I instrumented some Java bytecode. What I try to do is this:
InstrumentStackElem[] stack;
int stackpointer;
void foo(){
stackpointer = (stackpointer + 1) % stack.length;
InstrumentStackElem localref = stack[stackpointer];
localref.init();
//before each return
localref.clear();
stackpointer = (stackpointer -1 +stack.length) % stack.length;
So at the beginning of each method I increment the stackpointer and grab a local reference to the array element. Before each return, I call a clear method on the element and decrease the counter again.
When I try to access the field for the call of localref.clear() I always get a java.lang.VerifyError.
Example method:
public static boolean isValidName(final String s) {
if (!s.matches("[_a-zA-Z][_a-zA-Z0-9]*")) {
return false;
}
if (s.toCharArray()[0] == '_') { // this check should not be -> this is
// a bug
return false;
}
return true;
}
Corresponding instrumented bytecode
public static boolean isValidName(java.lang.String);
Code:
0: getstatic #38 // Field instrumentation_static_stackpointer:I
3: iconst_1
4: iadd
5: bipush 16
7: irem
8: putstatic #38 // Field instrumentation_static_stackpointer:I
11: getstatic #40 // Field instrumentation_static_stack:[Lme/instrumentor/InstrumentStackElem;
14: getstatic #38 // Field instrumentation_static_stackpointer:I
17: aaload
18: astore_2
19: aload_2
20: bipush -1
22: ldc #113 // String isValidName_Ljava_lang_String__Z
24: invokevirtual #48 // Method me/instrumentor/InstrumentStackElem.init:(ILjava/lang/String;)V
27: aload_0
28: ldc #115 // String [_a-zA-Z][_a-zA-Z0-9]*
30: invokevirtual #118 // Method java/lang/String.matches:(Ljava/lang/String;)Z
33: ifne 56
36: iconst_0
37: aload_2
38: invokevirtual #110 // Method me/instrumentor/InstrumentStackElem.clear:()V
41: getstatic #38 // Field instrumentation_static_stackpointer:I
44: iconst_1
45: isub
46: bipush 16
48: iadd
49: bipush 16
51: irem
52: putstatic #38 // Field instrumentation_static_stackpointer:I
55: ireturn
56: aload_0
57: invokevirtual #122 // Method java/lang/String.toCharArray:()[C
60: iconst_0
61: caload
62: bipush 95
64: if_icmpne 87
67: iconst_0
68: aload_2
69: invokevirtual #110 // Method me/instrumentor/InstrumentStackElem.clear:()V
72: getstatic #38 // Field instrumentation_static_stackpointer:I
75: iconst_1
76: isub
77: bipush 16
79: iadd
80: bipush 16
82: irem
83: putstatic #38 // Field instrumentation_static_stackpointer:I
86: ireturn
87: iconst_1
88: aload_2
89: invokevirtual #110 // Method me/instrumentor/InstrumentStackElem.clear:()V
92: getstatic #38 // Field instrumentation_static_stackpointer:I
95: iconst_1
96: isub
97: bipush 16
99: iadd
100: bipush 16
102: irem
103: putstatic #38 // Field instrumentation_static_stackpointer:I
106: ireturn
This is the exact error thrown:
Unexpected error: Lexer: java.lang.VerifyError: Bad local variable type
Exception Details:
Location:
lexer/Lexer.isValidName(Ljava/lang/String;)Z @68: aload_2
Reason:
Type top (current frame, locals[2]) is not assignable to reference type
Current Frame:
bci: @68
flags: { }
locals: { 'java/lang/String' }
stack: { integer }
Bytecode:
0000000: b200 2604 6010 1070 b300 26b2 0028 b200
0000010: 2632 4d2c 10ff 1271 b600 302a 1273 b600
0000020: 769a 0017 032c b600 6eb2 0026 0464 1010
0000030: 6010 1070 b300 26ac 2ab6 007a 0334 105f
0000040: a000 1703 2cb6 006e b200 2604 6410 1060
0000050: 1010 70b3 0026 ac04 2cb6 006e b200 2604
0000060: 6410 1060 1010 70b3 0026 ac
Stackmap Table:
same_frame(@56)
same_frame(@87)
I use the asm library to instrument the methods and used this.method.visitLocalVariable("localstackref", "[Lme/instrumentor/InstrumentStackElem;",null, bl, el,this.stackelementindex); to add the local variable. stackelementindex in this case was 2.
Anybody can help me?
The generated bytecode is correct, except for the fact that you forgot to update the stackmap table.
For reference, here's pseudocode for what your generated bytecode does.
instrumentation_static_stackpointer = (instrumentation_static_stackpointer + 1) % 16
a2 = instrumentation_static_stack[instrumentation_static_stackpointer]
a2.init(-1, "isValidName_Ljava_lang_String__Z")
if (!s.matches("[_a-zA-Z][_a-zA-Z0-9]*")) goto L56
a2.clear()
instrumentation_static_stackpointer = (instrumentation_static_stackpointer - 1 + 16) % 16
return 0
L56:
if (s.toCharArray()[0] != 95) goto L87
a2.clear()
instrumentation_static_stackpointer = (instrumentation_static_stackpointer - 1 + 16) % 16
return 0
L87:
instrumentation_static_stackpointer = (instrumentation_static_stackpointer - 1 + 16) % 16
return 1
The problem is that in bytecode version 51.0+, which is presumably what you are using, it is mandatory to include type information about the bytecode variables as well in the StackMapTable attribute. However you did not update this to include the type for your new variable a2. The stack map table is only consulted whenever there is a jump or jump target. So when it gets to L56, it consults the table to see what the incoming types are, and the table says that the only variable is s with type String. Then it gets to a2.clear(), sees that you are using a variable which didn't exist in the stack map table, and fails verification.