Re: Re: Re: Re: Try-catch block injection

Sh... seems to be a conceptual problem on my side :(

To explain my issue. I'm trying to log the old values of local variables 
everytime a new value is assigned to that variable. Of cource, the first 
time a value is assigned, the "old" value does not exist as the variable 
in not initialized. To cope with that problem, i thought it would be a 
great idea to catch the VerifyError and pass default values to my 
logging component for the exceptional case. Obviously this does not work!

Eugene Kuleshov schrieb:
>   If you look at the stack trace of that VerifyError, you should see
> that it is thrown before your code is executed, i.e. when your
> transformed class is loaded. Basically the bytecode you generating is
> invalid and you can't recover from that within an invalid method code.
> So, you should make sure that all your transformed code pass trough
> CheckClassAdapter.verify()
>
>   regards,
>   Eugene
>
>
> On Mon, Mar 22, 2010 at 1:05 PM, Nikolas Nehmer
> <[email protected]> wrote:
>   
>> 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
>>>>>>
>>>>>>
>>>>>> --
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>>>>>> list.
>>>>>> To unsubscribe: mailto:[email protected]
>>>>>> For general help: mailto:[email protected]?subject=help
>>>>>> OW2 mailing lists service home page: http://www.ow2.org/wws
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>             
>>>> --
>>>> 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]
>>
>>
>>     


-- 
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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