sys_execve system call from Assembly - linux

Question

asm_execve.s:
.section .data
file_to_run:
.ascii "/bin/sh"
.section .text
.globl main
main:
pushl %ebp
movl %esp, %ebp
subl $0x8, %esp # array of two pointers. array[0] = file_to_run array[1] = 0
movl file_to_run, %edi
movl %edi, -0x4(%ebp)
movl $0, -0x8(%ebp)
movl $11, %eax # sys_execve
movl file_to_run, %ebx # file to execute
leal -4(%ebp), %ecx # command line parameters
movl $0, %edx # environment block
int $0x80
leave
ret
makefile:
NAME = asm_execve
$(NAME) : $(NAME).s
gcc -o $(NAME) $(NAME).s
Program is executed, but sys_execve is not called:
alex#alex32:~/project$ make
gcc -o asm_execve asm_execve.s
alex#alex32:~/project$ ./asm_execve
alex#alex32:~/project$
Expected output is:
alex#alex32:~/project$ ./asm_execve
$ exit
alex#alex32:~/project$
This Assembly program is supposed to work like the following C code:
char *data[2];
data[0] = "/bin/sh";
data[1] = NULL;
execve(data[0], data, NULL);
Something wrong in system call parameters?

Answer 1

The execve system call is being called, but you are indeed passing it bad parameters.
(You can see this by running your executable using strace.)
There are three problems:
.ascii does not 0-terminate the string. (You might get lucky, as there is nothing following it in your .data section in this example, but that's not guaranteed...) Add a 0, or use .asciz (or .string) instead.
movl file_to_run, %edi moves the value pointed to by the file_to_run symbol into %edi, i.e. the first 4 bytes of the string (0x6e69622f). The address of the string is just the value of the symbol itself, so you need to use the $ prefix for literal values: movl $file_to_run, %edi. Similarly, you need to say movl $file_to_run, %ebx a few lines further down. (This is a common source of confusion between AT&T syntax and Intel syntax!)
The parameters are placed on the stack in the wrong order: -0x8(%ebp) is a lower address than -0x4(%ebp). So the address of the command string should be written to -0x8(%ebp), the 0 should be written to -0x4(%ebp), and the leal instruction should be leal -8(%ebp), %ecx.
Fixed code:
.section .data
file_to_run:
.asciz "/bin/sh"
.section .text
.globl main
main:
pushl %ebp
movl %esp, %ebp
subl $0x8, %esp # array of two pointers. array[0] = file_to_run array[1] = 0
movl $file_to_run, %edi
movl %edi, -0x8(%ebp)
movl $0, -0x4(%ebp)
movl $11, %eax # sys_execve
movl $file_to_run, %ebx # file to execute
leal -8(%ebp), %ecx # command line parameters
movl $0, %edx # environment block
int $0x80
leave
ret

Answer 2

You actually don't need to load anything in the other arguments. If you are doing this in x86 the following simpler code will also work:
.global _main
.section .text
.data
file_to_run:
.asciz "/bin/sh"
.section .text
.globl main
_main:
pushl %ebp
movl %esp, %ebp
movl $11, %eax # sys_execve
movl $file_to_run, %ebx # file to execute
movl $0, %ecx # Null value will work too
movl $0, %edx # Null will works too
int $0x80
leave
ret
This will essentially open a shell terminal after invoking the system call.