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 _start: #exit(0) system call movl $1, %rax movl $0, %rbx int $0X80

Though I am a bit insure and have not been able to find info pertaining to how you put strings in a register. So as an example I wanted to do a exec system call and it as its first parameter needs a filename to run and I want to run "/bin/bash", but how do I get that in rbx. How do I even know that I have to use rbx, in X86 I know I would use ebx, is it the same relationship in amd64 ebx=rbx, ecx=rcs, etc.

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

Thanks all

Pascal Cuoq · Accepted Answer · 2009-11-20 21:28:08Z

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

Mehrdad Afshari · Accepted Answer · 2009-11-20 21:20:57Z

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.

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