diff --git a/compiler-rt/lib/xray/xray_utils.cpp b/compiler-rt/lib/xray/xray_utils.cpp
--- a/compiler-rt/lib/xray/xray_utils.cpp
+++ b/compiler-rt/lib/xray/xray_utils.cpp
@@ -69,6 +69,10 @@
     return;
   }
   Offset += TotalBytes;
+
+  // Record the data size as a property of the VMO.
+  _zx_object_set_property(Vmo, ZX_PROP_VMO_CONTENT_SIZE,
+                          &Offset, sizeof(Offset));
 }
 
 void LogWriter::Flush() XRAY_NEVER_INSTRUMENT {