Static Program Analysis of Embedded Software Ramakrishnan Venkitaraman Graduate Student, Computer Science Advisor: Dr. Gopal Gupta

Software Reuse & System Integration But, the Integrated System does not work Cost of Project Companies

Outline Importance of Software Standards. Static Analysis based tool developed by us to enforce software standard compliance. How the marriage between industry and university research increases software reuse.

Why Software Standard? Incompatibilities make integration difficult. Complexity in software reuse. COTS Marketplace. Time to Market.

TI XDAIS Standard Contains 35 rules and 15 guidelines. SIX General Programming Rules. No tool currently exists to check for compliance. We want to build a tool to ENFORCE software compliance for these rules.

Problem and Solution Problem: Detection of hard coded addresses in programs without accessing source code. Solution: “Static Program Analysis”

Interest in Static Analysis “We actually went out and bought for 30 million dollars, a company that was in the business of building static analysis tools and now we want to focus on applying these tools to large-scale software systems ” Remarks by Bill Gates, 17th Annual ACM Conference on Object-Oriented Programming, Systems, Languages and Application, November 2002.

Static Analysis Defined as any analysis of a program carried out without completely executing the program. Un-decidability: Impossible to build a tool that will precisely detect hard coding.

Hard Coded Addresses Bad Programming Practice. Results in non relocatable code. Results in non reusable code.

Overview Of Our Approach Input: Object Code of the Software Output: Compliant or Not Compliant status Activity Diagram for our Static Analyzer Disassemble Object Code Split Into Functions Obtain Basic Blocks Obtain Flow Graph Static Analysis Output the Result

Basic Aim Of Analysis Find a path to trace pointer origin. Problem: Exponential Complexity Static Analysis approximation makes it linear

Analyzing Source Code – Easy { { q } } { { p } } P IS HARD CODED So, the program is not compliant with the standard

Analyzing Assembly – Hard A0 main: A0 07BD09C2 SUB.D2 SP,0x8,SP A4 020FA02A MVK.S2 0x1f40,B A8 023C22F6 STW.D2T2 B4,*+SP[0x1] AC NOP B0 023C42F6 STW.D2T2 B4,*+SP[0x2] B NOP B8 0280A042 MVK.D2 5,B BC F6 STW.D2T2 B5,*+B4[0x0] C NOP C4 008C8362 BNOP.S2 B3, C8 07BD0942 ADD.D2 SP,0x8,SP CC NOP D NOP {{ }} { { B4 } } B4 = 0x1f40 So, B4 is HARD CODED Code is NOT Compliant

Phases In Analysis Phase 1: Find the set of dereferenced pointers. Phase 2: Check the safety of dereferenced pointers.

Building Unsafe Sets (Phase 1) The first element is added to the unsafe set during pointer dereferencing. E.g. If “*Reg” in the disassembled code, the unsafe set is initialized to {Reg}. ‘N’ Pointers Dereferenced  ‘N’ Unsafe sets Maintained as SOUS (Set Of Unsafe Sets)

Populating Unsafe Sets (Phase 2) For e.g., if Reg = reg1 + reg2, the element “Reg” is deleted from the unsafe set, and the elements “reg1”, “reg2”, are inserted into the unsafe set. Contents of the unsafe set will now become {reg1, reg2}.

Handling Loops Complex: # iterations of loop may not be known until runtime. Cycle the loop until the unsafe set reaches a “fixed point”. No new information is added to the unsafe set during successive iterations.

Merging Information If no merging, then exponential complexity. Mandatory when loops Information loss. If (Cond) Then Block B Else Block C Block D Block A Block E

Extensive Compliance Checking Handle all cases occurring in programs. Single pointer, double pointer, triple pointer… Global pointer variables. Static and Dynamic arrays.

Extensive Compliance Checking Loops – all forms (e.g. for, while…) Function calls. Pipelining and Parallelism. Merging information from multiple paths.

Analysis Stops when… Compliance of all the pointers are established. Errors and warnings are reported. Log file containing statistics of the analysis is created.

Analysis Results Program# Lines# * Ptrs # Hard Coded Chain Length Running Time (ms) t_read timer mcbsp figtest m_hdrv dat gui_codec codec stress demo

Current Status and Future Work Prototype Implementation done But, context insensitive, intra-procedural Extend to context sensitive, inter-procedural. Extend compliance check for other rules.

So… Hard Coding is a bad programming practice. Non relocatable/reusable code. A Static Analysis based technique is useful and practical.

Software Reuse & System Integration WOW!!!! It works… Select ONLY Compliant Software

Questions… More Information: Ramakrishnan Venkitaraman

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General Programming Rules No tool currently exists to check for compliance. SIX rules. 1)All programs should follow the runtime conventions of TI’s C programming language. 2)Algorithms must be re-entrant. 3)No hard coded data memory locations. 4)No hard coded program memory locations. 5)Algorithms must characterize their ROM-ability. 6)No peripheral device accesses.

Some examples showing hardcoding void main() { int * p = 0x8800; // Some code *p = …; } Example1: Directly Hardcoded void main() { int *p = 0x80; int *q = p; //Some code *q = …; } Example2: Indirectly Hardcoded void main() { int *p, val; p = ….; val = …; if(val) p = 0x900; else p = malloc(…); *p; } Example3: Conditional Hardcoding NOTE: We don’t care if a pointer is hard coded and is never dereferenced.

Sample Code

Fig. Flow Graph