sun/jvm/hotspot/oops/Symbol.java

[Andrew Haley]

I've been testing serviceability support for AArch64, and I keep getting
bad pointers and NullPointerExceptions.

I thought it was a bug in my back-end, but it's not.  The problem is
that we read a Symbol pointer out of the constant pool and then try to
use it.  Unfortunately, the bottom two bits of a symbol pointer in the
constant pool have special meanings and we must mask them before use.

This is my fix:

diff -r 11b7f6b12521 agent/src/share/classes/sun/jvm/hotspot/oops/Symbol.java
--- a/agent/src/share/classes/sun/jvm/hotspot/oops/Symbol.java Fri Apr 24 
16:28:29 2015 +0100
+++ b/agent/src/share/classes/sun/jvm/hotspot/oops/Symbol.java Tue Apr 28 
19:23:18 2015 +0100
@@ -43,6 +43,8 @@
       });
   }

+    static class TagBits { final static int _resolved_value = 0, _symbol_bit = 
1, _pseudo_bit = 2, _symbol_mask = 3; }
+
   private static synchronized void initialize(TypeDataBase db) throws 
WrongTypeException {
     Type type  = db.lookupType("Symbol");
     length     = type.getCIntegerField("_length");
@@ -60,7 +62,8 @@
     if (addr == null) {
       return null;
     }
-    return new Symbol(addr);
+    return new Symbol(addr.andWithMask(~ TagBits._symbol_mask));
   }

I'm not at all sure this is the right place to fix it, but it works.

I'm just really surprised no-one noticed this before.

Andrew.