David Evans CS551/651: Dependable Computing University of Virginia Computer Science Static Analysis

25 September 2003Dependable Computing Fall Menu Validation Why Static Analysis is Impossible Why we do it anyway Static Analysis Tools

25 September 2003Dependable Computing Fall How do you decide is a system is dependable?

25 September 2003Dependable Computing Fall Validation

25 September 2003Dependable Computing Fall Dictionary Definition val·i·date 1.To declare or make legally valid. 2.To mark with an indication of official sanction. 3.To establish the soundness of; corroborate. Can we do any of these with software?

25 September 2003Dependable Computing Fall 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. …

25 September 2003Dependable Computing Fall 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.

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall 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?

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall 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”

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

25 September 2003Dependable Computing Fall 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 }

25 September 2003Dependable Computing Fall 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) ?

25 September 2003Dependable Computing Fall 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;”))); }

25 September 2003Dependable Computing Fall Give Up?

25 September 2003Dependable Computing Fall 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)

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall Splint Annotation-assisted lightweight analysis tool for C

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

25 September 2003Dependable Computing Fall (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

25 September 2003Dependable Computing Fall Limitations of Standard Types Type of reference never changes State changes along program paths Language defines checking rules System or programmer defines checking rules One type per reference Many attributes per reference

25 September 2003Dependable Computing Fall Type of reference never changes State changes along program paths Language defines checking rules System or programmer defines checking rules One type per reference Many attributes per reference Attributes Limitations of Standard Types

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall Sample Annotation: only 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 *); extern only char *gptr; extern only out null void *malloc (int);

25 September 2003Dependable Computing Fall Example 1int dummy (void) { 2 int *ip= (int *) malloc (sizeof (int)); 3 *ip = 3; 4 return *ip; 5 } extern only null void *malloc (int); in library 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

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

25 September 2003Dependable Computing Fall Example: Buffer Overflows David 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

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall 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 Cannot abandon FILE in open state Object cannot be open on one path, closed on another

25 September 2003Dependable Computing Fall Specifying I/O Functions FILE *fopen (const char *filename, const char *mode); int fclose FILE *stream) closed ; char *fgets (char *s, int n, FILE *stream);

25 September 2003Dependable Computing Fall 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

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

25 September 2003Dependable Computing Fall 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.)

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

25 September 2003Dependable Computing Fall 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

25 September 2003Dependable Computing Fall 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: