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

VerifyError is raised when the verifier verifies a class,
i.e after the loading of the class and before
the static init is executed and

cheers,
Rémi


Le 22/03/2010 18:11, Nikolas Nehmer a écrit :
> 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.
>>>>> 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]
>>>
>>>
>
>

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