├── README.md ├── atexit.cpp └── LICENSE /README.md: -------------------------------------------------------------------------------- 1 | # Local implementation of `__cxa_atexit` and `__cxa_finalize` (for zygisk modules) 2 | 3 | In the [Itanium C++ ABI](https://itanium-cxx-abi.github.io/cxx-abi/abi.html#dso-dtor-runtime-api), global variable destructors are registered using `__cxa_atexit` and called by `__cxa_finalize` during dlclose. 4 | However, these functions are provided by libc, and registered destructors are stored in the global AtexitEntry array. 5 | Even after the destructors are called, gaps are left in the array. 6 | Therefore, C++ Zygisk modules could be detected using the AtexitEntry array. 7 | 8 | This repository provides a local implementation of `__cxa_atexit` and `__cxa_finalize` that ensures your Zygisk module does not interfere with the global AtexitEntry array while correctly implementing global object destruction. 9 | 10 | This implementation is inspired by bionic (https://android.googlesource.com/platform/bionic/+/master/libc/bionic/atexit.cpp), with necesscary modifications. 11 | 12 | ## Usage 13 | 14 | Add `atexit.cpp` to your zygisk module's source code set. 15 | -------------------------------------------------------------------------------- /atexit.cpp: -------------------------------------------------------------------------------- 1 | /* 2 | * Copyright (C) 2020 The Android Open Source Project 3 | * All rights reserved. 4 | * 5 | * Redistribution and use in source and binary forms, with or without 6 | * modification, are permitted provided that the following conditions 7 | * are met: 8 | * * Redistributions of source code must retain the above copyright 9 | * notice, this list of conditions and the following disclaimer. 10 | * * Redistributions in binary form must reproduce the above copyright 11 | * notice, this list of conditions and the following disclaimer in 12 | * the documentation and/or other materials provided with the 13 | * distribution. 14 | * 15 | * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 16 | * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 17 | * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 18 | * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 19 | * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 20 | * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 21 | * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS 22 | * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED 23 | * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 24 | * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 25 | * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 | * SUCH DAMAGE. 27 | */ 28 | 29 | #include 30 | #include 31 | 32 | namespace { 33 | struct AtexitEntry { 34 | void (*fn)(void *); // the __cxa_atexit callback 35 | void *arg; // argument for `fn` callback 36 | }; 37 | 38 | AtexitEntry *g_array = nullptr; 39 | size_t capacity = 8; 40 | size_t count = 0; 41 | pthread_mutex_t g_atexit_mutex = PTHREAD_MUTEX_INITIALIZER; 42 | inline void atexit_lock() { 43 | pthread_mutex_lock(&g_atexit_mutex); 44 | } 45 | 46 | inline void atexit_unlock() { 47 | pthread_mutex_unlock(&g_atexit_mutex); 48 | } 49 | 50 | // Register a function to be called either when a library is unloaded (dso != nullptr), or when the 51 | // program exits (dso == nullptr). The `dso` argument is typically the address of a hidden 52 | // __dso_handle variable. This function is also used as the backend for the atexit function. 53 | // 54 | // See https://itanium-cxx-abi.github.io/cxx-abi/abi.html#dso-dtor. 55 | // 56 | extern "C" [[gnu::used]] int __cxa_atexit(void (*func)(void *), void *arg, void * /* dso */) { // NOLINT(bugprone-reserved-identifier) 57 | int result = -1; 58 | 59 | if (func != nullptr) { 60 | atexit_lock(); 61 | count++; 62 | if (!g_array) { 63 | g_array = reinterpret_cast(malloc(capacity * sizeof(AtexitEntry))); 64 | } 65 | if (count > capacity) [[unlikely]] { 66 | capacity *= 2; 67 | g_array = reinterpret_cast(realloc(g_array, capacity * sizeof(AtexitEntry))); 68 | } 69 | g_array[count - 1].fn = func; 70 | g_array[count - 1].arg = arg; 71 | result = 0; 72 | atexit_unlock(); 73 | } 74 | 75 | return result; 76 | } 77 | 78 | // This function will be called by __on_dlclose, which is a destructor of dso 79 | // https://cs.android.com/android/platform/superproject/main/+/main:bionic/libc/arch-common/bionic/crtbegin_so.c;l=34;drc=5501003be73b73de59044b44b12f6e20ba6e0021 80 | extern "C" [[gnu::used]] void __cxa_finalize(void * /* dso */) { // NOLINT(bugprone-reserved-identifier) 81 | atexit_lock(); 82 | restart: 83 | if (count > 0) { 84 | size_t total = count; 85 | for (size_t i = count - 1;; --i) { 86 | if (g_array[i].fn == nullptr) { 87 | if (i == 0) break; 88 | continue; 89 | } 90 | 91 | // Clear the entry in the array to avoid calling an entry again 92 | // if __cxa_finalize is called recursively. 93 | const AtexitEntry entry = g_array[i]; 94 | g_array[i] = {}; 95 | 96 | atexit_unlock(); 97 | entry.fn(entry.arg); 98 | atexit_lock(); 99 | 100 | if (count != total) { 101 | goto restart; 102 | } 103 | if (i == 0) break; 104 | } 105 | 106 | free(g_array); 107 | } 108 | atexit_unlock(); 109 | } 110 | } 111 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | Apache License 2 | Version 2.0, January 2004 3 | http://www.apache.org/licenses/ 4 | 5 | TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 6 | 7 | 1. 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