image = *(GpBitmap *)new_image;
-    else if (image->type == ImageTypeMetafile)
-        *(GpMetafile *)image = *(GpMetafile *)new_image;
+    memcpy(image, new_image, FIELD_OFFSET(GpImage, busy));
+    body_offset = RTL_SIZEOF_THROUGH_FIELD(GpImage, busy);
+    memcpy((char *)image + body_offset, (char *)new_image + body_offset, obj_size - body_offset);
     new_image->type = ~0;
     heap_free(new_image);
     return Ok;