[jboss-jira] [JBoss JIRA] (JASSIST-204) Getting java.lang.VerifyError when adding try/catch to a method using CtMethod.insertAfter, if the method returns when the JVM stack is not empty - Inconsistent stack height 0 != 1, or Inconsistent stackmap frames at branch target

[Dordoriko Beanie (JIRA)]

    [ https://issues.jboss.org/browse/JASSIST-204?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12793554#comment-12793554 ] 

Dordoriko Beanie commented on JASSIST-204:
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Hello Chiba-san,

Thank you for responding.
I am not sure if I understand you correctly;  The stack is indeed not empty before invoking the RETURN instruction, and indeed in this case the .class file wasn't produced "natively" from a java source by the javac compiler.
However, this isn't 'broken' according to the JVM specification, because the documentation of the RETURN-type instructions specifically discusses the situation that the stack depth (beyond the base of the current frame) isn't zero - it says that these extra entries [beyond the return value, if applicable] are popped / discarded automatically.

You can find this sentence in the spec for areturn/freturn/...: "All other items on the current method's operand stack are discarded."
And this sentence for 'return' (no args): "All items on the current method's operand stack are discarded."

Also, several different JVM's are able to correctly run a .class file which has this behavior, which further stresses that this is acceptable by the JVM.

Please let me know if this clarifies the situation, or elaborate to me what you meant by broken.

Again thank you very much,
 Dori

                
> Getting java.lang.VerifyError when adding try/catch to a method using CtMethod.insertAfter, if the method returns when the JVM stack is not empty - Inconsistent stack height 0 != 1, or Inconsistent stackmap frames at branch target
> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: JASSIST-204
>                 URL: https://issues.jboss.org/browse/JASSIST-204
>             Project: Javassist
>          Issue Type: Bug
>    Affects Versions: 3.17.0-GA, 3.18.0-GA
>         Environment: Occurred on Windows with standard HotSpot JVM (1.6.0_24, 1.7.0_21, ..), also with IBM J9 JVM on AIX etc.
>            Reporter: Dordoriko Beanie
>            Assignee: Shigeru Chiba
>              Labels: VerifyError, height, insertBefore, stack
>         Attachments: popnopbug.zip
>
>
> After instrumenting certain classes with insertAfter(), I am consistently getting java.lang.VerifyError when trying to load the instrumented class(either "Inconsistent stack height 0 != 1" or "Inconsistent stackmap frames at branch target", depending on whether I'm on a newer 1.7 environment or an older one).  The class was perfectly loadable prior to instrumentation, even though it exhibits a slightly unusual property that its return-type instructions are not necessarily executed when the stack is empty (not counting the return parameter, if applicable).
> The problem originally occurred with some class file that was apparently weaved by AspectJ 1.5.2a, but it was then reproduced by combining the following circumstances:
> 1) That the instrumented method has the "odd" property that it has an areturn/ireturn/return instruction which is invoked when the stack is not empty (when there are one or more values that were pushed onto the current frame and weren't popped).  Note that I was unable to produce this with a freshly compiled .java source, so instead I took a simple .class file and changed one of the popping instructions to a nop instruction using a hex editor.
> 2) That the code added by insertAfter() contains a try/catch structure or something similar (which forces the stack height to be reset in case of catch).  It is possible that this wasn't really necessary and that similar symptoms could occur by just having multiple return statements with different stack heights.
> When this happens, javassist replaces the original return-type instruction with a goto (when the stack height is N), but also adds a goto to the same label from the end of the catch clause (when the stack height is 0).
> runtest_modified_nop_java17.bat
> Exception in thread "main" java.lang.VerifyError: Inconsistent stackmap frames at branch target 35 in method com.test.HelloWorld.g()Ljava/lang/String; at offset 20
>         at java.lang.Class.getDeclaredMethods0(Native Method)
>         at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
>         at java.lang.Class.getMethod0(Unknown Source)
>         at java.lang.Class.getMethod(Unknown Source)
>         at sun.launcher.LauncherHelper.getMainMethod(Unknown Source)
>         at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)
> runtest_modified_nop_java12.bat
> Exception in thread "main" java.lang.VerifyError: (class: com/test/HelloWorld, method: g signature: ()Ljava/lang/String;) Inconsistent stack height 0 != 1
>         at java.lang.Class.getDeclaredMethods0(Native Method)
>         at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
>         at java.lang.Class.getMethod0(Unknown Source)
>         at java.lang.Class.getMethod(Unknown Source)
>         at sun.launcher.LauncherHelper.getMainMethod(Unknown Source)
>         at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

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