NASM issue on strcpy to launch firefox

Question

I made the function strcpy in assembly, then I tried to launch several program like Firefox, emacs, ... But I have an issue on the copy and I don't know where the problem is.

Assembly code :

global strcpy

section .text

strcpy:
    push rbp
    push rdx
    mov rbp, rsp
    mov rdx, 0
strcpy_loop:
    cmp byte [rsi + rdx], 0
    je strcpy_end
    mov al, byte [rsi + rdx]
    mov byte [rdi + rdx], al
    inc rdx
    jmp strcpy_loop
strcpy_end:
    mov rax, rdi
    mov rsp, rbp
    pop rdx
    pop rbp
    ret

result when i try to launch a small program with my shared library:

s1 = hello
s2 before copy = 
s2 after copy = hello

result when i try to launch firefox with my shared library:

/usr/bin/firefox: line 52: $'basenamex\326g\002Lame $0`': command not found
/usr/bin/firefox: line 57: bad substitution: no closing "`" in `x�gL
/usr/bin/firefox: line 63: $'[\340\n\002': command not found
/usr/bin/firefox: line 73: fileL: command not found
/usr/bin/firefox: line 75: echoL: command not found
grep: invalid option -- 'g'
Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.
/usr/bin/firefox: line 78: $'[\351\n\002L': command not found
/usr/bin/firefox: line 83: export: `MOZILLA_FIVE_HOME�gL': not a valid identifier
/usr/bin/firefox: line 91: $'[\257\n\002': command not found
/usr/bin/firefox: line 95: /dev/null�gL: Permission denied
/usr/bin/firefox: line 97: $'[\t\v\002': command not found
/usr/bin/firefox: line 108: $'[\006\v\002': command not found
/usr/bin/firefox: line 131: $'[\023\v\002': command not found
/usr/bin/firefox: line 138: exec: =0
                                : not found

The small program is working, but not firefox.

From the `strcpy` documentation: _"The strcpy() function copies the string pointed to by src, **including the terminating null byte** ('\0'), to the buffer pointed to by dest."_ — Michael, Mar 14 '14 at 09:42

paxdiablo · Answer 1 · 2015-07-27T06:50:36.693

You're not actually copying the string terminator \0 at the end of the string, meaning that the destination string is left without one. In other words, if you do something like:

char buff[] = "Hello there";
strcpy (buff, "Bye");         // your strcpy, not a 'real' one.

then you'll end up with:

Byelo there

which is not what you want. In order to correct this, you can do it with something like:

strcpy_end:
    xor   al, al                  ; force al to `\0`.
    mov   byte [rdi + rdx], al    ; and store it.

    mov   rax, rdi                ; rest of original
    mov   rsp, rbp                ;    code here ...
    pop   rdx
    pop   rbp
    ret

Alternatively, you could leave the finishing code as-is and integrate the final transfer into your main loop, making it a do-X-while-condition rather than a while-condition-do-X:

strcpy_loop:
    mov   al, byte [rsi + rdx]
    mov   byte [rdi + rdx], al
    inc   rdx
    cmp   al, 0
    jne   strcpy_loop

    mov   rax, rdi                ; rest of original
    mov   rsp, rbp                ;    code here ...
    pop   rdx
    pop   rbp
    ret

NASM issue on strcpy to launch firefox

1 Answers1