patch.c

#include <dlfcn.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>

typedef const char *(*CountryFn)(void *);

static const char *GetIPCountry(void *self) {
  fprintf(stderr, "[hook] GetIPCountry called\n");
  return "US";
}

static int already_patched = 0;

static void hook(void *obj) {
  if (!obj || already_patched) {
    fprintf(stderr, "[hook] skip (null or already patched)\n");
    return;
  }

  void ***p = (void ***)obj;
  if (!p || !*p) {
    fprintf(stderr, "[hook] invalid object layout\n");
    return;
  }

  void **vt = *p;

  /* basic sanity check */
  if (!vt[4]) {
    fprintf(stderr, "[hook] slot4 is null\n");
    return;
  }

  fprintf(stderr, "[hook] original slot4 = %p\n", vt[4]);

  /* copy only first 8 entries to avoid OOB reads */
  static void *vtable[8];

  for (int i = 0; i < 8; i++) {
    vtable[i] = vt[i];
  }

  vtable[4] = (void *)GetIPCountry;

  *p = vtable;
  already_patched = true;

  fprintf(stderr, "[hook] patch complete\n");
}

void *CreateInterface(const char *name, int *rc) {
  static void *(*real)(const char *, int *) = NULL;

  if (!real)
    real = dlsym(RTLD_NEXT, "CreateInterface");

  void *obj = real(name, rc);

  fprintf(stderr, "[hook] requested: %s\n", name ? name : "(null)");

  if (name && strcmp(name, "SteamUtils013") == 0 && obj) {
    fprintf(stderr, "[hook] steamutils found\n");
    hook(obj);
  }

  return obj;
}

__attribute__((constructor)) static void init() {
  fprintf(stderr, "[hook] loaded\n");
}