Ensure special static slots respect alignment (case 1266322)

Without proper alignment, this may lead to reference types
being stored at non-pointer aligned offsets. Among other
issues this may lead to the GC not scanning those pointers
properly.

Author: joncham

mono/metadata/threads.c

@@ -113,6 +113,7 @@ struct _StaticDataFreeList {
 	StaticDataFreeList *next;
 	guint32 offset;
 	guint32 size;
+	gint32 align;
 };
 
 typedef struct {
@@ -4381,12 +4382,12 @@ alloc_context_static_data_helper (gpointer key, gpointer value, gpointer user)
 }
 
 static StaticDataFreeList*
-search_slot_in_freelist (StaticDataInfo *static_data, guint32 size, guint32 align)
+search_slot_in_freelist (StaticDataInfo *static_data, guint32 size, gint32 align)
 {
 	StaticDataFreeList* prev = NULL;
 	StaticDataFreeList* tmp = static_data->freelist;
 	while (tmp) {
-		if (tmp->size == size) {
+		if (tmp->size == size && tmp->align == align) {
 			if (prev)
 				prev->next = tmp->next;
 			else
@@ -4544,7 +4545,7 @@ free_context_static_data_helper (gpointer key, gpointer value, gpointer user)
 }
 
 static void
-do_free_special_slot (guint32 offset, guint32 size)
+do_free_special_slot (guint32 offset, guint32 size, gint32 align)
 {
 	guint32 static_type = ACCESS_SPECIAL_STATIC_OFFSET (offset, type);
 	MonoBitSet **sets;
@@ -4577,6 +4578,7 @@ do_free_special_slot (guint32 offset, guint32 size)
 
 		item->offset = offset;
 		item->size = size;
+		item->align = align;
 
 		item->next = info->freelist;
 		info->freelist = item;
@@ -4591,7 +4593,7 @@ do_free_special (gpointer key, gpointer value, gpointer data)
 	gint32 align;
 	guint32 size;
 	size = mono_type_size (field->type, &align);
-	do_free_special_slot (offset, size);
+	do_free_special_slot (offset, size, align);
 }
 
 void