How to retrieve the file that is outside of current directory using format specifier?

Question

char * read_file(char * filename) {
  char * file_contents = malloc(4096 * sizeof(char));

  FILE * file;
  file = fopen(filename, "r");

  fread(file_contents, 4096, sizeof(char), file);
  fclose(file);

  return file_contents;
}

char * read_flag() {
  return read_file("/flag.txt");  // outside of current working directory ;)
}

int main(int argc, char* argv[]) {
  setvbuf(stdin,  NULL, _IONBF, 0);
  setvbuf(stdout, NULL, _IONBF, 0);

  char * flag = read_flag();
  char input_filename[40];

  //Current directory is /home/problem
  printf("Current working directory is: ");
  system("pwd");

  printf("Enter a filename to print the contents of the file => ");
  scanf("%39s", input_filename);

  while ((directory_entry = readdir(directory)) != NULL) {
    if (strcmp(input_filename, directory_entry->d_name) == 0) {
      printf("File contents:\n");
      printf("%s\n", read_file(input_filename));

      return 0;
    }
  }
}

I need to open a file that is outside of this directory ("/flag.txt"). I have tried something like "../" in the input to get out from this directory but it is not working. I am not sure how do i enter the filename such that it can retrieve the file that is outside of the /home/problem directory. I am currently using Ubuntu to do this. I think the idea should be using something like %s%d when i enter my input. Is this possible to use any specifier or exploit this program in order to read the entire contents?

You know what `./` means? you know what it refers to ? BTW: in your progam fragment `directory` is never defined nor initialized. — joop, Nov 19 '18 at 12:57
@joop sorry i typo. Supposed to be /flag.txt without the "." — Y.M, Nov 19 '18 at 14:31

ats · Answer 1 · 2018-11-19T10:55:43.667

1

You need to pass the full path to your file if it is outside the solution directory either with \\ or one /. On a windows based system this would be for example C:\\folder\\file.txt. I do not use linux currently, but it should be /home/folder/file.txt.

edited Nov 19 '18 at 10:55

answered Nov 19 '18 at 09:45

ats

109
4

For an absolute pathname, add a leading slash: `/home/folder/file.txt.` – joop Nov 19 '18 at 10:53
@joop The current directory when i execute the code is /home/problem. However the file that i want to access is outside of this directory which is ./flag.txt – Y.M Nov 19 '18 at 12:48
@Y.M. You can't refer to ./flag.txt without knowing the current directory, since the first part `./` means "in the current directory". So when you say it is outside `/home/problem`, and the name is `./file.txt` , noone can know where that file is. Find out which directory `flag.txt` is in. Say e.g. it is the folder `/somwhere/else/` , then you just open `/somwhere/else/flag.txt` – nos Nov 19 '18 at 13:30

Basile Starynkevitch · Answer 2 · 2018-11-19T16:59:34.093

0

The fopen function can fail, and you should handle that. Read fopen(3), open(2), path_resolution(7), errno(3) to understand the possible failure reasons. Details could be file system and computer specific (and could include hardware failures).

I recommend using perror(3) and exit(3) on failure (don't forget to include both <stdio.h> for perror and <stdlib.h> for exit):

FILE* file = fopen(filename, "r");
if (!file) {
    perror(filename);
    exit(EXIT_FAILURE);
}

then you'll get a meaningful error message (into stderr) on failure

My guess: your root file system (and root directory / ...) don't have a flag.txt file and you might want to retrieve what your shell understands from ~/flag.txt. Perhaps you want to retrieve it in your home directory (then build its file path, using getenv("HOME") on Linux or Unix; see this).

How to retrieve the file that is outside of current directory using format specifier?

2 Answers2