0x07 Yasm: Echo with libc.

Let us write something with standard C functions like printf or scanf. What I plan to learn from this is how to link the program with libc and do a little refactor to program structure.

The mini topic for this post is 'echo'. We want to read some string and print it. To do this with libc functions one needs to let known which functions are external. For this purpose, we can use extern.

I'm still using ld. This is the reason why I use _start label and have to finish the program with syscall instead of, for example, ret. But now _start function calls main. main do full stack frame and do 'echo' job.

; asm01.s - scanf version
global _start
extern printf
extern scanf

section .data
    msg     db  "write something for echo:",0x0a,0x00
    frm     db  "%127s\n",0x00
    frm2    db  "%s",0x00
    emg     db  0x0a,"the end...",0x0a,0x00

section .text
main:
    push rbp
    mov rbp, rsp
    sub rsp, 0x80 

    lea rdi, [msg]
    call printf

    lea rdi, [frm]
    mov rsi, rsp
    call scanf

    lea rdi, [frm2]
    mov rsi, rsp
    call printf

    lea rdi, [emg]
    call printf

    leave
    ret

_start:
    call main

_exit:
    mov rax, 60
    mov rdi, 0
    syscall

To build this I created Makefile. In yasm section -g dwarf2 is for debug information. Linker line is also longer. It linked additional c library (-lc). But this was not enough. When I tried to run a program linked in this way I've got: 'no such file or directory'. I've read somewhere that ld has some problems with libc on some distros. The way to solve this is to link dynamically real lib.

# Makefile
all: prog

prog: asm01.o
    ld -dynamic-linker /lib64/ld-linux-x86-64.so.2 -lc -o asm01 asm01.o

asm01.o: asm01.s
    yasm -f elf64 -g dwarf2 asm01.s

clean:
    rm -rf asm01.o
    rm -rf asm01

$ ./asm01 
write something for echo:
Echo is a nice function.... print whatever you put in...
Echo
the end...

Hmmm, ok something goes wrong... Ah, yes, %s reads string to a first white character. Not what I intended. First I go for fgets. But it needs FILE* structure for stdin. I didn't want to handle standard descriptions so I have changed my mind to read. So change scanf to read.

; asm01.s
global _start
extern printf
extern scanf
extern read

section .data
    msg     db  "write something for echo:",0x0a,0x00
    frm     db  "%s",0x00
    emg     db  "the end...",0x0a,0x00

section .text
main:
    push rbp
    mov rbp, rsp
    sub rsp, 0x80 

    lea rdi, [msg]
    call printf

    mov rdi, 0
    mov rsi, rsp
    mov rdx, 0x80
    call read

    lea rdi, [frm]
    mov rsi, rsp
    call printf

    lea rdi, [emg]
    call printf

    leave
    ret

_start:
    call main

_exit:
    mov rax, 60
    mov rdi, 0
    syscall

Ok, this is what I want. And Makefile works like a charm...

  ...SQUEAK!