Android系统属性SystemProperties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：

1.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"

2.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);
}

3.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.cpp

namespace android {
		extern int register_android_os_SystemProperties(JNIEnv *env);
	}

frameworks/base/core/jni/android_os_SystemProperties.cpp

static 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);
	}
}

4.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;
	}
}

5.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.id

system/core/init/init.c

void 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;
}

FYI