dlopen.c

#define _GNU_SOURCE

#include <stdio.h>
#include <stddef.h>

#include <dlfcn.h>

struct dlopen_args
{
  /* The arguments for dlopen_doit.  */
  const char *file;
  int mode;
  /* The return value of dlopen_doit.  */
  void *new;
  /* Address of the caller.  */
  const void *caller;
};

struct dlfcn_hook
{
  void *(*dlopen) (const char *file, int mode, void *dl_caller);
  int (*dlclose) (void *handle);
  void *(*dlsym) (void *handle, const char *name, void *dl_caller);
  void *(*dlvsym) (void *handle, const char *name, const char *version,
                   void *dl_caller);
  char *(*dlerror) (void);
  int (*dladdr) (const void *address, Dl_info *info);
  int (*dladdr1) (const void *address, Dl_info *info,
                  void **extra_info, int flags);
  int (*dlinfo) (void *handle, int request, void *arg, void *dl_caller);
  void *(*dlmopen) (Lmid_t nsid, const char *file, int mode, void *dl_caller);
  void *pad[4];
};

/* NOTE: These two functions are usually defined as static
   You'll want to preload a version of libdl.so where they're not.
     ./rebind.py -b global /path/to/libdl.so -o ./libdl.so -- _dlerror_run dlopen_doit
 */
extern void *_dlerror_run(void (*)(void *), void *);
extern void dlopen_doit(void *);

extern void _rtld_global_ro;
extern struct dlfcn_hook *_dlfcn_hook;

static inline int rtld_active() {
  return *(void **)(&_rtld_global_ro + 0x218) != NULL;
}

void *dlopen_from(const char *file, int mode, void *dl_caller) {
  if (!rtld_active())
    return _dlfcn_hook->dlopen(file, mode, dl_caller);

  struct dlopen_args args;
  args.file = file;
  args.mode = mode;
  args.caller = dl_caller;

  return _dlerror_run(dlopen_doit, &args) ? NULL : args.new;
}

void *dlopen(const char *file, int mode) {
  return dlopen_from(file, mode, __builtin_extract_return_addr(__builtin_return_address(0)));
}

example.c

#include <stdio.h>

void *dlopen_from(const char *, int, void *);

void *dlopen(const char *file, int mode) {
  void *caller = __builtin_extract_return_addr(__builtin_return_address(0));

  printf("Loading file %s with mode %d from 0x%lx\n", file, mode, (size_t)caller);

  return dlopen_from(file, mode, caller);
}