Null-terminated string, opening file for reading

问题

I'm experimenting with sys_open syscall and I get file descriptor for reading. Here is my program:

SYS_exit equ 0x3C

SYS_open equ 0x02
O_RDONLY equ 0x00
O_WRONLY equ 0x01
O_RDWR equ 0x02

section .text
    global _start

_start:
    mov eax, SYS_open
    mov rdi, file_name
    mov rsi, O_RDONLY
    mov rdx, 0x00
    syscall

    mov eax, SYS_exit
    mov rdi, 0x00

    syscall

section .data
    file_name: db '/path/to/test\0'

So when I ran strace ./bin I got the output:

open("/path/to/test\\0", O_RDONLY) = -1 ENOENT (No such file or directory)
exit(0)   

After deleting the null-terminal it seemed to work fine:

open("/path/to/test", O_RDONLY) = 3
exit(0)                                 = ?

I'm curious about how does the assembler knows the lenght of my string. The content of data section in the binary is this:

Contents of section .data:
 6000d8 2f706174 682f746f 2f746573 74        /path/to/test

I expected the string is read till reaching the null-terminator. How does it work?

回答1:

The problem is in the way you defined the following data:

section .data
    file_name: db '/path/to/test\0'

The trailing NUL character is missing, since the \0 inside the string corresponds to the characters \ and 0, it should be defined instead as:

section .data
    file_name: db '/path/to/test', 0

来源：https://stackoverflow.com/questions/49613697/null-terminated-string-opening-file-for-reading