Java class file verification

[Alex Buckley]

Hi John,

Per 
http://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.7, 
LocalVariableTable is not critical to correct interpretation of the 
class file by the JVM, so a JVM implementation like HotSpot is permitted 
to leave constraint checking up to the libraries. (This happens with the 
Runtime*Annotations attributes, for example.)

That said, I believe HotSpot in JDK 8 does go to the trouble of throwing 
ClassFormatError for oddball LocalVariableTable attributes:

     LocalVariableTable - Throws CFE if:
         start_pc >= code_length
         start_pc + length > code_length
         name_index or descriptor_index is invalid
         the name at name_index is not a legal field name
         the signature at descriptor_index is not a legal field signature
         index is a valid index in the local variable array

Probably best if you file a HotSpot bug on bugs.java.com so we can 
diagnose details there, not on a general discuss list.

Alex

On 1/14/2015 12:35 PM, John Bergin wrote:
> Hi all.
>
> I have a query regarding Java class file verification in particular
> verification of the LocalVariableTable attribute defined by the Java
> Virtual Machine 7 specification here,
>
> http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.13
>
> The specification states that "The value of start_pc must be a valid
> index into the code array of this Code attribute and must be the index
> of the opcode of an instruction." And in the example below the
> start_pc for args is 0 and for s it is 3 and both these indexes are
> valid code array offsets.
>
>    public static void main(java.lang.String[]);
>      flags: ACC_PUBLIC, ACC_STATIC
>      Code:
>        stack=1, locals=2, args_size=1
>           0: ldc           #2                  // String A string
>           2: astore_1
>           3: return
>        LineNumberTable:
>          line 3: 0
>          line 4: 3
>        LocalVariableTable:
>          Start  Length  Slot  Name   Signature
>                 0       4     0  args   [Ljava/lang/String;
>                 3       1     1     s   Ljava/lang/String;
>
> My question is: a program I'm helping to develop is dynamically
> generating Java class files where the start_pc indexes are NOT valid
> indexes into the code array (in fact they're off by one) and OpenJDK
> is not rejecting the class via a ClassFormatException. Has anyone got
> any idea why this is the case?
>
> Kind regards,
> John.
>