Write comments on each line in assemmbly code below that explain what each line

Question

Write comments on each line in assemmbly code below that explain what each line does.

1. .section .text

2. .globl _start

3. _start:

4. xor %eax, %eax // your comments go here

5. push %eax // and here, and so on...

6. push $0x68732f2f

7. push $0x6e69622f

8. mov %esp, %ebx

9. push %eax

10. mov %esp, %ecx

11. push %ebx

12. mov %eax, %edx

13. mov $0xb, %al

14. int $0x80

----------------------------------------------------

This is the C code version to help you write your comment

#include
int main()
{
char *command=”/bin/sh”; // Path to the command
char *args[2];
args[0]=command;
args[1] = NULL; // The array must be NULL-terminated
execve(command, args, NULL); // NULL for envp since we aren’t passing environment variables
}

Explanation / Answer

1. .section .text
2. .globl _start
3. _start:
4. xor %eax, %eax            // create a NULL in eax. This will be used for terminating the string
5. push %eax               // push zero (null) into stack:
6. push $0x68732f2f // push "//sh"
7. push $0x6e69622f // push "/bin"
8. mov %esp, %ebx           //   At this moment, esp points at the starting address of "/bin/sh". We can safely write this into ebx
9. push %eax               //   eax is still zero. We can use this to terminate char **args
10. mov %esp, %ecx           //   write the address of args into ecx
11. push %ebx // push ebx to stack
12. mov %eax, %edx // copy edx to eax
13. mov $0xb, %al // sys_execve = 0xb. That should be in eax
14. int $0x80 // Trigger the interrupt and enter kernel mode
----------------------------------------------------
This is the C code version to help you write your comment
#include
int main()
{
char *command=”/bin/sh”;     // Path to the command
char *args[2];
args[0]=command;
args[1] = NULL;            // The array must be NULL-terminated
execve(command, args, NULL); // NULL for envp since we aren’t passing environment variables
}

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