[repacker] get repacker fuzzer working.

Additionally add helper method that allows a graph to be saved as a fuzzer seed.

src/graph/graph.hh

@@ -953,6 +953,56 @@ struct graph_t
     return true;
   }
 
+#if 0
+  /*
+   * Saves the current graph to a packed binary format which the repacker fuzzer takes
+   * as a seed.
+   */
+  void save_fuzzer_seed (hb_tag_t tag) const
+  {
+    FILE* f = fopen ("./repacker_fuzzer_seed", "w");
+    fwrite ((void*) &tag, sizeof (tag), 1, f);
+
+    uint16_t num_objects = vertices_.length;
+    fwrite ((void*) &num_objects, sizeof (num_objects), 1, f);
+
+    for (const auto& v : vertices_)
+    {
+      uint16_t blob_size = v.table_size ();
+      fwrite ((void*) &blob_size, sizeof (blob_size), 1, f);
+      fwrite ((const void*) v.obj.head, blob_size, 1, f);
+    }
+
+    uint16_t link_count = 0;
+    for (const auto& v : vertices_)
+      link_count += v.obj.real_links.length;
+
+    fwrite ((void*) &link_count, sizeof (link_count), 1, f);
+
+    typedef struct
+    {
+      uint16_t parent;
+      uint16_t child;
+      uint16_t position;
+      uint8_t width;
+    } link_t;
+
+    for (unsigned i = 0; i < vertices_.length; i++)
+    {
+      for (const auto& l : vertices_[i].obj.real_links)
+      {
+        link_t link {
+          (uint16_t) i, (uint16_t) l.objidx,
+          (uint16_t) l.position, (uint8_t) l.width
+        };
+        fwrite ((void*) &link, sizeof (link), 1, f);
+      }
+    }
+
+    fclose (f);
+  }
+#endif
+
   void print_orphaned_nodes ()
   {
     if (!DEBUG_ENABLED(SUBSET_REPACK)) return;

src/hb-repacker.hh

@@ -378,7 +378,7 @@ hb_resolve_overflows (const T& packed,
   graph_t sorted_graph (packed);
   if (!sorted_graph.is_fully_connected ())
   {
-    DEBUG_MSG (SUBSET_REPACK, nullptr, "Input graph is not fully connected.");
+    sorted_graph.print_orphaned_nodes ();
     return nullptr;
   }
 

test/fuzzing/hb-repacker-fuzzer.cc

@@ -31,7 +31,10 @@ bool read(const uint8_t** data, size_t* size, T* out)
 void cleanup (hb_object_t* objects, uint16_t num_objects)
 {
   for (uint32_t i = 0; i < num_objects; i++)
+  {
+    free (objects[i].head);
     free (objects[i].real_links);
+  }
 }
 
 void add_links_to_objects (hb_object_t* objects, uint16_t num_objects,
@@ -56,7 +59,7 @@ void add_links_to_objects (hb_object_t* objects, uint16_t num_objects,
   for (uint32_t i = 0; i < num_links; i++)
   {
     uint16_t parent_idx = links[i].parent;
-    uint16_t child_idx = links[i].child;
+    uint16_t child_idx = links[i].child + 1; // All indices are shifted by 1 by the null object.
     hb_link_t* link = &(objects[parent_idx].real_links[link_count[parent_idx] - 1]);
 
     link->width = links[i].width;
@@ -89,8 +92,10 @@ extern "C" int LLVMFuzzerTestOneInput (const uint8_t *data, size_t size)
     if (!read<uint16_t> (&data, &size, &blob_size)) goto end;
     if (size < blob_size) goto end;
 
-    objects[i].head = (char*) data;
-    objects[i].tail = (char*) (data + blob_size);
+    char* copy = (char*) calloc (1, blob_size);
+    memcpy (copy, data, blob_size);
+    objects[i].head = (char*) copy;
+    objects[i].tail = (char*) (copy + blob_size);
 
     size -= blob_size;
     data += blob_size;
@@ -103,7 +108,7 @@ extern "C" int LLVMFuzzerTestOneInput (const uint8_t *data, size_t size)
     if (!read<link_t> (&data, &size, &links[i])) goto end;
 
     if (links[i].parent >= num_objects
-        || links[i].child >= links[i].parent) // Enforces DAG graph
+        || links[i].child > links[i].parent) // Enforces DAG graph
       goto end;
 
     if (links[i].width < 2 || links[i].width > 4) goto end;