Speaker 1: Welcome back listener. Today we're going to be taking a
Speaker 1: deep dive into into the world beneath the surface of
Speaker 1: CC plus plus programs. Yeah, have you ever wondered what's
Speaker 1: really going on when your code runs?
Speaker 2: It's a great question. We're talking beyond the you know,
Speaker 2: the the elegant syntax and the results you see on screen,
Speaker 2: right down to the bare metal of how computers actually think. Yeah,
Speaker 2: kind of like cracking open the hood of a high
Speaker 2: performance engine, but except instead of pistons and gears, we're
Speaker 2: going to be looking at bits and bytes and the
Speaker 2: fascinating logic of assembly code exactly. And our guidebook for
Speaker 2: this adventure is Dmitri Vostakov's Foundations of Linux, Debugging, disassembling,
Speaker 2: and reversing.
Speaker 1: And why should you care about this? Well, Understanding how
Speaker 1: your code interacts with the hardware can help you write
Speaker 1: more efficient programs, debug those tricky issues right, and even
Speaker 1: give you like a new appreciation for the elegance of
Speaker 1: software design.
Speaker 2: It's really about gaining a deeper understanding of the tools
Speaker 2: that you use every single day. Yeah, So imagine being
Speaker 2: a carpenter who only knows how to use power tools. Okay, yeah,
Speaker 2: but understanding the hand tools underneath gives you more control,
Speaker 2: more precision, a deeper understanding of your craft.
Speaker 1: That's a great analogy. Okay, so let's unpack this. The
Speaker 1: book starts with kind of a simplified model of a computer, yes,
Speaker 1: almost like a blueprint, and it uses this analogy of
Speaker 1: a city, uh huh, with buildings representing memory cells exactly.
Speaker 2: Each building or memory cell has a unique address, like
Speaker 2: a street number, and these cells hold the data, the
Speaker 2: raw numbers that your program works with. But imagine if
Speaker 2: every time your program needed data it had to drive
Speaker 2: across town to fetch it right from these buildings. It
Speaker 2: would be incredibly inefficient.
Speaker 1: Yeah. That's where registers come in, right. They're like having
Speaker 1: a little backpack, yeah, to carry you the most frequently
Speaker 1: used data precisely.
Speaker 2: Registers are small, super fast storage locations right on the CPU, okay,
Speaker 2: instead of driving across town for every little errand you
Speaker 2: keep your most essential items like your wallet and keys
Speaker 2: close at hand, so.
Speaker 1: In like a real sixty four bit PC, these backpacks
Speaker 1: are registers like RAX yes, and it's smaller counterpart, EAX.
Speaker 2: Yes, and each register has its own specific purpose. Think
Speaker 2: of RAX as kind of your main workhourse capable of
Speaker 2: handling a variety of operations. We'll see how it's used
Speaker 2: in our example a little bit later.
Speaker 1: Now I'm curious to see this all in action. The
Speaker 1: book walks through a simple program that adds two numbers yes,
Speaker 1: and it really breaks down how that interplay of memory
Speaker 1: registers and assembly instructions makes it happen.
Speaker 2: It's a great illustration. So let's say we have two
Speaker 2: memory locations labeled A and B, each containing the number one.
Speaker 2: The program first needs to load these values into registers
Speaker 2: to work with them. It's kind of like bringing the
Speaker 2: ingredients from the pantry with the countertop before you start cooking.
Speaker 1: So we'd use instructions to move the value of A
Speaker 1: into eax, yes, and the value of B into another register,
Speaker 1: maybe edX exactly.
Speaker 2: Now the magic happens. Okay, an assembly instruction called AD
Speaker 2: comes into play. Okay, this instruction adds the value in
Speaker 2: edX to the value in EAX, storing the result back
Speaker 2: in EAX. It's like combining the contents of our containers.
Speaker 1: So now EAX holds the sum which is two. But
Speaker 1: we're not done yet, right, we need to store that
Speaker 1: result back into memory.
Speaker 2: The program uses another instruction to move the value from
Speaker 2: eax back into memory, updating the value of b to two. Gotcha,
Speaker 2: And that, in essence is how a computer executes. Wow,
Speaker 2: even the simplest arithmetic operation, fetching the values, performing the calculation,
Speaker 2: and storing the result.
Speaker 1: It's amazing how much is happening under the hood for
Speaker 1: such a basic task.
Speaker 2: Yeah, it is.
Speaker 1: But things get even more interesting when we start talking
Speaker 1: about code optimization. The book shows how the compiler can
Speaker 1: be surprisingly clever, sometimes taking shortcuts that produce drastically different
Speaker 1: assembly code for the same outcome.
Speaker 2: Compilers are like master chefs who can find the most
Speaker 2: efficient way to prepare a dish. Let's say the compiler
Speaker 2: knows the values of A and B at compile time,
Speaker 2: even before the program runs. It can simply calculate the
Speaker 2: sum directly and store the final result, skipping all of
Speaker 2: those steps of moving values between memory and registers.
Speaker 1: So the optimized code might be just a single instruction
Speaker 1: that assigns the value too. To be directly exactly, It's
Speaker 1: like the compiler did the addition for us before the
Speaker 1: program even started running.
Speaker 2: That's right, and that's why understanding assembly can be so valuable.
Speaker 2: It gives you insight into the compiler's decision making process.
Speaker 2: It can help you write more efficient code yourself.
Speaker 1: This whole idea of efficiency reminds me of another crucial concept,
Speaker 1: the way computers represent numbers using binary and hexadecimal. I
Speaker 1: remember finding this a bit intimidating it first.
Speaker 2: They can seem cryptic, but they're just different ways of
Speaker 2: expressing the same information. Okay, Binary uses only two symbols
Speaker 2: zero in one, like a light switch that can be
Speaker 2: either on or off R. Hexodesimal, on the other hand,
Speaker 2: uses sixteen symbols, making it a more compact way to
Speaker 2: represent larger numbers.
Speaker 1: So why is hexadesimal so important in this world of debugging.
Speaker 1: It seems like we're constantly looking at memory addresses and
Speaker 1: instructions represented in hes.
Speaker 2: Imagine trying to debug a program by staring at a
Speaker 2: giant wall of zeros and ones. Hexadesimal makes things more
Speaker 2: manageable by grouping those bits into chunks, each represented by
Speaker 2: a single hexodesimal symbol. It's like breaking that wall into
Speaker 2: smaller and more readable sections.
Speaker 1: It's all about making the information more digestible for us
Speaker 1: humans exactly right now, I know we can't avoid talking
Speaker 1: about one of the most powerful and sometimes misunderstood features
Speaker 1: of CC plus plus. Pointers.
Speaker 2: Pointers are the key yes to unlocking the true power
Speaker 2: and potential pitfalls of these languages. But to understand pointers,
Speaker 2: we need to go back to our city analogy.
Speaker 1: All right, let's go back to our city. How do
Speaker 1: pointers fit into this picture?
Speaker 2: We'll imagine instead of carrying around bulky packages of data,
Speaker 2: you could simply share their locations. Okay, that's what a
Speaker 2: pointer does. It holds the address of a memory location,
Speaker 2: like a street address in our city. Instead of copying
Speaker 2: an entire file, you can just use a pointer to
Speaker 2: tell the program where to find it.
Speaker 1: So a pointer is like a map that tells us
Speaker 1: where to find the actual data. That makes sense, But
Speaker 1: I also hear horror stories about pointers causing programs to
Speaker 1: crash and burn. What's the deal with that?
Speaker 2: Pointers are powerful, Yeah, but they can be dangerous if
Speaker 2: they're misused. Right, If you have the wrong address, you'll
Speaker 2: end up at the wrong place, maybe somewhere you shouldn't be. Similarly,
Speaker 2: an invalid pointer, one that points to a protected or
Speaker 2: non existent memory location, can cause your program to crash.
Speaker 1: It sounds like we need to tread carefully in this
Speaker 1: world of pointers. Yes, they're like a double edged sword exactly.
Speaker 1: The book also mentions different types of addressing, like white word,
Speaker 1: double word, and quad word. And there are these suffixes
Speaker 1: used in assembly code b wl and Q. That's all
Speaker 1: that about.
Speaker 2: Those refer to the size of the data that we're
Speaker 2: working with. Think back to our city analogy. A byte
Speaker 2: is the smallest unit, like a single apartment. A quadword
Speaker 2: is the largest, like a massive skyscraper. Those suffixes in
Speaker 2: assembly code tell the computer how much data to load, store,
Speaker 2: or manipulate at a time.
Speaker 1: So move would move a single bite, while mufkq would
Speaker 1: move a whole quadward. It's like specifying whether we're moving
Speaker 1: a single box or an entire truckload of stuff.
Speaker 2: Precisely and using the correct addressing mode is crucial for
Speaker 2: avoiding errors. Imagine trying to cram the contents of a
Speaker 2: skyscraper into a tiny apartment. It just wouldn't work.
Speaker 1: Now, to really see how pointers work in practice, let's
Speaker 1: look at a slightly more complex example from the book.
Speaker 2: This program actually manipulates memory using poinkers okay, and the
Speaker 2: book uses GDB. The doughbugger to show us exactly how
Speaker 2: the memory layout changes step by step. It's like watching
Speaker 2: a slow motion replay of the action.
Speaker 1: This is getting exciting. Before before we dive into that,
Speaker 1: let's take a quick break to let all this information
Speaker 1: sync in. When we come back, we'll explore this example
Speaker 1: in detail and see the power of pointers in action.
Speaker 1: Stay tuned, Welcome back. Now let's get our hands dirty
Speaker 1: with this pointer example. Okay, the book sets up a
Speaker 1: program that assigns the address of a variable let's call
Speaker 1: it A, to a pointer called PAW. Then he uses
Speaker 1: PAW to store the value one at the memory location
Speaker 1: pointed two by PA.
Speaker 2: It's like we're creating a treasure map okay, where PA
Speaker 2: holds the coordinates to the treasure chest, which is the
Speaker 2: memory location of A, and then we use that map
Speaker 2: to put a gold coin or value of one into
Speaker 2: the jest.
Speaker 1: So the assembly code first needs to figure out the
Speaker 1: address of A and store it in PA. How does
Speaker 1: that happen? At this low level?
Speaker 2: That's where an instruction called LEE, which stands for a
Speaker 2: load effective address comes in. It figures out the address
Speaker 2: of A and puts it into a register. Let's say
Speaker 2: our ax.
Speaker 1: Okay.
Speaker 2: Then another instruction move move copies that address from RAX
Speaker 2: into the memory location of our pointer PAW.
Speaker 1: Got it. So now PAW is holding the address of
Speaker 1: A like our treasure map pointing to the right spot. Yes,
Speaker 1: but how do we actually use PAW to change the
Speaker 1: value of A right? It seems indirect.
Speaker 2: That's the beauty of pointers. We can use PAW to
Speaker 2: indirectly access and modify the value at the memory location
Speaker 2: it points to.
Speaker 1: Okay.
Speaker 2: The assembly code would use another move instruction, but this
Speaker 2: time it uses RAX, which holds the address stored in PAW,
Speaker 2: to figure out where to put the new value.
Speaker 1: So we're essentially using RAX as a temporary go between
Speaker 1: holding the address from PAW while the move instruction puts
Speaker 1: the value one into the correct memory spot.
Speaker 2: Which is where A lives precisely.
Speaker 1: Okay.
Speaker 2: This indirect access is what makes pointers so versatile. You
Speaker 2: can change the value of PAW to point to a
Speaker 2: different memory location, and suddenly you're manipulating a different variable
Speaker 2: altogether just by changing the map.
Speaker 1: That's incredibly powerful, But as we talked about before, it
Speaker 1: also comes with risks. What happens if our treasure map
Speaker 1: our pointer is wrong or worse, what if it points
Speaker 1: to nowhere at all.
Speaker 2: That's when we run into the dreaded segmentation faults and
Speaker 2: program crashes. If we try to use a NL pointer
Speaker 2: which essentially points to nothing, or an invalid pointer that
Speaker 2: points to a protected area of memory, the operating system
Speaker 2: steps in and says, Nope, not allowed.
Speaker 1: It's like trying to dig for treasure in the middle
Speaker 1: of a busy street. You're bound to hit something important cause.
Speaker 2: Chaos, exactly, And that's why understanding how pointers work and
Speaker 2: using them carefully is crucial. They can unlock tremendous power
Speaker 2: and efficiency, but require a deep respect for their potential consequences.
Speaker 1: Speaking of understanding how things work, let's shift gears a
Speaker 1: bit and talk about reverse engineering. Okay, the book dedicates
Speaker 1: a significant section to this, and I must admit it
Speaker 1: feels a bit like stepping into the shoes of a detective.
Speaker 2: Reverse engineering is like being handed a disassembled puzzle, a
Speaker 2: jumbled mess of ambly instructions, and your task is to
Speaker 2: figure out what the original picture the program's purpose looked like.
Speaker 1: So we're not looking at nicely formatted CC plus plus
Speaker 1: code anymore. No, we're scaring at the raw output the
Speaker 1: compiler generates. Right where do you even begin to make
Speaker 1: sense of that?
Speaker 2: It's all about pattern recognition and connecting the dots. You
Speaker 2: start by identifying familiar landmarks in the assembly code.
Speaker 1: Okay.
Speaker 2: For example, function prologs and epilogues, which mark the beginning
Speaker 2: and end of a function, are like signposts telling you
Speaker 2: where one function ends and another begins.
Speaker 1: Wait back up a second, what exactly are these prologs
Speaker 1: and epilogus you mentioned?
Speaker 2: Think of them as the setup and cleanup crew for
Speaker 2: a function. The prologue sets up the stack frame, reserving
Speaker 2: space for local variables. Okay, well, the epilogue cleans up
Speaker 2: the stack and restores the previous state. They ensure that
Speaker 2: functions can operate in their own little sandboxes without interfering
Speaker 2: with each other.
Speaker 1: Ah. So they're like the stage hands setting up into
Speaker 1: taking down the scenery for each act in a play. Yeah,
Speaker 1: that makes sense, But how do we go from recognizing
Speaker 1: these sign posts to actually understanding what the code is doing?
Speaker 2: From there, you look for instructions that load and store values,
Speaker 2: perform calculations, jump to different parts of the code, and
Speaker 2: so on. Right, you gradually piece together the puzzle by
Speaker 2: understanding the role of each instruction and how they fit together.
Speaker 1: It sounds like you're tracing the flow of data through
Speaker 1: the program, figuring out where it comes from, how it's manipulated,
Speaker 1: and where it ends up.
Speaker 2: Precisely, It's like following a trail of breadcrumbs, only in
Speaker 2: this case, the bread crumbs are assembly instructions, and with practice,
Speaker 2: you start to develop an intuition for how CC plus
Speaker 2: constructs are translated into assembly language, making it easier to
Speaker 2: reverse engineer even complex programs.
Speaker 1: So by understanding assembly code, we can not only appreciate
Speaker 1: how computers think, but also delve into the inner workings
Speaker 1: of existing programs, potentially uncovering vulnerabilities or optimize performance. It's
Speaker 1: like having a secret decoder ring for the digital world exactly.
Speaker 2: Reverse engineering isn't just an academic exercise. It has practical
Speaker 2: applications in security analysis, software development, and even understanding legacy
Speaker 2: code that might lack proper documentation. It's a powerful skill
Speaker 2: that opens up a whole new layer of understanding.
Speaker 1: Now, as we delve deeper into this low level world,
Speaker 1: we keep encountering this concept of the stack. The book
Speaker 1: uses the analogy of a stack of plates and I'm
Speaker 1: starting to grasp how it works. But why is this
Speaker 1: stack structure so important in the grand scheme of things?
Speaker 2: The stack is crucial for managing memory efficiently, especially when
Speaker 2: dealing with function calls. Imagine you're hosting a dinner party
Speaker 2: and each guest needs their own set of plates and cutlery.
Speaker 1: Okay, I can picture that. Everyone gets their own space
Speaker 1: on the table exactly.
Speaker 2: In a computer program, each function gets its own section
Speaker 2: of memory on the stack, called a stack frame. The
Speaker 2: stack frame holds the function's local variables, parameters pass to
Speaker 2: the function, and some bookkeeping information.
Speaker 1: So it's like each guest having their own designated area
Speaker 1: to keep their belongings organized.
Speaker 2: Right.
Speaker 1: But what happens if we invite too many guests, or
Speaker 1: in our case, make too many function calls? What we
Speaker 1: run out of table space or in this case, stack space.
Speaker 2: That's a great analogy, and you're right. We can indeed
Speaker 2: run out of stack space. It's called a stack overflow,
Speaker 2: and it's a common programming error, especially when you have
Speaker 2: functions calling themselves recursively, or when dealing with large data
Speaker 2: structures on the stack.
Speaker 1: It's like piling on more and more plates until the
Speaker 1: whole stack topples over not a good scenario. So how
Speaker 1: do we avoid these stack overflows in our programs?
Speaker 2: Careful memory management is key. We need to be mindful
Speaker 2: of how much data we're putting on the stack and
Speaker 2: make sure functions clean up after themselves, removing their data
Speaker 2: when they're done, just like responsible dinner guests clearing their
Speaker 2: own plates.
Speaker 1: That makes sense now. The book also mentions something called
Speaker 1: the frame pointer. What role does this play in our
Speaker 1: stack of plates analogy?
Speaker 2: The frame pointer is like a place card that marks
Speaker 2: the beginning of each guest space on the table. Okay,
Speaker 2: it provides a stable reference point for the function to
Speaker 2: find its local variables and parameters, even as the stack
Speaker 2: grows and shrinks with each function call in return.
Speaker 1: Ah, so it's like having a map of the table
Speaker 1: so that each guest can easily find their belongings. Yeah,
Speaker 1: this is getting quite technical, but I'm really starting to
Speaker 1: appreciate the intricate mechanisms at play behind the scenes of
Speaker 1: a running program.
Speaker 2: It is fascinating, isn't it. And there's still more to explore.
Speaker 2: We haven't even touched upon the CPU flags register, which
Speaker 2: acts like a hidden dashboard revealing the state of the
Speaker 2: processor after each operation.
Speaker 1: Okay, now you've peaked my curiosity. CPU flags register sounds
Speaker 1: like something out of a spy movie. What secrets does
Speaker 1: it hold?
Speaker 2: Well? For that, we'll need to continue our deep dive
Speaker 2: in the next part. Stay tuned.
Speaker 1: Welcome back to the final part of our deep dive.
Speaker 1: We've journeyed through memory and registers and pointers, and even
Speaker 1: dabbled in the art of reverse engineering.
Speaker 2: Yeah, it's been quite a journey.
Speaker 1: Now it's time to unveil the secrets of this CPU
Speaker 1: flags register right, and uncover how function parameters work at
Speaker 1: this granular level.
Speaker 2: Let's start with function parameters. They're like the specialized tools
Speaker 2: a function needs to do its job. So when you
Speaker 2: call a function, you're essentially handing it these tools, these
Speaker 2: pieces of data, so it can perform that specific task.
Speaker 1: So, if we call a function to calculate the area
Speaker 1: of a rectangle, we'd pass at the lengthen with those parameters,
Speaker 1: right exactly, But how does the function actually receive and
Speaker 1: organize these parameters down at the assembly level.
Speaker 2: Imagine you're a chef and someone hands you a basket
Speaker 2: of ingredients. You wouldn't just start cooking without first organizing
Speaker 2: those ingredients on your countertop.
Speaker 1: Right, of course, not a well organized workspace is essential
Speaker 1: for any chef.
Speaker 2: Exactly, the stack, our trustee stack of plates acts like
Speaker 2: that countertoption is called its parameters are carefully placed on
Speaker 2: the stack in a specific order. This organized placement allows
Speaker 2: the function to know precisely where to find each parameter.
Speaker 1: So it's not just a random jumble of data on
Speaker 1: the stack. There's a method to the madness, right, But
Speaker 1: how does the function actually locate and access these neatly
Speaker 1: arranged parameters.
Speaker 2: Remember our trustee frame pointer are placecard that marks the
Speaker 2: beginning of the function stack frame. Yes, well, the function
Speaker 2: uses the frame pointer as a reference to calculate the
Speaker 2: location of each parameter relative to its own stack frame.
Speaker 1: It's like having a seating chart that tells each guest
Speaker 1: each parameter exactly where their place is at the table. Yeah,
Speaker 1: and this process happens seamlessly every time a function is called,
Speaker 1: ensuring that everything is in its right place precisely.
Speaker 2: And this mechanism is crucial for making programs modular and reusable.
Speaker 2: We can call the same function with different parameters, just
Speaker 2: like a chef can use the same recipe with different ingredients.
Speaker 2: To create a variety of dishes.
Speaker 1: Okay, that makes sense. Now let's move on to this
Speaker 1: mysterious CPU flags register you mentioned earlier. What kind of
Speaker 1: dashboard are we talking about here?
Speaker 2: Imagine a control panel with a series of lights, each
Speaker 2: representing a specific condition inside the CPU. Okay, These lights
Speaker 2: or flags are individual bits within the CPU flags register,
Speaker 2: and they get flipped on or off based on the
Speaker 2: outcome of various operations.
Speaker 1: So some flags might indicate whether the result of an
Speaker 1: arithmetic operation was zero, negative, or caused and overflow. Its
Speaker 1: like the CPU is sending us signals about what.
Speaker 2: Just happened exactly. Other flags track things like whether a
Speaker 2: comparison resulted in equality, or if a carry occurred during
Speaker 2: an addition. It's a treasure trove of information about the
Speaker 2: CPU's internal state.
Speaker 1: But why should we as programmers care about these kind
Speaker 1: of cryptic CPU signals? How do they actually impact our programs?
Speaker 2: They play a crucial role in conditional branching, where the
Speaker 2: program's flow can change based on certain conditions. Right For instance,
Speaker 2: if a flag indicating a zero result to set, the
Speaker 2: program might jump to a specific section of code. If
Speaker 2: it's not set, it might take a different path I see.
Speaker 1: So the CPU flags act like signposts, guiding the program's
Speaker 1: execution based on the outcome of previous operations. Exactly, it's
Speaker 1: like having to choose your own adventure story, where the
Speaker 1: flags determine which page to turn to next.
Speaker 2: Precisely, this ability to make decisions based on conditions gives
Speaker 2: programs incredible flexibility and power to handle a wide range
Speaker 2: of situations.
Speaker 1: So understanding these flags can help us decipher the CPU's
Speaker 1: internal logic and even anticipate how our programs will behave
Speaker 1: under different circumstances.
Speaker 2: Exactly, it's like learning the secret language of the processor,
Speaker 2: allowing us to trace the execution flow and potentially even
Speaker 2: optimize our code for better performance.
Speaker 1: Wow, we've covered so much ground in this deep dive,
Speaker 1: from memory layouts and registers to pointers in reverse engineering,
Speaker 1: and even the CPU's internal flags. It feels like we've
Speaker 1: peeled back the layers of this complex machine and glimpsed
Speaker 1: its inner workings.
Speaker 2: And while this might seem like a lot to absorb,
Speaker 2: remember it's just the tip of the iceberg. The world
Speaker 2: of programming is vast and ever evolving, but the fundamental
Speaker 2: principles that we've explored here will serve you well as
Speaker 2: you continue your journey.
Speaker 1: Absolutely, this deep dive has given me a whole new
Speaker 1: appreciation for the intricate dance of bits and bites and
Speaker 1: instructions that make the magic of computing possible.
Speaker 2: And who knows, maybe someday you'll be writing your own
Speaker 2: assembly code, or even diving into the fascinating world of
Speaker 2: low level security research. The possibilities are endless.
Speaker 1: To our listener, we encourage you to keep exploring, keep
Speaker 1: asking questions, and never stop learning. The deeper you dive,
Speaker 1: the more you'll discover about the power and elegance of
Speaker 1: the digital world. Until next time, happy coding.