HACKING: THE ART OF EXPLOITATION
I N T R O D U C T I O N
The idea of hacking may conjure stylized images of electronic vandalism, espionage, dyed hair, and body piercings. Most people associate hacking with breaking the law and assume that everyone who engages in hacking activities is a criminal. Granted, there are people out
there who use hacking techniques to break the law, but hacking isn�t really about that. In fact, hacking is more about following the law than breaking it. The essence of hacking is finding unintended or overlooked uses for the laws and properties of a given situation and then applying them in new and inventive ways to solve a problem�whatever it may be.
The following math problem illustrates the essence of hacking:
Use each of the numbers 1, 3, 4, and 6 exactly once with any of the four basic math operations (addition, subtraction, multiplication, and division) to total 24. Each number must be used once and only once, and you may define the order of operations; for example, 3 * (4 + 6) + 1 = 31 is valid, however incorrect, since it doesn�t total 24.
The rules for this problem are well defined and simple, yet the answer eludes many. Like the solution to this problem (shown on the last page of this book), hacked solutions follow the rules of the system, but they use those rules in counterintuitive ways. This gives hackers their edge, allowing them to solve problems in ways unimaginable for those confined to conventional thinking and methodologies.
�Since the infancy of computers, hackers have been creatively solving problems. In the late 1950s, the MIT model railroad club was given a donation of parts, mostly old telephone equipment. The club�s members used this equipment to rig up a complex system that allowed multiple operators to control different parts of the track by dialing in to the appropriate sections. They called this new and inventive use of telephone equipment hacking; many people consider this group to be the original hackers. The group moved on to programming on punch cards and ticker tape for early computers like the IBM 704 and the TX-0. While others were content with writing programs that just solved problems, the early hackers were obsessed with writing programs that solved problems well. A new program that could achieve the same result as an existing one but used fewer punch cards was considered better, even though it did the same thing. The key difference was how the program achieved its results�elegance.
Being able to reduce the number of punch cards needed for a program showed an artistic mastery over the computer. A nicely crafted table can hold a vase just as well as a milk crate can, but one sure looks a lot better than the other. Early hackers proved that technical problems can have artistic solutions, and they thereby transformed programming from a mere engineering task into an art form.
Like many other forms of art, hacking was often misunderstood. The few who got it formed an informal subculture that remained intensely focused on learning and mastering their art. They believed that information should be free and anything that stood in the way of that freedom should be circumvented. Such obstructions included authority figures, the bureaucracy of college classes, and discrimination. In a sea of graduation-driven students, this unofficial group of hackers defied conventional goals and instead pursued knowledge itself. This drive to continually learn and explore transcended even the conventional boundaries drawn by discrimination, evident in the MIT model railroad club�s acceptance of 12-year-old Peter Deutsch when he demonstrated his knowledge of the TX-0 and his desire to learn. Age, race, gender, appearance, academic degrees, and social status were not primary criteria for judging another�s worth�not because of a desire for equality, but because of a desire to advance the emerging art of hacking.
�The original hackers found splendor and elegance in the conventionally dry sciences of math and electronics. They saw programming as a form of artistic expression and the computer as an instrument of that art. Their desire to dissect and understand wasn�t intended to demystify artistic endeavors; it was simply a way to achieve a greater appreciation of them. These knowledgedriven values would eventually be called the Hacker Ethic: the appreciation of logic as an art form and the promotion of the free flow of information, surmounting conventional boundaries and restrictions for the simple goal of
0x100
better understanding the world. This is not a new cultural trend; the Pythagoreans in ancient Greece had a similar ethic and subculture, despite not owning computers. They saw beauty in mathematics and discovered many core concepts in geometry. That thirst for knowledge and its beneficial byproducts would continue on through history, from the Pythagoreans to Ada Lovelace to Alan Turing to the hackers of the MIT model railroad club. Modern hackers like Richard Stallman and Steve Wozniak have continued the hacking legacy, bringing us modern operating systems, programming languages, personal computers, and many other technologies that we use every day.
�How does one distinguish between the good hackers who bring us the wonders of technological advancement and the evil hackers who steal our credit card numbers? The term cracker was coined to distinguish evil hackers from the good ones. Journalists were told that crackers were supposed to be the bad guys, while hackers were the good guys. Hackers stayed true to the Hacker Ethic, while crackers were only interested in breaking the law and making a quick buck. Crackers were considered to be much less talented than the elite hackers, as they simply made use of hacker-written tools and scripts without understanding how they worked. Cracker was meant to be the catch-all label for anyone doing anything unscrupulous with a computer� pirating software, defacing websites, and worst of all, not understanding what they were doing. But very few people use this term today.
�The term�s lack of popularity might be due to its confusing etymology� cracker originally described those who crack software copyrights and reverse engineer copy-protection schemes. Its current unpopularity might simply result from its two ambiguous new definitions: a group of people who engage in illegal activity with computers or people who are relatively unskilled hackers. Few technology journalists feel compelled to use terms that most of their readers are unfamiliar with. In contrast, most people are aware of the mystery and skill associated with the term hacker, so for a journalist, the decision to use the term hacker is easy. Similarly, the term script kiddie is sometimes used to refer to crackers, but it just doesn�t have the same zing as the shadowy hacker. There are some who will still argue that there is a distinct line between hackers and crackers, but I believe that anyone who has the hacker spirit is a hacker, despite any laws he or she may break.
The current laws restricting cryptography and cryptographic research further blur the line between hackers and crackers. In 2001, Professor Edward Felten and his research team from Princeton University were about to publish a paper that discussed the weaknesses of various digital watermarking schemes. This paper responded to a challenge issued by the Secure Digital Music Initiative (SDMI) in the SDMI Public Challenge, which encouraged the public to attempt to break these watermarking schemes. Before Felten and his team could publish the paper, though, they were threatened by both the SDMI Foundation and the Recording Industry Association of America (RIAA). The Digital Millennium Copyright Act (DCMA) of 1998 makes it illegal to discuss or provide technology that might be used to bypass industry consumer controls. This same law was used against Dmitry Sklyarov, a Russian computer programmer and hacker. He had written software to circumvent
Introduction��� 3
overly simplistic encryption in Adobe software and presented his findings at a hacker convention in the United States. The FBI swooped in and arrested him, leading to a lengthy legal battle. Under the law, the complexity of the industry consumer controls doesn�t matter�it would be technically illegal to reverse engineer or even discuss Pig Latin if it were used as an industry consumer control. Who are the hackers and who are the crackers now? When laws seem to interfere with free speech, do the good guys who speak their minds suddenly become bad? I believe that the spirit of the hacker transcends governmental laws, as opposed to being defined by them.
The sciences of nuclear physics and biochemistry can be used to kill, yet they also provide us with significant scientific advancement and modern medicine. There�s nothing good or bad about knowledge itself; morality lies in the application of knowledge. Even if we wanted to, we couldn�t suppress the knowledge of how to convert matter into energy or stop the continued technological progress of society. In the same way, the hacker spirit can never be stopped, nor can it be easily categorized or dissected. Hackers will constantly be pushing the limits of knowledge and acceptable behavior, forcing us to explore further and further.
Part of this drive results in an ultimately beneficial co-evolution of security through competition between attacking hackers and defending hackers. Just as the speedy gazelle adapted from being chased by the cheetah, and the cheetah became even faster from chasing the gazelle, the competition between hackers provides computer users with better and stronger security, as well as more complex and sophisticated attack techniques. The introduction and progression of intrusion detection systems (IDSs) is a prime example of this co-evolutionary process. The defending hackers create IDSs to add to their arsenal, while the attacking hackers develop IDS-evasion techniques, which are eventually compensated for in bigger and better IDS products. The net result of this interaction is positive, as it produces smarter people, improved security, more stable software, inventive problem-solving techniques, and even a new economy.
The intent of this book is to teach you about the true spirit of hacking. We will look at various hacker techniques, from the past to the present, dissecting them to learn how and why they work. Included with this book is a bootable LiveCD containing all the source code used herein as well as a preconfigured Linux environment. Exploration and innovation are critical to the art of hacking, so this CD will let you follow along and experiment on your own. The only requirement is an x86 processor, which is used by all Microsoft Windows machines and the newer Macintosh computers�just insert the CD and reboot. This alternate Linux environment will not disturb your existing OS, so when you�re done, just reboot again and remove the CD. This way, you will gain a hands-on understanding and appreciation for hacking that may inspire you to improve upon existing techniques or even to invent new ones. Hopefully, this book will stimulate the curious hacker nature in you and prompt you to contribute to the art of hacking in some way, regardless of which side of the fence you choose to be on.
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0x200
P R O G R A M M I N G
Hacker is a term for both those who write code and those who exploit it. Even though these two groups of hackers have different end goals, both groups use similar problem-solving techniques. Since an understanding of programming helps those who exploit, and an understanding of exploitation helps those who program, many
hackers do both. There are interesting hacks found in both the techniques used to write elegant code and the techniques used to exploit programs. Hacking is really just the act of finding a clever and counterintuitive solution to a problem.
The hacks found in program exploits usually use the rules of the computer to bypass security in ways never intended. Programming hacks are similar in that they also use the rules of the computer in new and inventive ways, but the final goal is efficiency or smaller source code, not necessarily a security compromise. There are actually an infinite number of programs that can be written to accomplish any given task, but most of these solutions are unnecessarily large, complex, and sloppy. The few solutions that remain are small, efficient, and neat. Programs that have these qualities are said to have elegance, and the clever and inventive solutions that tend to lead to this efficiency are called hacks. Hackers on both sides of programming appreciate both the beauty of elegant code and the ingenuity of clever hacks.
In the business world, more importance is placed on churning out functional code than on achieving clever hacks and elegance. Because of the tremendous exponential growth of computational power and memory, spending an extra five hours to create a slightly faster and more memoryefficient piece of code just doesn�t make business sense when dealing with modern computers that have gigahertz of processing cycles and gigabytes of memory. While time and memory optimizations go without notice by all but the most sophisticated of users, a new feature is marketable. When the bottom line is money, spending time on clever hacks for optimization just doesn�t make sense.
True appreciation of programming elegance is left for the hackers: computer hobbyists whose end goal isn�t to make a profit but to squeeze every possible bit of functionality out of their old Commodore 64s, exploit writers who need to write tiny and amazing pieces of code to slip through narrow security cracks, and anyone else who appreciates the pursuit and the challenge of finding the best possible solution. These are the people who get excited about programming and really appreciate the beauty of an elegant piece of code or the ingenuity of a clever hack. Since an understanding of programming is a prerequisite to understanding how programs can be exploited, programming is a natural starting point.
0x210 What Is Programming?
Programming is a very natural and intuitive concept. A program is nothing more than a series of statements written in a specific language. Programs are everywhere, and even the technophobes of the world use programs every day. Driving directions, cooking recipes, football plays, and DNA are all types of programs. A typical program for driving directions might look something like this:
Start out down Main Street headed east. Continue on Main Street until you see a church on your right. If the street is blocked because of construction, turn right there at 15th Street, turn left on Pine Street, and then turn right on 16th Street. Otherwise, you can just continue and make a right on 16th Street. Continue on 16th Street, and turn left onto Destination Road. Drive straight down Destination Road for 5 miles, and then you'll see the house on the right. The address is 743 Destination Road.
Anyone who knows English can understand and follow these driving directions, since they�re written in English. Granted, they�re not eloquent, but each instruction is clear and easy to understand, at least for someone who reads English.
0 x200
But a computer doesn�t natively understand English; it only understands machine language. To instruct a computer to do something, the instructions must be written in its language. However, machine language is arcane and difficult to work with�it consists of raw bits and bytes, and it differs from architecture to architecture. To write a program in machine language for an Intel x86 processor, you would have to figure out the value associated with each instruction, how each instruction interacts, and myriad low-level details. Programming like this is painstaking and cumbersome, and it is certainly not intuitive.
What�s needed to overcome the complication of writing machine language is a translator. An assembler is one form of machine-language translator�it is a program that translates assembly language into machine-readable code. Assembly language is less cryptic than machine language, since it uses names for the different instructions and variables, instead of just using numbers. However, assembly language is still far from intuitive. The instruction names are very esoteric, and the language is architecture specific. Just as machine language for Intel x86 processors is different from machine language for Sparc processors, x86 assembly language is different from Sparc assembly language. Any program written using assembly language for one processor�s architecture will not work on another processor�s architecture. If a program is written in x86 assembly language, it must be rewritten to run on Sparc architecture. In addition, in order to write an effective program in assembly language, you must still know many low-level details of the processor architecture you are writing for.
These problems can be mitigated by yet another form of translator called a compiler. A compiler converts a high-level language into machine language. High-level languages are much more intuitive than assembly language and can be converted into many different types of machine language for different processor architectures. This means that if a program is written in a highlevel language, the program only needs to be written once; the same piece of program code can be compiled into machine language for various specific architectures. C, C++, and Fortran are all examples of high-level languages. A program written in a high-level language is much more readable and English-like than assembly language or machine language, but it still must follow very strict rules about how the instructions are worded, or the compiler won�t be able to understand it.
0x220 Pseudo-code
Programmers have yet another form of programming language called pseudo-code. Pseudo-code is simply English arranged with a general structure similar to a high-level language. It isn�t understood by compilers, assemblers, or any computers, but it is a useful way for a programmer to arrange instructions. Pseudo-code isn�t well defined; in fact, most people write pseudo-code slightly differently. It�s sort of the nebulous missing link between English and high-level programming languages like C. Pseudo-code makes for an excellent introduction to common universal programming concepts.
Programming
0x230 Control Structures
Without control structures, a program would just be a series of instructions executed in sequential order. This is fine for very simple programs, but most programs, like the driving directions example, aren�t that simple. The driving directions included statements like, Continue on Main Street until you see a church on your right and If the street is blocked because of construction. . . . These statements are known as control structures, and they change the flow of the program�s execution from a simple sequential order to a more complex and more useful flow.
0x231������� If-Then-Else
In the case of our driving directions, Main Street could be under construction. If it is, a special set of instructions needs to address that situation. Otherwise, the original set of instructions should be followed. These types of special cases can be accounted for in a program with one of the most natural control structures: the if-then-else structure. In general, it looks something like this:
If ( condition) then
{
� Set of instructions to execute if the condition is met;
}
Else
{
� Set of instruction to execute if the condition is not met;
}
For this book, a C-like pseudo-code will be used, so every instruction will end with a semicolon, and the sets of instructions will be grouped with curly braces and indentation. The if-then-else pseudo-code structure of the preceding driving directions might look something like this:
Drive down Main Street;
If (street is blocked)
{
� Turn right on 15th Street;�� Turn left on Pine Street;
� Turn right on 16th Street;
}
Else
{
� Turn right on 16th Street;
}
Each instruction is on its own line, and the various sets of conditional instructions are grouped between curly braces and indented for readability. In C and many other programming languages, the then keyword is implied and therefore left out, so it has also been omitted in the preceding pseudo-code.
0 x200
Of course, other languages require the then keyword in their syntax� BASIC, Fortran, and even Pascal, for example. These types of syntactical differences in programming languages are only skin deep; the underlying structure is still the same. Once a programmer understands the concepts these languages are trying to convey, learning the various syntactical variations is fairly trivial. Since C will be used in the later sections, the pseudocode used in this book will follow a C-like syntax, but remember that pseudo-code can take on many forms.
Another common rule of C-like syntax is when a set of instructions bounded by curly braces consists of just one instruction, the curly braces are optional. For the sake of readability, it�s still a good idea to indent these instructions, but it�s not syntactically necessary. The driving directions from before can be rewritten following this rule to produce an equivalent piece of pseudo-code:
Drive down Main Street;
If (street is blocked)
{
� Turn right on 15th Street;�� Turn left on Pine Street;
� Turn right on 16th Street;
}
Else
� Turn right on 16th Street;
This rule about sets of instructions holds true for all of the control structures mentioned in this book, and the rule itself can be described in pseudo-code.
If (there is only one instruction in a set of instructions)
� The use of curly braces to group the instructions is optional;
Else
{
� The use of curly braces is necessary;
� Since there must be a logical way to group these instructions;
}
Even the description of a syntax itself can be thought of as a simple program. There are variations of if-then-else, such as select/case statements, but the logic is still basically the same: If this happens do these things, otherwise do these other things (which could consist of even more if-then statements).
0x232������� While/Until Loops
Another elementary programming concept is the while control structure, which is a type of loop. A programmer will often want to execute a set of instructions more than once. A program can accomplish this task through looping, but it requires a set of conditions that tells it when to stop looping,
Programming
lest it continue into infinity. A while loop says to execute the following set of instructions in a loop while a condition is true. A simple program for a hungry mouse could look something like this:
While (you are hungry)
{
� Find some food;
� Eat the food;
}
The set of two instructions following the while statement will be repeated while the mouse is still hungry. The amount of food the mouse finds each time could range from a tiny crumb to an entire loaf of bread. Similarly, the number of times the set of instructions in the while statement is executed changes depending on how much food the mouse finds.
Another variation on the while loop is an until loop, a syntax that is available in the programming language Perl (C doesn�t use this syntax). An until loop is simply a while loop with the conditional statement inverted. The same mouse program using an until loop would be:
Until (you are not hungry)
{
� Find some food;
� Eat the food;
}
Logically, any until-like statement can be converted into a while loop.
The driving directions from before contained the statement Continue on Main Street until you see a church on your right. This can easily be changed into a standard while loop by simply inverting the condition.
0x233�� For Loops
Another looping control structure is the for loop. This is generally used when a programmer wants to loop for a certain number of iterations. The driving direction Drive straight down Destination Road for 5 miles could be converted to a for loop that looks something like this:
In reality, a for loop is just a while loop with a counter. The same statement can be written as such:
Set the counter to 0;
While (the counter is less than 5)
{
� Drive straight for 1 mile;
� Add 1 to the counter;
}
The C-like pseudo-code syntax of a for loop makes this even more apparent:
In this case, the counter is called i, and the for statement is broken up into three sections, separated by semicolons. The first section declares the counter and sets it to its initial value, in this case 0. The second section is like a while statement using the counter: While the counter meets this condition, keep looping. The third and final section describes what action should be taken on the counter during each iteration. In this case, i++ is a shorthand way of saying, Add 1 to the counter called i.
Using all of the control structures, the driving directions from page 6 can be converted into a C-like pseudo-code that looks something like this:
Begin going East on Main Street;
While (there is not a church on the right)
� Drive down Main Street;
If (street is blocked)
{
� Turn right on 15th Street;�� Turn left on Pine Street;
� Turn right on 16th Street;
}
Else
� Turn right on 16th Street;
Turn left on Destination Road;
For (i=0; i<5; i++)
� Drive straight for 1 mile;
Stop at 743 Destination Road;
0x240 More Fundamental Programming Concepts
In the following sections, more universal programming concepts will be introduced. These concepts are used in many programming languages, with a few syntactical differences. As I introduce these concepts, I will integrate them into pseudo-code examples using C-like syntax. By the end, the pseudocode should look very similar to C code.
x241���� Variables
The counter used in the for loop is actually a type of variable. A variable can simply be thought of as an object that holds data that can be changed� hence the name. There are also variables that don�t change, which are aptly called constants. Returning to the driving example, the speed of the car would be a variable, while the color of the car would be a constant. In pseudocode, variables are simple abstract concepts, but in C (and in many other languages), variables must be declared and given a type before they can be used. This is because a C program will eventually be compiled into an executable program. Like a cooking recipe that lists all the required ingredients before giving the instructions, variable declarations allow you to make preparations before getting into the meat of the program. Ultimately, all variables are stored in memory somewhere, and their declarations allow the compiler to organize this memory more efficiently. In the end though, despite all of the variable type declarations, everything is all just memory.
In C, each variable is given a type that describes the information that is meant to be stored in that variable. Some of the most common types are int (integer values), float (decimal floating-point values), and char (single character values). Variables are declared simply by using these keywords before listing the variables, as you can see below.
int a, b; float k; char z;
The variables a and b are now defined as integers, k can accept floatingpoint values (such as 3.14), and z is expected to hold a character value, like A �or w. Variables can be assigned values when they are declared or anytime afterward, using the = operator.
int a = 13, b; float k; char z = 'A';
k = 3.14; z = 'w'; b = a + 5;
After the following instructions are executed, the variable a will contain the value of 13, k will contain the number 3.14, z will contain the character w, and b will contain the value 18, since 13 plus 5 equals 18. Variables are simply a way to remember values; however, with C, you must first declare each variable�s type.
0x242�� Arithmetic Operators
The statement b = a + 7 is an example of a very simple arithmetic operator.
In C, the following symbols are used for various arithmetic operations.
The first four operations should look familiar. Modulo reduction may seem like a new concept, but it�s really just taking the remainder after division. If a is 13, then 13 divided by 5 equals 2, with a remainder of 3, which means that a % 5 = 3. Also, since the variables a and b are integers, the statement b = a / 5 will result in the value of 2 being stored in b, since that�s the integer portion of it. Floating-point variables must be used to retain the more correct answer of 2.6.
Addition� +��������������������� b = a + 5
Subtraction������������ -���������������������� b = a - 5
Multiplication�������� *��������������������� b = a * 5
Division�� /��������������������� b = a / 5
Modulo reduction %����������������������� b = a % 5
To get a program to use these concepts, you must speak its language. The C language also provides several forms of shorthand for these arithmetic operations. One of these was mentioned earlier and is used commonly in for loops.
i = i + 1��������� i++ or ++i Add 1 to the variable. i = i - 1����������� i-- or --i Subtract 1 from the variable.
These shorthand expressions can be combined with other arithmetic operations to produce more complex expressions. This is where the difference between i++ and ++i becomes apparent. The first expression means Increment the value of i by 1 after evaluating the arithmetic operation, while the second expression means Increment the value of i by 1 before evaluating the arithmetic operation. The following example will help clarify.
int a, b; a = 5; b = a++ * 6;
At the end of this set of instructions, b will contain 30 and a will contain 6, since the shorthand of b = a++ * 6; is equivalent to the following statements:
However, if the instruction b = ++a * 6; is used, the order of the addition to a changes, resulting in the following equivalent instructions:
Since the order has changed, in this case b will contain 36, and a will still contain 6.
Quite often in programs, variables need to be modified in place. For example, you might need to add an arbitrary value like 12 to a variable, and store the result right back in that variable (for example, i = i + 12). This happens commonly enough that shorthand also exists for it.
i = i + 12������ i+=12������������ Add some value to the variable. i = i - 12���� i-=12������������� Subtract some value from the variable. i = i * 12�������� i*=12������������ Multiply some value by the variable. i = i / 12����������������� i/=12������������� Divide some value from the variable.
0x243�� Comparison Operators
Variables are frequently used in the conditional statements of the previously explained control structures. These conditional statements are based on some sort of comparison. In C, these comparison operators use a shorthand syntax that is fairly common across many programming languages.
| Less than | < | ( a < b ) |
| Greater than | > | ( a > b ) |
| Less than or equal to | <= | ( a <= b ) |
| Greater than or equal to | >= | ( a >= b ) |
| Equal to | == | ( a == b ) |
| Not equal to | != | ( a != b ) |
Most of these operators are self-explanatory; however, notice that the shorthand for equal to uses double equal signs. This is an important distinction, since the double equal sign is used to test equivalence, while the single equal sign is used to assign a value to a variable. The statement a = 7 means Put the value 7 in the variable a, while a == 7 means Check to see whether the variable a is equal to 7. (Some programming languages like Pascal actually use := for variable assignment to eliminate visual confusion.) Also, notice that an exclamation point generally means not. This symbol can be used by itself to invert any expression.
These comparison operators can also be chained together using shorthand for OR and AND.
The example statement consisting of the two smaller conditions joined with OR logic will fire true if a is less than b, OR if a is less than c. Similarly, the example statement consisting of two smaller comparisons joined with AND logic will fire true if a is less than b AND a is not less than c. These statements should be grouped with parentheses and can contain many different variations.
Many things can be boiled down to variables, comparison operators, and control structures. Returning to the example of the mouse searching for food, hunger can be translated into a Boolean true/false variable. Naturally, 1 means true and 0 means false.
While (hungry == 1)
{
� Find some food;
� Eat the food;
}
Here�s another shorthand used by programmers and hackers quite often. C doesn�t really have any Boolean operators, so any nonzero value is considered true, and a statement is considered false if it contains 0. In fact, the comparison operators will actually return a value of 1 if the comparison is true and a value of 0 if it is false. Checking to see whether the variable hungry �is equal to 1 will return 1 if hungry equals 1 and 0 if hungry equals 0. Since the program only uses these two cases, the comparison operator can be dropped altogether.
While (hungry)
{
� Find some food;
� Eat the food;
}
A smarter mouse program with more inputs demonstrates how comparison operators can be combined with variables.
While ((hungry) && !(cat_present))
{
� Find some food;
� If(!(food_is_on_a_mousetrap))
��� Eat the food;
}
This example assumes there are also variables that describe the presence of a cat and the location of the food, with a value of 1 for true and 0 for false. Just remember that any nonzero value is considered true, and the value of 0 is considered false.
0x244�� Functions
Sometimes there will be a set of instructions the programmer knows he will need several times. These instructions can be grouped into a smaller subprogram called a function. In other languages, functions are known as subroutines or procedures. For example, the action of turning a car actually consists of many smaller instructions: Turn on the appropriate blinker, slow down, check for oncoming traffic, turn the steering wheel in the appropriate direction, and so on. The driving directions from the beginning of this chapter require quite a few turns; however, listing every little instruction for every turn would be tedious (and less readable). You can pass variables as arguments to a function in order to modify the way the function operates. In this case, the function is passed the direction of the turn.
Function Turn(variable_direction)
{
� Activate the variable_direction blinker;
� Slow down;
� Check for oncoming traffic;�� while(there is oncoming traffic)
� {
��� Stop;
��� Watch for oncoming traffic;
� }
� Turn the steering wheel to the variable_direction;�� while(turn is not complete)
� {
��� if(speed < 5 mph)
����� Accelerate;
� }
� Turn the steering wheel back to the original position;
� Turn off the variable_direction blinker;
}
This function describes all the instructions needed to make a turn. When a program that knows about this function needs to turn, it can just call this function. When the function is called, the instructions found within it are executed with the arguments passed to it; afterward, execution returns to where it was in the program, after the function call. Either left or right can be passed into this function, which causes the function to turn in that direction.
By default in C, functions can return a value to a caller. For those familiar with functions in mathematics, this makes perfect sense. Imagine a function that calculates the factorial of a number�naturally, it returns the result.
In C, functions aren�t labeled with a �function� keyword; instead, they are declared by the data type of the variable they are returning. This format looks very similar to variable declaration. If a function is meant to return an integer (perhaps a function that calculates the factorial of some number x), the function could look like this:
int factorial(int x)
{�� int i;�� for(i=1; i < x; i++)���� x *= i;�� return x;
}
This function is declared as an integer because it multiplies every value from 1 to x and returns the result, which is an integer. The return statement at the end of the function passes back the contents of the variable x and ends the function. This factorial function can then be used like an integer variable in the main part of any program that knows about it.
At the end of this short program, the variable b will contain 120, since the factorial function will be called with the argument of 5 and will return 120.
Also in C, the compiler must �know� about functions before it can use them. This can be done by simply writing the entire function before using it later in the program or by using function prototypes. A function prototype is simply a way to tell the compiler to expect a function with this name, this return data type, and these data types as its functional arguments. The actual function can be located near the end of the program, but it can be used anywhere else, since the compiler already knows about it. An example of a function prototype for the factorial() function would look something like this:
Usually, function prototypes are located near the beginning of a program. There�s no need to actually define any variable names in the prototype, since this is done in the actual function. The only thing the compiler cares about is the function�s name, its return data type, and the data types of its functional arguments.
If a function doesn�t have any value to return, it should be declared as void, as is the case with the turn() function I used as an example earlier. However, the turn() function doesn�t yet capture all the functionality that our driving directions need. Every turn in the directions has both a direction and a street name. This means that a turning function should have two variables: the direction to turn and the street to turn on to. This complicates the function of turning, since the proper street must be located before the turn can be made. A more complete turning function using proper C-like syntax is listed below in pseudo-code.
void turn(variable_direction, target_street_name)
{
� Look for a street sign;
� current_intersection_name = read street sign name;�� while(current_intersection_name != target_street_name)
� {
��� Look for another street sign;
��� current_intersection_name = read street sign name;�� }
� Activate the variable_direction blinker;
� Slow down;
� Check for oncoming traffic;�� while(there is oncoming traffic)
� {
��� Stop;
��� Watch for oncoming traffic;
� }
� Turn the steering wheel to the variable_direction;�� while(turn is not complete)
� {
��� if(speed < 5 mph)
����� Accelerate;
� }
� Turn the steering wheel right back to the original position;
� Turn off the variable_direction blinker;
}
This function includes a section that searches for the proper intersection by looking for street signs, reading the name on each street sign, and storing that name in a variable called current_intersection_name. It will continue to look for and read street signs until the target street is found; at that point, the remaining turning instructions will be executed. The pseudo-code driving instructions can now be changed to use this turning function.
Begin going East on Main Street; while (there is not a church on the right)��� Drive down Main Street; if (street is blocked)
{
� Turn(right, 15th Street);
� Turn(left, Pine Street);
� Turn(right, 16th Street);
} else
� Turn(right, 16th Street); Turn(left, Destination Road); for (i=0; i<5; i++)�� Drive straight for 1 mile;
Stop at 743 Destination Road;
Functions aren�t commonly used in pseudo-code, since pseudo-code is mostly used as a way for programmers to sketch out program concepts before writing compilable code. Since pseudo-code doesn�t actually have to work, full functions don�t need to be written out�simply jotting down Do some complex stuff here will suffice. But in a programming language like C, functions are used heavily. Most of the real usefulness of C comes from collections of existing functions called libraries.
0x250 Getting Your Hands Dirty
Now that the syntax of C feels more familiar and some fundamental programming concepts have been explained, actually programming in C isn�t that big of a step. C compilers exist for just about every operating system and processor architecture out there, but for this book, Linux and an x86-based processor will be used exclusively. Linux is a free operating system that everyone has access to, and x86-based processors are the most popular consumer-grade processor on the planet. Since hacking is really about experimenting, it�s probably best if you have a C compiler to follow along with.
Included with this book is a LiveCD you can use to follow along if your computer has an x86 processor. Just put the CD in the drive and reboot your computer. It will boot into a Linux environment without modifying your existing operating system. From this Linux environment you can follow along with the book and experiment on your own.
Let�s get right to it. The firstprog.c program is a simple piece of C code that will print �Hello, world!� 10 times.
firstprog.c
#include <stdio.h>
int main() {�� int i;�� for(i=0; i < 10; i++)������ // Loop 10 times.
� {���� puts("Hello, world!\n");� // put the string to the output.
� }�� return 0;������������������ // Tell OS the program exited without errors.
}
�The main execution of a C program begins in the aptly named main() function. Any text following two forward slashes (//) is a comment, which is ignored by the compiler.
The first line may be confusing, but it�s just C syntax that tells the compiler to include headers for a standard input/output (I/O) library named stdio. This header file is added to the program when it is compiled. It is located at /usr/include/stdio.h, and it defines several constants and function prototypes for corresponding functions in the standard I/O library. Since the main() function uses the printf() function from the standard I/O library, a function prototype is needed for printf() before it can be used. This function prototype (along with many others) is included in the stdio.h header file. A lot of the power of C comes from its extensibility and libraries. The rest of the code should make sense and look a lot like the pseudo-code from before. You may have even noticed that there�s a set of curly braces that can be eliminated. It should be fairly obvious what this program will do, but let�s compile it using GCC and run it just to make sure.
The GNU Compiler Collection (GCC) is a free C compiler that translates C into machine language that a processor can understand. The outputted translation is an executable binary file, which is called a.out by default. Does the compiled program do what you thought it would?
reader@hacking:~/booksrc $ gcc firstprog.c reader@hacking:~/booksrc $ ls -l a.out
- rwxr-xr-x 1 reader reader 6621 2007-09-06 22:16 a.out
reader@hacking:~/booksrc $ ./a.out Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world! Hello, world! reader@hacking:~/booksrc $
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