[BCEL-283] Support for StackMap should be different from StackMapTable

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 6.1
• Assignee: Benedikt Ritter
• Reporter: Mark Roberts
• Created: 09/Nov/16 17:04
• Updated: 18/Sep/17 09:12
• Resolved: 30/Aug/17 19:23

Description

It turns out we made a mistake with the StackMap renaming changes. I ran into a problem decoding some old .class files and went back to the Java Specification Requests (JSRs) to try and understand the history of the StackMap attribute. While I was not able to get the complete story, I have a pretty good guess at what happened and it does explain the problem I have seen.

The official introduction of StackMaps appears to be JSR 202 from (approx.) Sept. 2006 and it is part of JDK6 (see attached spec) . However, StackMaps were originally proposed much earlier (approx.) March 2002 - see the attached verifier spec. It appears there were some implementations of this earlier specification - the ASM tool was one and there may be others.

The key differences are:
Typechecker Spec: StackMap attribute with fixed format and no frame type byte
JDK 6 Spec: StackMapTable attribute with variable format including a frame type byte.

The JVM and the javap tool support BOTH formats. BCEL recognizes both attribute names, but treats them the same, as JDK6 version.

I don't think the changes will be too difficult. I will create a proposed fix within the next few days.