Slae - Demo Videos

Front 1

What file contains the 32bit system calls for reference?

Back 1

On BT5:

/usr/include/asm/unistd_32.h

Front 2

How do you define the following places within the asm source?

1. The start of "main"
2. Text section
3. Data section
4. A label

Back 2

1. _start:
-You'll also need global _start at the top of the source

2. section .text

3. section .data

4. <label name>: label data

Front 3

When invoking a system call, which register contains the syscall number, and what is the appropriate order for the following arguments?

Back 3

eax - syscall number
ebx - 1st argument
ecx - 2nd argument
edx - 3rd argument
esi - 4th argument
edi - 5th argument

Front 4

Which register typically contains the function's return value?

Back 4

EAX

Front 5

If you had a label which contained a string, how can you have nasm automatically count the length for you?

Back 5

message: db "Hello World!" <---- label for string
mlen eq $-message <----- mlen now has the value 12

Front 6

How do you assemble and link the program with nasm and ld?

Back 6

How to assemble with nasm:

nasm -f elf32 -o hello_world.o hello_world.asm

How to link:

ld -o hello_world hello_world.o

Front 7

What are the 4 things to know when calling libc functions from within nasm?

Back 7

1. define all libc functions with "extern" without quotes

2. All arguments must be pushed onto the stack in reverse order.

3. Adjust the stack after calling libc functions

4. Link with gcc instead of ld (use main instead of _start)

Front 8

Here is an example of calling libc functions from within nasm.

Back 8

extern printf
extern exit

global main

section .text
main:
push message
call printf
add esp, 0x4 ; adjust the stack by 4 bytes for the pushed pointer to message string