David Evanshttp://www.cs.virginia.edu/evans

CS551/651: Dependable ComputingUniversity of VirginiaComputer Science

Static Analysis

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Menu

• Validation• Why Static Analysis is Impossible• Why we do it anyway• Static Analysis Tools

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How do you decide is a system is dependable?

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Validation

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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?

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Sun’s Java License5.  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.  …

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Java’s License2.  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. 

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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

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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

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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?

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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

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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”

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Hopelessness of Analysis

It is impossible to correctly decide if any interesting property is true for an arbitrary program!

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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}

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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) ?

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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;”)));}

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Give Up?

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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)

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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

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SplintAnnotation-assisted lightweight analysis tool for C

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A Gross Oversimplification

Effort RequiredLow Unfathomable

Formal Verifiers

Bug

s D

etec

ted

none

all

Compilers

SplintSplint

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(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

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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

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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

AttributesLimitations of

Standard Types

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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

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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);

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Example

1 int 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 nulldummy.c:4:14: Fresh storage ip not released before return

dummy.c:2:43: Fresh storage ip allocated

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Security Flaws

Malformed Input16%

Resource Leaks

6%

Format Bugs6%

Buffer Overflows

19%

Access16%

Pathnames10%

Symbolic Links11%

Other16%

Reported flaws in Common Vulnerabilities and Exposures Database, Jan-Sep 2001.[Evans & Larochelle, IEEE Software, Jan 2002.]

190 VulnerabilitiesOnly 4 having to do with crypto108 of them could have been

detected with simple static analyses!

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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

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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

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Defining Opennessattribute 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 ==> openend

Cannot abandon FILE in open state

Object cannot be openon one path, closed on another

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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);

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Reading, ‘Riting, ‘Rithmeticattribute 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 ==> rweitherend

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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@*/ ;

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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.)

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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 = openf:rwness = rweither

Branches join in incompatible states: f is closed on true branch,open on false branch

f:openness = closed, f:rwness = rwnone

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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

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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