do you know how "return" works under the hood? (are you SURE?)

Summary

This video demystifies how the 'return' statement works in programming, explaining its role in function calls and how it interacts with the CPU's registers and memory. By diving into assembly language, it reveals how the program counter (PC) and the stack are used to track return locations and handle function execution. It also touches on how return values are stored in processor-specific registers, such as RAX in Intel assembly. The video highlights the potential vulnerabilities of return addresses in the context of hacking and security, offering a glimpse into how malicious code can exploit these mechanisms.

Takeaways

• 😀 Functions in programming are blocks of code that perform a single task, and they can return values after being called.
• 😀 When a function is called, the program temporarily stops executing and jumps to the function’s code, but must know where to return to once the function finishes.
• 😀 The return address is crucial for understanding where the program should continue executing after a function completes.
• 😀 Hardcoding return addresses into machine code doesn’t work when functions are called multiple times or dynamically (e.g., virtual methods in C++).
• 😀 Function calls in assembly use the 'CALL' instruction, which saves the program counter (PC) — the return address — onto the stack.
• 😀 The stack is a region of memory used to store return addresses and local variables for each function, and is managed with two special registers: Stack Pointer (SP) and Base Pointer (BP).
• 😀 When a function finishes, the 'RET' instruction pops the return address from the stack and redirects the program back to the calling location.
• 😀 In addition to the return address, the return value of a function is stored in a specific register (e.g., RAX in Intel assembly).
• 😀 The return value stored in a register can be accessed by the calling code once the function execution is complete.
• 😀 The manipulation of return addresses can be a potential security risk, as attackers may attempt to alter the stack to execute malicious code.
• 😀 Understanding how return addresses and function calls work is vital for both optimizing program performance and preventing security vulnerabilities.

Q & A

What is the primary purpose of using functions in programming?

Functions in programming are used to organize code into blocks that perform a specific action, allowing for more manageable and reusable code that can solve problems or run tasks.

How does a function work when it is called in programming?

When a function is called, arguments are passed to it, and it returns a value. The execution of the program is paused at the function call, and the function itself begins execution.

What happens when a function call is made in a program?

When a function is called, the program saves the return address (where the program should continue after the function finishes) to the stack. Then, the function's code begins execution.

What is the role of the return statement in a function?

The return statement in a function indicates where the function should return after its execution is complete. It pops the return address from the stack and continues execution from that point.

What challenges arise with return addresses in multi-function programs?

In multi-function programs, hardcoding a return address becomes problematic because a function might be called from several places. This requires a dynamic way to manage return addresses.

How do return address lookup tables work in handling return addresses?

A return address lookup table stores offsets and return locations, allowing the return instruction to look up the correct return address. However, this method fails when dealing with computed calls or dynamic dispatch (like virtual methods in C++).

What is dynamic dispatch, and why is it important for handling return addresses?

Dynamic dispatch is a mechanism in object-oriented languages, such as C++, where the method to be called is determined at runtime. This makes it impossible to determine the return address at compile-time, complicating return address management.

How does the Program Counter (PC) or Instruction Pointer (IP) relate to function calls?

The Program Counter (PC), also known as the Instruction Pointer (IP), keeps track of the next instruction to be executed. When a function is called, the address of the next instruction is saved in the PC so the program can return to it after the function finishes.

What is a stack frame, and what role does it play in a function call?

A stack frame is a region of memory used by a function to store its local variables and other runtime information. It helps manage function calls by maintaining the context of the function's execution, including the return address.

How does the return instruction work in assembly language?

In assembly, the return instruction pops the return address from the stack into the Program Counter, causing the CPU to continue execution from the point where the function was originally called.

Where does the return value of a function go in Intel assembly?

In Intel assembly, the return value of a function is placed in the RAX register (or A register). This allows the caller to access the returned value once the function finishes execution.

Why are return addresses a potential target for hackers?

Return addresses are a critical part of the program's control flow. By exploiting vulnerabilities in return address handling, attackers can redirect program execution to malicious code, which is why this area is a frequent target for hacking attempts.