Assembly and System Calls

Question

Im having a bit of trouble understanding the more complex system calls in assembly. I wrote a exec system call and it worked great

 .bss

.text

.globl _start

         


        

Answer 1

Here's a trick to make progress quickly with these aspects of assembly: ask a C compiler to show you how it does it! Write a C program that does what you want to do and type gcc -S.

Example:

Manzana:ppc pascal$ cat t.c
#define NULL ((void*)0)
char *args[] = { "foo", NULL } ;
char *env[] = { "PATH=/bin", NULL } ;


int execve(const char *filename, char *const argv[], char *const envp[]);

int main()
{

  execve("/bin/bash", args, env);

} 

then:

Manzana:ppc pascal$ gcc -S -fno-PIC t.c  # added no-PIC for readability of generated code
Manzana:ppc pascal$ cat t.s
.globl _args
    .cstring
LC0:
    .ascii "foo\0"
    .data
    .align 2
_args:
    .long   LC0
    .long   0
.globl _env
    .cstring
LC1:
    .ascii "PATH=/bin\0"
    .data
    .align 2
_env:
    .long   LC1
    .long   0
    .cstring
LC2:
    .ascii "/bin/bash\0"
    .text
.globl _main
_main:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $24, %esp
    movl    $_env, 8(%esp)
    movl    $_args, 4(%esp)
    movl    $LC2, (%esp)
    call    _execve
    leave
    ret
    .subsections_via_symbols

Answer 2

You don't put strings in a register. You should pass a pointer (the address) to a null (0) terminated string (C style) in the register for this function. Some system calls (like write) take a pointer (not necessarily terminated by '\0') and length in two registers.

# somewhere in the data section:
myString:
   .asciz "/bin/bash"

and pass $myString using the register.