Bytecode Verification on Java Smart cards

1. Bytecode Verification on Java Smart cards Xavier Leroy Presentation(Day 2) - Nithya

2. JSR • Subroutines -> mostly used for compiling the try-finally construct • Subroutines and Sun’s Verfn algorithm • Subroutines and our Verfn Algorithm • JSR instruction • Ret instruction • Role of local variable

3. Algorithm

4. JSR vs invoke • Methods : invokevirtual, invokenonvirtual, return, areturn, Ireturn • JSR instruction and ret instruction

5. OffCard code Transformations • Two methods to ensure that all correct applets pass verification: • Using special Java compiler • Using a std Java Compiler and Java Card Converter (for Off-card code Transformation) and pass it to on card verifier

6. Applet Conversion

7. Applet Installation

8. Architecture of the System

9. Transformations • Stack Normalisation • Register Reallocation

10. Stack Normalisation For a branch with non-empty stack: • Insert stores to fresh registers before the branch • Loads from the same registers at the branch target

11. Example : C.m(b ? x : y);

12. Second pass of SN • Case 1: if i is a branch target with non-empty stack • Case 2: if i is a branch to instruction j and the operand stack is not empty at j.

13. 1) i ->Branch Target • Case a: If instruction before i doesnt fall thru (uncndl branch/return/throw), • Insert loads from l1…ln before i • Redirect the branches to the first load thus inserted

14. Case a

15. 1)i ->Branch Target • Case b: If the instruction before i falls thru: • Insert stores to ln..l1 • Load from l1..ln, before i

16. Case b

17. 2) i ->Branch to instruction j • Case a: If instruction i does not fall through (unconditional branch): • Insert before i code to swap the top k words of the stack with the n words • Insert stores ln…l1

18. Case a

19. 2)I ->Branch to instruction j • Case b: If instruction I can fall thru (conditional branch) • Insert after I, loads from l1…ln

20. Case b

21. Worst case • Example of combination of two transformations: • The instruction before i falls through • i itself falls through

22. Worst case

23. Tunneling optimizations • Idea: reduce the number of branches • Replace branches “goto lbl” by a direct branch to lbl • Replace unconditional branches “return” or “throw” by a copy of the return or a throw instruction itself

24. Example

25. Tunneling optimization • Conforms to Requirement R1 • No stack Normalisation needed for this code

26. Before Register reallocation

27. After Register reallocation Number of registers stays constant

28. Chaitin’s graph coloring allocator • Compute live ranges for every register • Compute principal type for every live range • Build the interference graph between live ranges • Nodes -> live ranges • Add interference edges between live ranges that dont have same principal type • Coalescing: Detect reg-to-reg copies • Color the interference graph: • Assign a new reg number to every live range that 2 interfering live edges have distinct reg numbers

29. After compilation and stack normalisation ->JCVM code:

30. After coalescingsload Rtmp, sstore Rs : Short s; if (b) {s=x;} else{s=y;}

31. Effect of offcard code transformation on code size and register

32. Comments?