David Evans cs.virginia/evans

1. Static Analysis David Evans http://www.cs.virginia.edu/evans CS551/651: Dependable Computing University of Virginia Computer Science

2. Menu • Validation • Why Static Analysis is Impossible • Why we do it anyway • Static Analysis Tools Dependable Computing Fall 2003

3. How do you decide is a system is dependable? Dependable Computing Fall 2003

4. Validation Dependable Computing Fall 2003

5. Dictionary Definition val·i·date • To declare or make legally valid. • To mark with an indication of official sanction. • To establish the soundness of; corroborate. Can we do any of these with software? Dependable Computing Fall 2003

6. Sun’s Java License 5.  LIMITATION OF LIABILITY.  TO THE EXTENT NOT PROHIBITED BY LAW, IN NO EVENT WILL SUN OR ITS LICENSORS BE LIABLE FOR ANY LOST REVENUE, PROFIT OR DATA, OR FOR SPECIAL, INDIRECT, CONSEQUENTIAL, INCIDENTAL OR PUNITIVE DAMAGES, HOWEVER CAUSED REGARDLESS OF THE THEORY OF LIABILITY, ARISING OUT OF OR RELATED TO THE USE OF OR INABILITY TO USE SOFTWARE, EVEN IF SUN HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.  … Dependable Computing Fall 2003

7. Java’s License 2.  RESTRICTIONS.  … Unless enforcement is prohibited by applicable law, you may not modify, decompile, or reverse engineer Software.  You acknowledge that Software is not designed, licensed or intended for use in the design, construction, operation or maintenance of any nuclear facility.  Sun disclaims any express or implied warranty of fitness for such uses.  Dependable Computing Fall 2003

8. Software Validation • Process designed to increase our confidence that a program works as intended • For complex programs, cannot often make guarantees • This is why typical software licenses don’t make any claims about their program working Dependable Computing Fall 2003

9. Increasing Confidence • Testing • Run the program on set of inputs and check the results • Verification • Argue formally or informally that the program always works as intended • Analysis • Poor programmer’s verification: examine the source code to increase confidence that it works as intended Dependable Computing Fall 2003

10. Testing • If all the test cases produce the correct results, you know that a particular execution of the program on each of the test cases produced the correct result • Concluding that this means the program is correct is like concluding there are no fish in the river because you didn’t catch one! • What makes a good test case? Dependable Computing Fall 2003

11. Analysis • Make claims about all possible paths by examining the program code directly, not executing it • Use formal semantics of programming language to know what things mean • Use formal specifications of procedures to know that they do Dependable Computing Fall 2003

12. Example Software Properties • Does what the customer wants • Does what the programmer intends • Doesn’t do anything dangerous • Always eventually halts • Never dereferences null • Always opens a file before writing to it • Never prints “3” Dependable Computing Fall 2003

13. Hopelessness of Analysis It is impossible to correctly decide if any interesting property is true for an arbitrary program! Dependable Computing Fall 2003

14. Halting Problem • Can we write a program that takes any program as input and returns true if that program always halts, and returns false if it sometimes doesn’t halt. bool alwaysHalts (Program p) { … // returns true iff p will halt } Dependable Computing Fall 2003

15. Informal Proof • Suppose we could write alwaysHalts. • Proof by contradiction: bool contradictHalts () { if (alwaysHalts (contradictHalts)) { while (true) ; // loop forever } else { return false; } } What is alwaysHalts (contradictHalts) ? Dependable Computing Fall 2003

16. Hopelessness of Analysis • But this means, we can’t write a program that decides any other interesting property either: bool dereferencesNull (Program p) // EFFECTS: Returns true if p ever dereferences null, // false otherwise. bool alwaysHalts (Program p) { return (derefencesNull (new Program (“p (); *NULL;”))); } Dependable Computing Fall 2003

17. Give Up? Dependable Computing Fall 2003

18. Compromises • Only work for some programs • Accept unsoundness and incompleteness • False positives: sometimes an analysis tool will report warnings for a program, when the program is actually okay (incompleteness – can’t prove a property that is true) • False negatives: sometimes an analysis tool will report no warnings for a program, even when the program violates properties it checks (unsoundness – proves a property that is not true) Dependable Computing Fall 2003

19. Properties to Analyze • Generic Properties • Dangerous Code • C: memory leaks, dereferencing null, type mismatches, undefined behavior, etc. • Concurrency: race conditions, deadlocks • Don’t need a specification (but it may help across procedure boundaries) • Application-Specific Properties • Need some way of describing the properties we want Dependable Computing Fall 2003

20. Splint Annotation-assisted lightweight analysis tool for C Dependable Computing Fall 2003

21. A Gross Oversimplification all Formal Verifiers Bugs Detected Splint Compilers none Low Unfathomable Effort Required Dependable Computing Fall 2003

22. (Almost) Everyone Likes Types • Easy to Understand • Easy to Use • Quickly Detect Many Programming Errors • Useful Documentation • …even though they are lots of work! • 1/4 of text of typical C program is for types Dependable Computing Fall 2003

23. Limitations of Standard Types Dependable Computing Fall 2003

24. Attributes Limitations of Standard Types Dependable Computing Fall 2003

25. Approach • Programmers add annotations (formal specifications) • Simple and precise • Describe programmers intent: • Types, memory management, data hiding, aliasing, modification, null-ity, buffer sizes, security, etc. • Splint detects inconsistencies between annotations and code • Simple (fast!) dataflow analyses Dependable Computing Fall 2003

26. Sample Annotation: only extern only char *gptr; extern only out null void *malloc (int); • Reference (return value) owns storage • No other persistent (non-local) references to it • Implies obligation to transfer ownership • Transfer ownership by: • Assigning it to an external only reference • Return it as an only result • Pass it as an only parameter: e.g., extern void free (only void *); Dependable Computing Fall 2003

27. Example • extern only null void *malloc (int); in library 1 int dummy (void) { 2 int *ip= (int *) malloc (sizeof (int)); 3 *ip = 3; 4 return *ip; 5 } Splint output: dummy.c:3:4: Dereference of possibly null pointer ip: *ip dummy.c:2:13: Storage ip may become null dummy.c:4:14: Fresh storage ip not released before return dummy.c:2:43: Fresh storage ip allocated Dependable Computing Fall 2003

28. Security Flaws 190 Vulnerabilities Only 4 having to do with crypto 108 of them could have been detected with simple static analyses! Reported flaws in Common Vulnerabilities and Exposures Database, Jan-Sep 2001. [Evans & Larochelle, IEEE Software, Jan 2002.] Dependable Computing Fall 2003

29. Example: Buffer OverflowsDavid Larochelle • Most commonly exploited security vulnerability • 1988 Internet Worm • Still the most common attack • Code Red exploited buffer overflow in IIS • >50% of CERT advisories, 23% of CVE entries in 2001 • Attributes describe sizes of allocated buffers • Heuristics for analyzing loops • Found several known and unknown buffer overflow vulnerabilities in wu-ftpd Dependable Computing Fall 2003

30. Defining Properties to Check • Many properties can be described in terms of state attributes • A file is open or closed • fopen: returns an open file • fclose: openclosed • fgets, etc. require open files • Reading/writing – must reset between certain operations Dependable Computing Fall 2003

31. Object cannot be open on one path, closed on another Cannot abandon FILE in open state Defining Openness attribute openness context reference FILE * oneof closed, open annotations open ==> open closed ==> closed transfers open as closed ==> error closed as open ==> error merge open + closed ==> error losereference open ==> error "file not closed" defaults reference ==> open end Dependable Computing Fall 2003

32. Specifying I/O Functions /*@open@*/ FILE *fopen (const char *filename, const char *mode); int fclose (/*@open@*/ FILE *stream) /*@ensures closed stream@*/ ; char *fgets (char *s, int n, /*@open@*/ FILE *stream); Dependable Computing Fall 2003

33. Reading, ‘Riting, ‘Rithmetic attribute rwness context reference FILE * oneof rwnone, rwread, rwwrite, rweither annotations read ==> rwread write ==> rwwrite rweither ==> rweither rwnone ==> rwnone merge rwread + rwwrite ==> rwnone rwnone + * ==> rwnone rweither + rwread ==> rwread rweither + rwwrite ==> rwwrite transfers rwread as rwwrite ==> error "Must reset file between read and write." rwwrite as rwread ==> error "Must reset file between write and read." rwnone as rwread ==> error "File in unreadable state." rwnone as rwwrite ==> error "File in unwritable state." rweither as rwwrite ==> rwwrite rweither as rwread ==> rwread defaults reference ==> rweither end Dependable Computing Fall 2003

34. Reading, ‘Righting /*@rweither@*/ FILE *fopen (const char *filename, const char *mode) ; int fgetc (/*@read@*/ FILE *f) ; int fputc (int, /*@write@*/ FILE *f) ; /* fseek resets the rw state of a stream */ int fseek (/*@rweither@*/ FILE *stream, long int offset, int whence) /*@ensures rweither stream@*/ ; Dependable Computing Fall 2003

35. Checking • Simple dataflow analysis • Intraprocedural – except uses annotations to alter state around procedure calls • Integrates with other Spint analyses (e.g., nullness, aliases, ownership, etc.) Dependable Computing Fall 2003

36. f:openness = open f:rwness = rweither Possibly null reference f passed where non-null expected f:openness = open, f:rwness = rwread Attribute mismatch – passed read where write FILE * expected. f:openness = closed, f:rwness = rwnone Branches join in incompatible states: f is closed on true branch,open on false branch Example FILE *f = fopen (fname, “rw”); int i = fgetc (f); if (i != EOF) { fputc (i, f); fclose (f); } Dependable Computing Fall 2003

37. Other Static Analysis Tools • PREfix (Microsoft) • C/C++ defect detection, no user annotations (models of library functions) • Runs on Windows, Office, etc. code base • Thousands of warnings, prioritize those most likely to be interesting • ESC/Java (Compaq SRC) • Annotations describe invariants • Warnings where Java programs could raise RunTime exceptions, concurrency issues Dependable Computing Fall 2003

38. Summary • Redundancy is good for dependability • Static analysis tools can check redundant information is consistent • Any useful property is impossible to decide soundly and completely (but, lots of useful checking can still be done) • For more on Splint: www.splint.org Dependable Computing Fall 2003