Re: Re: Re: Try-catch block injection

Thanks for your answer. Some points are getting clearer now. But I still 
have some issues. I can't manage to catch the VerifyError even including 
the try-catch block on source code level.
If i use the following code snippet:
try {
   int xxx = 5;
} catch (VerifyError e) {
   System.out.println("Hello");
}

injecting an xLOAD before the xSTORE statement without inserting 
additional labels the vm is terminated because of an uncaught exception 
(VerifiyError). The catch-block is not executed!

InsnList list = instructions;
//currentInsnNode represents xSTORE
list.insertBefore(currentInsnNode,new VarInsnNode(Opcodes.ILOAD, 
((VarInsnNode)currentInsnNode).var));

Instead injecting an exception initialization at the same place, works 
out perfectly, i.e. the surrounding try-catch block catches and handles 
the exception.

list.insertBefore(currentInsnNode, new TypeInsnNode(Opcodes.NEW, 
"java/lang/VerifyError"));
list.insertBefore(currentInsnNode, new InsnNode(Opcodes.DUP));
list.insertBefore(currentInsnNode, new 
MethodInsnNode(Opcodes.INVOKESPECIAL, 
"java/lang/VerifyError","<init>","()V"));
list.insertBefore(currentInsnNode, new InsnNode(Opcodes.ATHROW));

Any ideas what the problem might be?

Cheers
Nikolas

Eugene Kuleshov schrieb:
>   In regards to your example, if it wasn't clear from my previous
> email you need to insert start label BEFORE wrapped block and end
> label - AFTER that block. Also your code always fall into the catch
> block (with or without the exception), so content of the stack doesn't
> match for those two cases. So, as I already suggested before, use
> ASMifier on a code compiled from a Java snipped before and after your
> transformation and you should see what I mean.
>
>   BTW, instead of loading variable you can also duplicate value used
> by xLOAD opcodes from the stack, but that will certainly affect the
> stackmap structure and maxstack value.
>
>   regards,
>   Eugene
>
>
> On Mon, Mar 22, 2010 at 10:05 AM, Nikolas Nehmer
> <[email protected]> wrote:
>   
>> By basic idea is as follows:
>> I try to log local variable access...logging the "old" variable value. To
>> achieve that goal, a method's bytecode is parsed and the following code is
>> injected (in this case for an ISTORE op) - passing the variable's stack
>> frame position and the old value to my logger. The currentIsnsNode in that
>> case would be the ISTORE.
>>
>> tempList.add(new LdcInsnNode(((VarInsnNode)currentInsnNode).var));
>> tempList.add(new VarInsnNode(Opcodes.ILOAD,
>> ((VarInsnNode)currentInsnNode).var));
>> tempList.add(new MethodInsnNode(Opcodes.INVOKESTATIC, logger,localLogger,
>> ("(II)V")));
>>
>> Unfortunately, thr ILOAD might throw a VerifyError if the local variable is
>> not initialized. To cope with that situation, I would like to surround the
>> code block inserted for logging, with a try catch block, catching the
>> VerifyError and passing a "default" value to the logger instead of loading
>> the current value.
>>
>> thanks in advance!
>> Nikolas
>>
>> Eugene Kuleshov schrieb:
>>     
>>>  Not sure what suggestion you are looking for, but your code is
>>> incomplete to drive any conclusions (e.g. what's the
>>> currentInsnNode?).
>>>  I'd suggest to forget about stackmap frames for now, it is going to
>>> work without them just fine and you can get back to them when you get
>>> a better grip on a bytecode.
>>>  Also, write your java code before and after transformation and use
>>> ASMifier to get the code to generate bytecodes, so you can compare
>>> what changes.
>>>
>>>  Anyways, from what I read, your code is transforming this:
>>>
>>>  currentInsnNode
>>>
>>>  into this:
>>>
>>>
>>>  {
>>>    startLabel (created by you)
>>>    endLabel (created by you)
>>>  } catch(Throwable e) {
>>>     logger.testTryCatch();
>>>  }
>>>  currentInsnNode
>>>  ...
>>>
>>>  Note that you also are NOT consuming the exception value off the stack.
>>>
>>>  regards,
>>>  Eugene
>>>
>>>
>>> On Mon, Mar 22, 2010 at 9:03 AM, Nikolas Nehmer
>>> <[email protected]> wrote:
>>>
>>>       
>>>> Hi,
>>>>
>>>> I'm trying to do the following transformation based on bytecode
>>>> injection:
>>>>
>>>> startLabel:
>>>> foo...;
>>>> endLabel
>>>>
>>>> into
>>>>
>>>> try {
>>>>  startLabel:
>>>>  foo...;
>>>>  endLabel
>>>> } catch (VerifyError e) {
>>>>  //doSomething;
>>>> }
>>>>
>>>> Currently, my code looks like this (raising a
>>>> java.lang.ClassFormatError):
>>>>
>>>> LabelNode startLabel = new LabelNode();
>>>> tempList.add(startLabel);
>>>>                 LabelNode endLabel = new LabelNode();
>>>> tempList.add(endLabel);
>>>>                 //catch block
>>>> int numberOfLocalVariables = 0;
>>>> Object[] localVariableTypes = new Object[0];
>>>> int numberOfStackElements = 0;
>>>> Object[] stackElementTypes = new Object[0];
>>>> tempList.add(new FrameNode(Opcodes.F_FULL, numberOfLocalVariables,
>>>> localVariableTypes, numberOfStackElements, stackElementTypes));
>>>> tempList.add(new MethodInsnNode(Opcodes.INVOKESTATIC,
>>>> logger,"testTryCatch",
>>>> ("()V")));
>>>>
>>>> list.insertBefore(currentInsnNode, tempList);
>>>>         list.insert(new FrameNode(Opcodes.F_SAME, numberOfLocalVariables,
>>>> localVariableTypes, numberOfStackElements, stackElementTypes));
>>>> tryCatchBlocks.add(new TryCatchBlockNode(startLabel, endLabel, endLabel,
>>>> "java/lang/Throwable"));
>>>>
>>>> Any suggestions?
>>>>
>>>> Cheers
>>>> Nikolas
>>>>
>>>>
>>>> --
>>>> You receive this message as a subscriber of the [email protected] mailing list.
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>>>> For general help: mailto:[email protected]?subject=help
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>>>>
>>>>
>>>>
>>>>         
>> --
>> Dipl.-Inf. Nikolas Nehmer,
>> Dept. of Computer Science. University of Kaiserslautern P.O. Box 3049, 67653
>> Kaiserslautern, Germany.
>> room 36/308, phone: +49 (0) 631 - 205 2644, fax: +49 (0) 631 - 205 3299
>> mail: [email protected]
>>
>>
>>     


-- 
Dipl.-Inf. Nikolas Nehmer,
Dept. of Computer Science. University of Kaiserslautern 
P.O. Box 3049, 67653 Kaiserslautern, Germany.
room 36/308, phone: +49 (0) 631 - 205 2644, fax: +49 (0) 631 - 205 3299
mail: [email protected]

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