Handling .class files insite java process

Question

Hello folks
I have a question regarding JVMPI and JVM: I have created a small proff-of-concept program that handles JVMPI_EVENT_CLASS_LOAD_HOOK and JVMPI_EVENT_CLASS_LOAD events. After my class (simple Test.java with System.out.println) has beed loaded (*CLASSLOAD_HOOK event) I remembered pointer and size of .class file in memory of java process. Then, after CLASSLOAD event I filled this memory with zeroes (memset) to see if Test.class will execute normally. And it did worked fine. My question is: does java process need this memory after JVMPI_EVENT_CLASS_LOAD event? Can I safely override .class file data?
Best Regards
Boloek

Answer 1

At some point, the JVM writes the contents of the
class file into memory that's accessible via coredump.
Otherwise it couldn't execute the bytecodes. If you
zero out the memory where the physical .class file was
loaded, it won't affect the memory where the actual
class is stored.you are wrong :-) I did some testing and I caught three events:
- LOADHOOK where I remembered pointer to .class file and size of it
- _LOAD where I zeroed this memory
- JVMSHUT_DOWN where I caused SIGSEGV to force coredump.
I tested two cases: with and without clearing memory and then searched two different core-files for content of *.class file. I only found it in core dumped in a situation when I did not use memset to clear memory. It is because (as I assume) java process after loading .class file makes some optimisations to efficiently execute bytecode so representation of my code exists in memory of proces but in unreadable, undocummented and undecompilable (at least for non jvm developers) format which is characteristic to particular JVM implementation
Someone who wants your code badly enough is going to
get it.sure but I try to make him this task as difficult as I can ;)
Best Regards
Boloek