Handy LD_PRELOAD-able library to override libc functions

liboverride.c

#define _GNU_SOURCE
#include <dirent.h>
#include <dlfcn.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <syslog.h>
#include <unistd.h>

#ifndef DEBUG
#define DEBUG 0
#endif

#ifndef DEBUG_2_SYSLOG
#define DEBUG_2_SYSLOG 1
#endif

#ifndef DEBUG_2_STDERR
#define DEBUG_2_STDERR 0
#endif

#define DEBUG_PRINT(fmt, ...)                                              \
  do {                                                                         \
    if (DEBUG) {                                                           \
      if (DEBUG_2_SYSLOG) {                                                \
        syslog(LOG_INFO | LOG_USER, "override|%s: " fmt, __func__,             \
               __VA_ARGS__);                                                   \
        closelog();                                                            \
      }                                                                        \
      if (DEBUG_2_STDERR)                                                  \
        fprintf(stderr, "override|%s: " fmt, __func__, __VA_ARGS__);           \
    }                                                                          \
  } while (0)

/* Function pointers to hold the value of the glibc functions */
static DIR *(*real_opendir)(const char *str);
static int (*real_access)(const char *str, int mode);
static int (*real_open)(const char *str, int flags, mode_t mode);
static int (*real_stat)(const char *str, struct stat *buf);
static int (*real_lstat)(const char *str, struct stat *buf);
static int (*real_fstatat)(int dirfd, const char *str, struct stat *buf,
                           int flags);
static int (*real___fxstatat)(int ver, int dirfd, const char *str,
                              struct stat *buf, int flags);
static int (*real___xstat)(int ver, const char *str, struct stat *buf);
static int (*real___xstat64)(int ver, const char *str, struct stat64 *buf);
static int (*real___lxstat)(int ver, const char *str, struct stat *buf);
static int (*real___lxstat64)(int ver, const char *str, struct stat64 *buf);
static FILE *(*real_fopen)(const char *str, const char *mode);
static FILE *(*real_fopen64)(const char *str, const char *mode);
static ssize_t (*real_getxattr)(const char *path, const char *name, void *value,
                          size_t size);
static ssize_t (*real_lgetxattr)(const char *path, const char *name, void *value,
                           size_t size);

void translate_path(const char **old_str, const char **new_str, char *flexi_str,
                    int flexi_str_len) {
  char *old_str_at = (char *)*old_str;
  *new_str = (char *)*old_str; /* do something to me? */;
}

#define TRANSLATE_STR(old_str, new_str)                                        \
  DEBUG_PRINT("%s\n", old_str);                                            \
  char flexi_str[PATH_MAX];                                                \
  translate_path(&old_str, &new_str, flexi_str, sizeof(flexi_str));            \
  DEBUG_PRINT("%s\n", new_str);

int open(const char *str, int flags, mode_t mode) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_open = dlsym(RTLD_NEXT, __func__);
  return real_open(__str, flags, mode);
}

DIR *opendir(const char *str) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_opendir = dlsym(RTLD_NEXT, __func__);
  return real_opendir(__str);
}

int access(const char *str, int mode) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_access = dlsym(RTLD_NEXT, __func__);
  return real_access(__str, mode);
}

FILE *fopen(const char *str, const char *mode) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_fopen = dlsym(RTLD_NEXT, __func__);
  return real_fopen(__str, mode);
}

FILE *fopen64(const char *str, const char *mode) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_fopen64 = dlsym(RTLD_NEXT, __func__);
  return real_fopen64(__str, mode);
}

int stat(const char *str, struct stat *buf) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_stat = dlsym(RTLD_NEXT, __func__);
  return real_stat(__str, buf);
}

int lstat(const char *str, struct stat *buf) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_lstat = dlsym(RTLD_NEXT, __func__);
  return real_lstat(__str, buf);
}

int __xstat(int ver, const char *str, struct stat *buf) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real___xstat = dlsym(RTLD_NEXT, __func__);
  return real___xstat(ver, __str, buf);
}

int __xstat64(int ver, const char *str, struct stat64 *buf) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real___xstat64 = dlsym(RTLD_NEXT, __func__);
  return real___xstat64(ver, __str, buf);
}

int __lxstat(int ver, const char *str, struct stat *buf) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real___lxstat = dlsym(RTLD_NEXT, __func__);
  return real___lxstat(ver, __str, buf);
}

int __lxstat64(int ver, const char *str, struct stat64 *buf) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real___lxstat64 = dlsym(RTLD_NEXT, __func__);
  return real___lxstat64(ver, __str, buf);
}

int fstatat(int dirfd, const char *str, struct stat *buf, int flags) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_fstatat = dlsym(RTLD_NEXT, __func__);
  return real_fstatat(dirfd, __str, buf, flags);
}

int __fxstatat(int ver, int dirfd, const char *str, struct stat *buf,
               int flags) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real___fxstatat = dlsym(RTLD_NEXT, __func__);
  return real___fxstatat(ver, dirfd, __str, buf, flags);
}

ssize_t getxattr(const char *str, const char *name, void *value, size_t size) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_getxattr = dlsym(RTLD_NEXT, __func__);
  return real_getxattr(__str, name, value, size);
}

ssize_t lgetxattr(const char *str, const char *name, void *value, size_t size) {
  const char *__str = str;
  TRANSLATE_STR(str, __str)
  real_lgetxattr = dlsym(RTLD_NEXT, __func__);
  return real_lgetxattr(__str, name, value, size);
}

PoC will be continued at https://github.com/nehaljwani/libcoverride

This is great, i currently trying to override do_system (const char *line) function in system.c I managed to override __libc_system (const char *line) but not do_system (const char *line), do you have any idea how to do it? Thank you