Android 的系统属性(SystemProperties)设置分析

Android 的系统属性包括两部分：文件保存的持久属性和每次开机导入的cache属性。前者主要保存在下面几个文件中：bionic/libc/include/sys/_system_properties.h#define PROP_SERVICE_NAME "property_service"#define PROP_PATH_RAMDISK_DEFAULT "/default.prop"#define PROP_PATH_SYSTEM_BUILD "/system/build.prop"#define PROP_PATH_SYSTEM_DEFAULT "/system/default.prop"#define PROP_PATH_LOCAL_OVERRIDE "/data/local.prop"后者则通过frameworks/base/core/java/android/os/SystemProperties.java的接口定义，private static native String native_get(String key);private static native String native_get(String key, String def);private static native void native_set(String key, String def);public static void set(String key, String val) { if (key.length() > PROP_NAME_MAX) { throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX); } if (val != null && val.length() > PROP_VALUE_MAX) { throw new IllegalArgumentException("val.length > " + PROP_VALUE_MAX); } native_set(key, val);}该接口类在初始化运行环境中注册对应的cpp接口android_os_SystemProperties.cpp，实际操作通过JNI调用的是cpp文件对应的接口：frameworks/base/core/jni/AndroidRuntime.cppnamespace android { extern int register_android_os_SystemProperties(JNIEnv *env);}frameworks/base/core/jni/android_os_SystemProperties.cppstatic void SystemProperties_set(JNIEnv *env, jobject clazz, jstring keyJ, jstring valJ){ int err; const char* key; const char* val; key = env->GetStringUTFChars(keyJ, NULL); if (valJ == NULL) { val = ""; /* NULL pointer not allowed here */ } else { val = env->GetStringUTFChars(valJ, NULL); } err = property_set(key, val); env->ReleaseStringUTFChars(keyJ, key); if (valJ != NULL) { env->ReleaseStringUTFChars(valJ, val); }}设置key的value时，需要作鉴权，根据设置程序所在进程的fd获知uid值，比如system server进程可以设置net打头的key，不可以设置gsm打头的key，相关的定义如下：system/core/include/private/android_filesystem_config.h#define AID_ROOT 0 /* traditional unix root user */#define AID_SYSTEM 1000 /* system server */#define AID_RADIO 1001 /* telephony subsystem, RIL */#define AID_DHCP 1014 /* dhcp client */#define AID_SHELL 2000 /* adb and debug shell user */#define AID_CACHE 2001 /* cache access */#define AID_APP 10000 /* first app user */system/core/init/property_service.c#define PERSISTENT_PROPERTY_DIR "/data/property"struct { const char *prefix; unsigned int uid;} property_perms[] = { { "net.rmnet0.", AID_RADIO }, { "net.gprs.", AID_RADIO }, { "ril.", AID_RADIO }, { "gsm.", AID_RADIO }, { "net.dns", AID_RADIO }, { "net.usb0", AID_RADIO }, { "net.", AID_SYSTEM }, { "dev.", AID_SYSTEM }, { "runtime.", AID_SYSTEM }, { "hw.", AID_SYSTEM }, { "sys.", AID_SYSTEM }, { "service.", AID_SYSTEM }, { "wlan.", AID_SYSTEM }, { "dhcp.", AID_SYSTEM }, { "dhcp.", AID_DHCP }, { "debug.", AID_SHELL }, { "log.", AID_SHELL }, { "service.adb.root", AID_SHELL }, { "persist.sys.", AID_SYSTEM }, { "persist.service.", AID_SYSTEM }, { NULL, 0 }};int property_set(const char *name, const char *value){ property_changed(name, value); return 0;}int start_property_service(void){ int fd; load_properties_from_file(PROP_PATH_SYSTEM_BUILD); load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT); load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE); /* Read persistent properties after all default values have been loaded. */ load_persistent_properties(); fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666, 0, 0); if(fd < 0) return -1; fcntl(fd, F_SETFD, FD_CLOEXEC); fcntl(fd, F_SETFL, O_NONBLOCK); listen(fd, 8); return fd;}void handle_property_set_fd(int fd){ switch(msg.cmd) { case PROP_MSG_SETPROP: msg.name[PROP_NAME_MAX-1] = 0; msg.value[PROP_VALUE_MAX-1] = 0; if(memcmp(msg.name,"ctl.",4) == 0) { if (check_control_perms(msg.value, cr.uid)) { handle_control_message((char*) msg.name + 4, (char*) msg.value); } else { ERROR("sys_prop: Unable to %s service ctl [%s] uid: %d pid:%d/n", msg.name + 4, msg.value, cr.uid, cr.pid); } } else { if (check_perms(msg.name, cr.uid)) { property_set((char*) msg.name, (char*) msg.value); } else { ERROR("sys_prop: permission denied uid:%d name:%s/n", cr.uid, msg.name); } } break; default: break; }}在开机启动后的init操作中，会执行一个loop循环，当检测到有新的设置时，进入设置流程，鉴权失败会提示相关的异常，如sys_prop: permission denied uid:1000 name:gsm.phone.idsystem/core/init/init.cvoid property_changed(const char *name, const char *value){ if (property_triggers_enabled) { queue_property_triggers(name, value); drain_action_queue(); }}int main(int argc, char **argv){ parse_config_file("/init.rc"); qemu_init(); device_fd = device_init(); property_init(); fd = open(console_name, O_RDWR); property_set_fd = start_property_service(); ufds[0].fd = device_fd; ufds[0].events = POLLIN; ufds[1].fd = property_set_fd; ufds[1].events = POLLIN; ufds[2].fd = signal_recv_fd; ufds[2].events = POLLIN; fd_count = 3; for(;;) { if (ufds[0].revents == POLLIN) handle_device_fd(device_fd); if (ufds[1].revents == POLLIN) handle_property_set_fd(property_set_fd); if (ufds[3].revents == POLLIN) handle_keychord(keychord_fd); } return 0;}