Introduction to Static Analysis David Evans 1 March

Introduction to Static Analysis David Evans 1 March 2004 http: //www. cs. virginia. edu/evans These slides: http: //www. cs. virginia. edu/evans/talks/cs 340 -s 04. ppt Static Analysis

How do you decide if a system is dependable? 1 March 2004 Static Analysis 2

Validation: 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? 1 March 2004 Static Analysis 3

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 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. … 1 March 2004 Static Analysis 4

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. 1 March 2004 Static Analysis 5

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 1 March 2004 Static Analysis 6

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 1 March 2004 Static Analysis 7

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? 1 March 2004 Static Analysis 8

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 1 March 2004 Static Analysis 9

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” 1 March 2004 Static Analysis 10

Hopelessness of Analysis It is impossible to correctly decide if any interesting property is true for an arbitrary program! 1 March 2004 Static Analysis 11

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 always. Halts (Program p) { … // returns true iff p will halt } 1 March 2004 Static Analysis 12

Informal Proof • Suppose we could write always. Halts. • Proof by contradiction: bool contradict. Halts () { if (always. Halts (contradict. Halts)) { while (true) ; // loop forever } else { return false; } } What is always. Halts (contradict. Halts) ? 1 March 2004 Static Analysis 13

Hopelessness of Analysis • But this means, we can’t write a program that decides any other interesting property either: bool dereferences. Null (Program p) // EFFECTS: Returns true if p ever dereferences null, // false otherwise. bool always. Halts (Program p) { return (derefences. Null (new Program (“p (); *NULL; ”))); } 1 March 2004 Static Analysis 14

Give Up and go to the Beach? 1 March 2004 Static Analysis 15

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) 1 March 2004 Static Analysis 16

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 1 March 2004 Static Analysis 17

Splint Annotation-assisted lightweight analysis tool for C 1 March 2004 Static Analysis 18

A Gross Oversimplification all Bugs Detected Formal Verifiers nt i l Sp Compilers none Low 1 March 2004 Effort Required Static Analysis Unfathomable 19

(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 1 March 2004 Static Analysis 20

Limitations of Standard Types Type of reference never changes Language defines checking rules One type per reference 1 March 2004 State changes along program paths System or programmer defines checking rules Many attributes per reference Static Analysis 21

Limitations of Standard Types Type of reference never changes Language defines checking rules One type per reference 1 March 2004 Attributes State changes along program paths System or programmer defines checking rules Many attributes per reference Static Analysis 22

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 1 March 2004 Static Analysis 23

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 *); 1 March 2004 Static Analysis 24

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 1 March 2004 Static Analysis 25

Security Flaws 190 Vulnerabilities Only 4 having to do with crypto 108 of them could have been Reported flaws in Common Vulnerabilities and detected with simple Exposures Database, Jan-Sep 2001. [Evans & Larochelle, IEEE Software, Jan 2002. ] static analyses! 1 March 2004 Static Analysis 26

Example: Buffer Overflows • 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 1 March 2004 Static Analysis 27

Other Static Analysis Tools • ESC/Java (DEC/Compaq/HP SRC) – Annotations describe invariants – Detects possible Run. Time execptions • Coverity SWAT (Coverity, Inc. , was Stanford research project) – Statistical analysis of large code bases to find anomalies – Has found thousands of flaws in Linux • PREfix (Microsoft) – No user annotations, but models library functions – Used on Windows, Office, etc. code base • Heuristics to prioritize thousands of warnings 1 March 2004 Static Analysis 28

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 http: //www. cs. virginia. edu/evans/talks/cs 340 -s 04. ppt 1 March 2004 Static Analysis 29