Re: java/5794: gcj fails to verify .class file generated by Sun JDK 1.4 compiler

[Adam Dingle <adam@medovina.org>]

The following reply was made to PR java/5794; it has been noted by GNATS.

From: Adam Dingle <adam@medovina.org>
To: tromey@redhat.com
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: java/5794: gcj fails to verify .class file generated by Sun JDK
 1.4 compiler
Date: Fri, 01 Mar 2002 00:01:18 -0800

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 Tom,
 
 > Could you upload the resulting .class file?
 > Just reply to this email and attach it, keeping gcc-gnats on the CC
 > line.
 
 No problem - I've attached the .class file to this reply.
 
 > Adam> When Sun's Java compiler in JDK 1.4 compiles a try...finally
 > Adam> block, it emits a (useless and unused) exception table entry
 > Adam> whose target is within the range of protected instructions.
 > Adam> There is nothing inherently unsafe with this, but we check for
 > Adam> this case and reject it in verify.c.
 >
 > It seems to me that this must be invalid.
 
 Well, I'm not sure.  Section 4.9.5 of the Java Virtual Machine Specification 
 does include the following text: "Java virtual machine code produced by 
 Sun's compiler for the Java programming language always generates exception 
 handlers such that ... The handler for an exception will never be inside the 
 code that is being protected ... These restrictions are not enforced by the 
 class file verifier since they do not pose a threat to the integrity of the 
 Java virtual machine." (see 
 http://java.sun.com/docs/books/vmspec/2nd-edition/html/ClassFile.doc.html)
 
 > Adam> I tried that and then gcj complained about a stack overflow in
 > Adam> the same .class file.
 >
 > Yeah.  Suppose the VM encounters an internal error and throws an
 > exception after it has set the PC to the start of the exception
 > handler but before the first instruction of the handler (probably a
 > pop or an astore) is executed.  This leads inexorably to a stack
 > overflow.  I imagine the gij verifier will die on this too.
 
 That is a scary thought, although it's hard for me to imagine any sort of 
 error that might happen just at that moment.
 
 > I'd like to see the class file though.  It's possible there are
 > obscure extenuating circumstances.  Or perhaps they've tightened the
 > verification spec and I don't yet know about it.
 
 I'll be curious to see what you think.  I do seem to get this kind of unusual 
 exception table entry almost any time I compile a try...finally block with 
 JDK 1.4.
 
 -adam
 
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