How to pass array of strings as parameter to function?

Question

How can I pass array of strings as parameter to function in assembler? For example lets say I want to call execve() function which looks like this:

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

so I do this:

test.asm

format elf executable

entry main

main:
    mov eax, 11 ; execve - executes program
    mov ebx, filename   ; label name is address of string variable
    mov ecx, args       ; label name is address of array of strings?
    mov edx, 0          ; NULL
    int 80h

    mov eax, 1  ;exit
    int 80h
    ret

filename db '/bin/ls', 0        ; path to program
args db '/bin/ls', 0, '.', 0, 0 ; array should end with empty string to
                                ; indicate end of array

makefile

all:
    ~/apps/fasm/fasm ./test.asm

But when I run my program execve() fails to execute requested program and strace ./test shows this message:

execve("/bin/ls", [0x6e69622f, 0x736c2f, 0x2e], [/* 0 vars */]) = -1 EFAULT (Bad address)

How to properly pass "args" variable to execve function?

Thanks :)

To find out how something is done, write it in C and look at compiler output. — Peter Cordes
– Peter Cordes, Commented Mar 27, 2016 at 14:27
Use lea ebx, [ filename ] to move the address of a label into a register. — sivizius
– sivizius, Commented May 13, 2016 at 14:39

Jester · Accepted Answer · 2016-03-27 14:05:50Z

2

Do you know how this works in C? Strings are pointers, and a string array is an array of pointers. Thus you need to do something like:

filename db '/bin/ls', 0 
dot db '.', 0
args dd filename, dot, 0

Notice that args is dd to get pointer size items, and it is filled with the addresses of strings.

answered Mar 27, 2016 at 14:05

Jester

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