faker.c

#define _GNU_SOURCE

#include <dlfcn.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>


static int (*real_open)(const char* pathname, int flags, ...) = NULL;
// we need to hook __lxstat function because of some glibx stat calls magic
static int (*real_lxstat)(int ver, const char *pathname, struct stat *buf) = NULL;
static int (*real_xstat)(int ver, const char *pathname, struct stat *buf) = NULL;
static int (*real_chdir)(const char *path) = NULL;



int open(const char *pathname, int flags, ...)
{
  int fd;
  real_open = dlsym(RTLD_NEXT, "open");
  if (strcmp(pathname, "/xxx") == 0){
    FILE * tmpfile = NULL;
    //FIXME - we are opening tempfile insted of real one to lets kernel give use proper file descriptor
    char * tmpname;

    tmpname = tmpnam(NULL);

    tmpfile = fopen(tmpname, "w");
    
    fprintf(tmpfile, "TEST TEXTaaaaaaaaaaaaaaaaaaaaaa\n");
    fclose (tmpfile);
    
    fd = real_open(tmpname, flags);
    return fd;
  }
  return real_open(pathname,flags);
}

int __xstat(int ver, const char *pathname, struct stat *buf)
{
  real_xstat = dlsym(RTLD_NEXT, "__xstat");
  if (strcmp(pathname, "/xxx") == 0)
    {
      buf->st_mode = 0555; 
      return 0;
    }
  int rc = real_xstat(ver, pathname, buf);
  return rc;
}  

int __lxstat(int ver, const char *pathname, struct stat *buf)
{
  real_lxstat = dlsym(RTLD_NEXT, "__lxstat");
  if (strcmp(pathname, "/xxx") == 0)
    {
      buf->st_mode = 0555; 
      return 0;
    }
  int rc = real_lxstat(ver, pathname, buf);
  return rc;
  

}

int chdir(const char *path) {
  real_chdir = dlsym(RTLD_NEXT, "chdir");
  if (strcmp(path, "/xxx") == 0)
    return 0;
  int rc = real_chdir(path);
  return rc;
}