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.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);
}
}
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.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;
}