Dive Into Systems

8.5. Functions in Assembly

In the last several sections, we traced through simple functions in assembly. In this section, we discuss the interaction between multiple functions in assembly in the context of a larger program. We also introduce some new instructions involved with function management.

Let’s begin with a refresher on how the call stack is managed. Recall that %esp is the stack pointer and always points to the top of the stack. The register %ebp represents the base pointer (also known as the frame pointer) and points to the base of the current stack frame. The stack frame (also known as the activation frame or the activation record) refers to the portion of the stack allocated to a single function call. The currently executing function is always at the top of the stack, and its stack frame is referred to as the active frame. The active frame is bounded by the stack pointer (at the top of stack) and frame pointer (at the bottom of the frame). The activation record typically holds local variables and parameters for a function.

Figure 1 shows the stack frames for main() and a function it calls named fname(). Going forward, we will refer to the main() function as the caller function and fname() as the callee function.

Figure 1. Stack Frame Management

In Figure 1 the current active frame belongs to the callee function (fname()). The memory between the stack pointer and frame pointer are used for local variables. The stack pointer moves as local values are pushed and popped from the stack. In contrast, the frame pointer remains relatively constant, pointing to the beginning (e.g. bottom) of the current stack frame. As a result, compilers like gcc commonly reference values on the stack relative to the frame pointer. In Figure 1, the active frame is bounded below by the base pointer of fname() which contains stack address 0x418. The value stored at this address is the "saved" %ebp value (0x42c) which itself indicates the bottom of the activation frame for the main() function. The top of the activation frame of main() is bounded by the return address, which indicates the program address at which main() resumes execution once the callee function exits.

The return address points to program memory not stack memory

Recall that the call stack region (stack memory) of a program is different from its code region (code memory). While %ebp and %esp point to locations in the stack memory, %eip points to a location in code memory. In other words, the return address is an address in code memory, not stack memory:

Figure 2. The parts of a program’s address space.

Table 1 contains several additional instructions that the compiler uses for basic function management:

Table 1. Common Function Management Instructions
Instruction	Translation
`leave`	prepares the stack for leaving a function. Equivalent to: mov %ebp, %esp pop %ebp
`call addr <fname>`	switches active frame to callee function. Equivalent to: push %eip mov addr, %eip
`ret`	restores active frame to caller function. Equivalent to: pop %eip

For example, the leave instruction is a shorthand that the compiler uses to restore the stack and frame pointers as it prepares to leave a function. When the callee function finishes execution, leave ensures that the frame pointer is restored to its previous value.

The call and ret instructions play a prominent role in the process where one function calls another. Both instructions modify the instruction pointer (register %eip). When the caller function executes the call instruction, the current value of %eip is saved on the stack to represent the return address, or the program address at which the caller resumes executing once the callee function finishes. The call instruction also replaces the value of %eip with the address of the callee function.

The ret instruction restores the value of %eip to the value saved on the stack, ensuring that the program resumes execution at the program address specified in the caller function. Any value returned by the callee is stored in %eax. The ret instruction is usually the last instruction that executes in any function.

8.5.1. Tracing through an Example

Using our knowledge of function management, let’s trace through the code example first introduced at the beginning of this chapter:

#include <stdio.h>

int assign(){
    int y = 40;
    return y;
}

int adder(){
    int a;
    return a + 2;
}

int main(){
    int x;
    assign();
    x = adder();
    printf("x is: %d\n", x);
    return 0;
}

Compile the code with the -m32 flag and use objdump -d to view the underlying assembly. The latter command outputs a pretty big file that contains a lot of information that we don’t need. Use less and the search functionality to extract the adder, assign, and main functions:

804840d <assign>:
 804840d:       55                      push   %ebp
 804840e:       89 e5                   mov    %esp,%ebp
 8048410:       83 ec 10                sub    $0x10,%esp
 8048413:       c7 45 fc 28 00 00 00    movl   $0x28,-0x4(%ebp)
 804841a:       8b 45 fc                mov    -0x4(%ebp),%eax
 804841d:       c9                      leave
 804841e:       c3                      ret

0804841f <adder>:
 804841f:       55                      push   %ebp
 8048420:       89 e5                   mov    %esp,%ebp
 8048422:       83 ec 10                sub    $0x10,%esp
 8048425:       8b 45 fc                mov    -0x4(%ebp),%eax
 8048428:       83 c0 02                add    $0x2,%eax
 804842b:       c9                      leave
 804842c:       c3                      ret

0804842d <main>:
 804842d:       55                      push   %ebp
 804842e:       89 e5                   mov    %esp,%ebp
 8048433:       83 ec 20                sub    $0x14,%esp
 8048436:       e8 d2 ff ff ff          call   804840d <assign>
 804843b:       e8 df ff ff ff          call   804841f <adder>
 8048440:       89 44 24 1c             mov    %eax,0xc(%esp)
 8048444:       8b 44 24 1c             mov    0xc(%esp),%eax
 8048448:       89 44 24 04             mov    %eax,0x4(%esp)
 804844c:       c7 04 24 f4 84 04 08    movl   $0x80484f4,(%esp)
 8048453:       e8 88 fe ff ff          call   80482e0 <printf@plt>
 8048458:       b8 00 00 00 00          mov    $0x0,%eax
 804845d:       c9                      leave
 804845e:       c3                      ret

Each function begins with a symbolic label that corresponds to its declared name in the program. For example, <main>: is the symbolic label for the main() function. The address of a function label is also the address of the first instruction in that function. To save space in the figures below, we truncate addresses to the lower 12 bits. So, program address 0x804842d is shown as 0x42d.

8.5.2. Tracing through main

Figure 3 shows the execution stack immediately prior to the execution of main():

Figure 3. The initial state of the CPU registers and call stack prior to executing the main function.

Recall that the stack grows toward lower addresses. In this example, %ebp is address 0x140, and %esp is address 0x130 (both of these values are made up for this example). Registers %eax and %edx initially contain junk values. The arrow indicates the currently executing instruction. Initially, %eip contains address 0x42d, which is the program memory address of the first line in the main() function. Let’s trace through the program’s execution together.

The first instruction pushes the value of ebp on to the stack, saving address 0x140. Since the stack grows toward lower addresses, the stack pointer %esp updates to 0x12c, which is 4 bytes less than 0x130. Register %eip advances to the next instruction in sequence.

The next instruction (mov %esp, %ebp) updates the value of %ebp to be the same as %esp. The frame pointer (%ebp) now points to the start of the stack frame for the main() function. %eip advances to the next instruction in sequence.

The sub instruction subtracts 0x14 from the address of our stack pointer, "growing" the stack by 20 bytes. Register %eip advances to the next instruction, which is the first call instruction.

The call <assign> instruction pushes the value inside register %eip (which denotes the address of the next instruction to execute) onto the stack. Since the next instruction after call <assign> has an address of 0x43b, that value is pushed onto the stack as the return address. Recall that the return address indicates the program address where execution should resume when program execution returns to main().

Next, the call instruction moves address of the assign function (0x40d) into register %eip, signifying that program execution should continue into the callee function assign() and not the next instruction in program sequence.

The first two instructions that execute in the assign() function are the usual book-keeping that every function performs. The first instruction pushes the value stored in %ebp (memory address 0x12c) onto the stack. Recall that this address points to the beginning of the stack frame for main(). %eip advances to the second instruction in assign().

The next instruction (mov %esp, %ebp) updates %ebp to point to the top of the stack, marking the beginning of the stack frame for assign(). The instruction pointer (%eip) advances to the next instruction in the assign() function.

The sub instruction at address 0x410 grows the stack by 16 bytes, creating extra space on the stack frame to store local values and updating %esp. The instruction pointer again advances to the next instruction in the assign() function.

The mov instruction at address 0x413 moves the value $0x28 (or 40) onto the stack at address -0x4(%ebp), which is 4 bytes above the frame pointer. Recall that the frame pointer is commonly used to reference locations on the stack. %eip advances to the next instruction in the assign() function.

The mov instruction at address 0x41a places the value $0x28 into register %eax, which holds the return value for of the function. %eip advances to the leave instruction in the assign() function.

At this point, the assign() function has almost completed execution. The next instruction that executes is the leave instruction, which prepares the stack for returning from the function call. Recall that leave is analogous to the following pair of instructions:

mov %ebp, %esp
pop %ebp

In other words, the CPU overwrites the stack pointer with the frame pointer. In our example, the stack pointer is initially updated from 0x100 to 0x110. Next, the CPU executes pop %ebp, which takes the value located at 0x110 (in our example, the address 0x12c) and places it in %ebp. Recall that 0x12c is the start of stack frame for main(). %esp becomes 0x114 and %eip points to the ret instruction in the assign() function.

The last instruction in assign() is a ret instruction. When ret executes, the return address is popped off the stack into register %eip. In our example, %eip now advances to the call to the adder() function.

Some important things to notice at this juncture:

The stack pointer and frame pointer have been restored to their values prior to the call to assign(), reflecting that the stack frame for main() is once again the active frame.
The old values on the stack from the prior active stack frame are not removed. They still exist on the call stack.

The call to adder() overwrites the old return address on the stack with a new return address (0x440). This return address points to the next instruction to be executed after adder() returns, or mov %eax, 0xc(%ebp). %eip reflects the first instruction to execute in adder(), which is at address 0x41f.

The first instruction in the adder() function saves the caller’s frame pointer (%ebp of main()) on the stack.

The next instruction updates %ebp with the current value of %esp, or address 0x110. Together, these last two instructions establish the beginning of the stack frame for adder().

The sub instruction at address 0x422 "grows" the stack by 16 bytes. Notice again that growing the stack does not impact any previously created values on the stack. Again, old values will litter the stack until they are overwritten.

Pay close attention to the next instruction that executes: mov $-0x4(%ebp), %eax. This instruction moves an old value that is on the stack into register %eax! This is a direct result of the fact that the programmer forgot to initialize a in the function adder().

The add instruction at address 0x428 adds 2 to register %eax. Recall IA32 passes the return value through register %eax. Together the last two instructions are equivalent to the following code in adder():

int a;
return a + 2;

After leave executes, the frame pointer once again points to the beginning of the stack frame for main(), or address 0x12c. The stack pointer now stores the address 0x114.

The execution of ret pops the return address off of the stack, restoring the instruction pointer back to 0x440, or the address of the next instruction to execute in main(). The address of %esp is now 0x118.

The mov %eax, 0xc(%esp) instruction places the value in %eax in a location 12 bytes (3 spaces) below %esp.

Skipping ahead a little, the mov instructions at addresses 0x444 and 0x448 set %eax to the value saved at location %esp+12 (or 0x2A) and places 0x2A one spot below the top of the stack (address %esp + 4, or 0x11c).

The next instruction (mov $0x80484f4, (%esp)) copies a constant value that is a memory address to the top of the stack. This particular memory address, 0x80484f4, contains the string "x is %d\n". The instruction pointer advances to the call to the printf() function (which is denoted with the label <printf@plt>).

For the sake of brevity, we will not trace the printf() function, which is part of stdio.h. However, we know from the manual page (man -s3 printf) that printf() has the following format:

int printf(const char * format, ...)

In other words, the first argument is a pointer to a string specifying the format, and the second argument onwards specify the values that are used in that format. The instructions specified by addresses 0x444 - 0x45c correspond to the following line in the main() function:

printf("x is %d\n", x);

When the printf() function is called:

A return address specifying the instruction that executes after the call to printf() is pushed onto the stack.
The value of %ebp is pushed onto the stack, and %ebp is updated to point to the top of the stack, indicating the beginning of the stack frame for printf().

At some point, printf() references its arguments, which are string "x is %d\n" and value 0x2A. Recall that the return address is located directly below %ebp at location %ebp+4. The first argument is thus located at %ebp+8 (i.e, right below the return address), and the second argument is located at %ebp+12.

For any function with n arguments , gcc places the first argument at location %ebp+8, the second at %ebp+12, and the n^th argument at location (%ebp+8) + (4*(n-1)).

After the call to printf(), the value 0x2A is outputted to the user in integer format. Thus, the value 42 is printed to the screen!

After the call to printf(), the last few instructions clean up the stack and prepare a clean exit from the main() function. First the value 0x0 is placed in register %eax, signifying that the value 0 is returned from main(). Recall that a program returns 0 to indicate correct termination.

Once leave and ret are executed, the stack and frame pointers revert to their original values prior to the execution of main(). With 0x0 in the return register %eax, the program returns 0.

If you have carefully read through this section, you should understand why our program prints out the value 42. In essence, the program inadvertently uses old values on the stack to cause it to behave in a way that we didn’t expect. While this example was pretty harmless, we discuss in future sections how hackers have misused function calls to make programs misbehave in truly malicious ways.