警告
本文最后更新于 2023-06-27，文中内容可能已过时。
Magisk内部实现细节的第四篇，在前两篇着重讲了Magisk的三个重要功能的两个—su以及hide，这篇就来分析下最后一个重要功能—resetprop，这三个功能Magisk也分别导出了三个可执行文件
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// native/jni/core/applets.cpp
static main_fun applet_main[] = { su_client_main, resetprop_main, magiskhide_main, nullptr };
static int call_applet(int argc, char *argv[]) {
// Applets
string_view base = basename(argv[0]);
for (int i = 0; applet_names[i]; ++i) {
if (base == applet_names[i]) {
// 根据可执行文件的名称执行具体的类方法
return (*applet_main[i])(argc, argv);
}
}
#if ENABLE_INJECT
if (str_starts(base, "app_process")) {
return app_process_main(argc, argv);
}
#endif
fprintf(stderr, "%s: applet not found\n", base.data());
return 1;
}
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// native/jni/resetprop/resetprop.cpp
int resetprop_main(int argc, char *argv[]) {
log_cb.d = [](auto fmt, auto ap) -> int { return verbose ? vfprintf(stderr, fmt, ap) : 0; };
bool prop_svc = true;
bool persist = false;
char *argv0 = argv[0];
--argc;
++argv;
// Parse flags and -- options
while (argc && argv[0][0] == '-') {
for (int idx = 1; true; ++idx) {
switch (argv[0][idx]) {
case '-':
if (strcmp(argv[0], "--file") == 0 && argc == 2) {
load_prop_file(argv[1], prop_svc);
return 0;
} else if (strcmp(argv[0], "--delete") == 0 && argc == 2) {
return delprop(argv[1], persist);
} else if (strcmp(argv[0], "--help") == 0) {
usage(argv0);
}
case 'v':
verbose = true;
continue;
case 'p':
persist = true;
continue;
case 'n':
prop_svc = false;
continue;
case '\0':
break;
case 'h':
default:
usage(argv0);
}
break;
}
--argc;
++argv;
}
switch (argc) {
case 0:
print_props(persist);
return 0;
case 1: {
string prop = getprop(argv[0], persist);
if (prop.empty())
return 1;
printf("%s\n", prop.data());
return 0;
}
case 2:
return setprop(argv[0], argv[1], prop_svc);
default:
usage(argv0);
}
}
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功能不多，除了开始对命令行参数的解析，再就是print_props、getprop、setprop，从字面上很容易了解它们的含义，先从print_props类看
print_props也就是批量获取getprop
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// native/jni/resetprop/resetprop.cpp
static void print_props(bool persist) {
getprops([](const char *name, const char *value, auto) {
printf("[%s]: [%s]\n", name, value);
}, nullptr, persist);
}
void getprops(void (*callback)(const char *, const char *, void *), void *cookie, bool persist) {
get_impl()->getprops(callback, cookie, persist);
}
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首先先初始化impl
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// native/jni/resetprop/resetprop.cpp
static sysprop_stub *get_impl() {
static sysprop_stub *impl = nullptr;
if (impl == nullptr) {
// 判断/data/property/persistent_properties是否可读
use_pb = access(PERSISTENT_PROPERTY_DIR "/persistent_properties", R_OK) == 0;
#ifdef APPLET_STUB_MAIN
if (__system_properties_init()) {
LOGE("resetprop: __system_properties_init error\n");
exit(1);
}
impl = new resetprop();
#else
// Load platform implementations
// dlsym查找相关system_property方法
load_functions();
if (__system_properties_init()) {
LOGW("resetprop: __system_properties_init error\n");
impl = new sysprop();
} else {
impl = new resetprop();
}
#endif
}
return impl;
}
// native/jni/external/systemproperties/system_property_api.cpp
int __system_properties_init() {
// 传入/dev/__properties__
return system_properties.Init(PROP_FILENAME) ? 0 : -1;
}
// native/jni/external/systemproperties/system_properties.cpp
bool SystemProperties::Init(const char* filename) {
// This is called from __libc_init_common, and should leave errno at 0 (http://b/37248982).
ErrnoRestorer errno_restorer;
if (initialized_) {
/* resetprop remove */
// contexts_->ResetAccess();
return true;
}
if (strlen(filename) >= PROP_FILENAME_MAX) {
return false;
}
strcpy(property_filename_, filename);
if (is_dir(property_filename_)) {
if (access("/dev/__properties__/property_info", R_OK) == 0) {
contexts_ = new (contexts_data_) ContextsSerialized();
if (!contexts_->Initialize(false, property_filename_, nullptr)) {
return false;
}
} else {
contexts_ = new (contexts_data_) ContextsSplit();
if (!contexts_->Initialize(false, property_filename_, nullptr)) {
return false;
}
}
} else {
contexts_ = new (contexts_data_) ContextsPreSplit();
if (!contexts_->Initialize(false, property_filename_, nullptr)) {
return false;
}
}
initialized_ = true;
return true;
}
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