NOP ATHROW hack defeated !

Charles Nutter (a JRuby guy) was able to generate a code that defeat
the infamous NOP ... ATHROW hack.

see http://asm.ow2.org/doc/developer-guide.html#deadcode
for the background.

Basically, when a code is dead, not reachable, ASM replaces the 
offending code by
NOP ... ATHROW and generate a dedicated stackmap: [] [java/lang/Throwable].
But if the unreachable code is in a try/catch, we create a path where no 
local variable
are available anymore. So the split verifier will reject the bytecode if
a local variable is used in the exception handler of the try/catch.

The program below generates a bytecode which is rejected by the split 
verifier.
[forax@localhost asm4-test]$ java deadcode
Exception in thread "main" java.lang.VerifyError: Stack map does not 
match the one at exception handler 7 in method 
deadcode.foo(Ljava/lang/Object;)V at offset 3
     at java.lang.Class.getDeclaredMethods0(Native Method)
     at java.lang.Class.privateGetDeclaredMethods(Class.java:2440)
     at java.lang.Class.getMethod0(Class.java:2683)
     at java.lang.Class.getMethod(Class.java:1618)
     at sun.launcher.LauncherHelper.getMainMethod(LauncherHelper.java:484)
     at 
sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:476)


Eric, do you see a solution for that ?

Rémi

----------------------------------------------------------------------------------------------------------------

import static org.objectweb.asm.Opcodes.ACC_PUBLIC;
import static org.objectweb.asm.Opcodes.ACC_STATIC;
import static org.objectweb.asm.Opcodes.ASTORE;
import static org.objectweb.asm.Opcodes.ATHROW;
import static org.objectweb.asm.Opcodes.GOTO;
import static org.objectweb.asm.Opcodes.ISTORE;
import static org.objectweb.asm.Opcodes.RETURN;
import static org.objectweb.asm.Opcodes.V1_7;

import java.io.IOException;
import java.io.PrintWriter;
import java.nio.file.Files;
import java.nio.file.Paths;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.util.CheckClassAdapter;


public class DeadCode2 {
    public static void main(String[] args) throws IOException {
      ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
      writer.visit(V1_7, ACC_PUBLIC, "deadcode", null, 
"java/lang/Object", null);
      MethodVisitor mv = writer.visitMethod(ACC_PUBLIC|ACC_STATIC, 
"foo", "(Ljava/lang/Object;)V", null, null);
      mv.visitCode();

      Label L0, L1, L3;
      L0 = new Label(); L1 = new Label(); L3 = new Label();

      mv.visitTryCatchBlock(L0, L1, L1, "java/lang/RuntimeException");

      mv.visitLabel(L0);
      mv.visitJumpInsn(GOTO, L3);

      mv.visitLdcInsn("foo");
      mv.visitVarInsn(ASTORE, 0);
      mv.visitVarInsn(ISTORE, 0);

      mv.visitLabel(L1);

      mv.visitInsn(ATHROW);

      mv.visitLabel(L3);

      mv.visitInsn(RETURN);

      mv.visitMaxs(-1, -1);
      mv.visitEnd();

      writer.visitEnd();

      CheckClassAdapter.verify(new ClassReader(writer.toByteArray()), 
true, new PrintWriter(System.err));


      Files.write(Paths.get("deadcode.class"), writer.toByteArray());
   }
}

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