YesWeHack - LeHack challenge 2019

file_reader.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/sendfile.h>
#include <sys/stat.h>
#include <fcntl.h>

void die(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    vprintf(fmt, args);
    va_end(args);
    exit(1);
}

int file_belong_to_user(const char *filename)
{
  struct stat path_stat;
  stat(filename, &path_stat);
  return (path_stat.st_uid == getuid()) && (path_stat.st_gid == getgid())  ;
}


size_t get_file_size(const char *filename){
  struct stat path_stat;
  stat(filename, &path_stat);
  return path_stat.st_size;
}

void read_file(const char *filename){
  int fd = open(filename, O_RDONLY);
  if (fd < 0){
    die("Unable to open \"%s\"\n", filename);
  }

  sendfile(fileno(stdout), fd, 0, get_file_size((filename)));
}


int main(const int argc, const char **argv){
  if (argc < 2){
    die("Usage: ./file_reader FILENAME...\n");
  }

  for (int x = 0; x < argc - 1; ++x){
    const char *filename = argv[x + 1];
    if (!file_belong_to_user(filename)){
      die("[ERROR] \"%s\" does not belong to you.\n", filename);
    }
  }

  for (int x = 0; x < argc - 1; ++x){
    const char *filename = argv[x + 1];
    read_file(filename);
  }
  return 0;
}