picoctf 2017] just no

1.문제

A program at /problems/f6ae7c5798439ab46292af914f47b170 has access to a flag but refuses to share it. Can you convince it otherwise?

//justno.c

#include <stdio.h>                                                  
#include <stdlib.h>                                                 
#include <string.h>                                                 
#include <unistd.h>                                                 
#include <sys/types.h>                                              
                                                                    
int main(int argc, char **argv){                                    
  FILE* authf = fopen("../../problems/f6ae7c5798439ab46292af914f47b1
70/auth","r"); //access auth file in ../../../problems/f6ae7c5798439
ab46292af914f47b170                                                 
  if(authf == NULL){                                                
    printf("could not find auth file in ../../problems/f6ae7c5798439
ab46292af914f47b170/\n");                                           
    return 0;                                                       
  }                                                                 
  char auth[8];                                                     
  fgets(auth,8,authf);                                              
  fclose(authf);                                                    
  if(strcmp(auth,"no")!=0){                                         
    FILE* flagf;                                                    
    flagf = fopen("/problems/f6ae7c5798439ab46292af914f47b170/flag",
"r");                                                               
    char flag[64];                                                  
    fgets(flag,64,flagf);                                           
    printf("Oh. Well the auth file doesn't say no anymore so... Here
's the flag: %s",flag);                                             
    fclose(flagf);                                                  
  }else{                                                            
    printf("auth file says no. So no. Just... no.\n");              
  }                                                                 
  return 0; 

//auth

woounnan@shell-web:/problems/f6ae7c5798439ab46292af914f47b170$ ls   
auth  flag  justno  justno.c

*문제내용

auth에 no가 저장되어있는데

justno가 auth를 읽어서 확인했을 때 no가 아니어야 플래그를 출력해줌.

2.알게된 것

 -리눅스 fopen의 경로기준은 파일을 실행했을 때 실행한 사람의 현재위치가 기준이다.

(실행자가 /tmp/test 면 이 위치를 기준으로 상대경로가 적용됨)