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

1 #define PROP_SERVICE_NAME "property_service"
2 #define PROP_PATH_RAMDISK_DEFAULT "/default.prop"
3 #define PROP_PATH_SYSTEM_BUILD "/system/build.prop"
4 #define PROP_PATH_SYSTEM_DEFAULT "/system/default.prop"
5 #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，

1 private static native String native_get(String key);
2 private static native String native_get(String key, String def);
3 private static native void native_set(String key, String def);
4 public static void set(String key, String val) {
5 if (key.length() > PROP_NAME_MAX) {
6 throw new IllegalArgumentException( " key.length > " + PROP_NAME_MAX);
7 }
8 if (val != null && val.length() > PROP_VALUE_MAX) {
9 throw new IllegalArgumentException( " val.length > " +
10 PROP_VALUE_MAX);
11 }
12 native_set(key, val);
13 }

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

1 namespace android {
2 extern int register_android_os_SystemProperties(JNIEnv * env);
3 }

frameworks/base/core/jni/android_os_SystemProperties.cpp

1 static void SystemProperties_set(JNIEnv * env, jobject clazz, jstring keyJ, jstring valJ)
2 {
3 int err;
4 const char * key;
5 const char * val;
6 key = env -> GetStringUTFChars(keyJ, NULL);
7 if (valJ == NULL) {
8 val = "" ; /* NULL pointer not allowed here */
9 } else {
10 val = env -> GetStringUTFChars(valJ, NULL);
11 }
12 err = property_set(key, val);
13 env -> ReleaseStringUTFChars(keyJ, key);
14 if (valJ != NULL) {
15 env -> ReleaseStringUTFChars(valJ, val);
16 }
17 }

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

1 #define AID_ROOT 0 /* traditional unix root user */
2 #define AID_SYSTEM 1000 /* system server */
3 #define AID_RADIO 1001 /* telephony subsystem, RIL */
4 #define AID_DHCP 1014 /* dhcp client */
5 #define AID_SHELL 2000 /* adb and debug shell user */
6 #define AID_CACHE 2001 /* cache access */
7 #define AID_APP 10000 /* first app user */

system/core/init/property_service.c

1 #define PERSISTENT_PROPERTY_DIR "/data/property"
2 struct {
3 const char * prefix;
4 unsigned int uid;
5 } property_perms[] = {
6 { " net.rmnet0. " , AID_RADIO },
7 { " net.gprs. " , AID_RADIO },
8 { " ril. " , AID_RADIO },
9 { " gsm. " , AID_RADIO },
10 { " net.dns " , AID_RADIO },
11 { " net.usb0 " , AID_RADIO },
12 { " net. " , AID_SYSTEM },
13 { " dev. " , AID_SYSTEM },
14 { " runtime. " , AID_SYSTEM },
15 { " hw. " , AID_SYSTEM },
16 { " sys. " , AID_SYSTEM },
17 { " service. " , AID_SYSTEM },
18 { " wlan. " , AID_SYSTEM },
19 { " dhcp. " , AID_SYSTEM },
20 { " dhcp. " , AID_DHCP },
21 { " debug. " , AID_SHELL },
22 { " log. " , AID_SHELL },
23 { " service.adb.root " , AID_SHELL },
24 { " persist.sys. " , AID_SYSTEM },
25 { " persist.service. " , AID_SYSTEM },
26 { NULL, 0 }
27 };
28 int property_set( const char * name, const char * value)
29 {
30 property_changed(name, value);
31 return 0 ;
32 }
33 int start_property_service( void )
34 {
35 int fd;
36
37 load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
38 load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
39 load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
40 /* Read persistent properties after all default values have been loaded. */
41 load_persistent_properties();
42
43 fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666 , 0 , 0 );
44 if (fd < 0 ) return - 1 ;
45 fcntl(fd, F_SETFD, FD_CLOEXEC);
46 fcntl(fd, F_SETFL, O_NONBLOCK);
47
48 listen(fd, 8 );
49 return fd;
50 }
51 void handle_property_set_fd( int fd)
52 {
53 switch (msg.cmd) {
54 case PROP_MSG_SETPROP:
55 msg.name[PROP_NAME_MAX - 1 ] = 0 ;
56 msg.value[PROP_VALUE_MAX - 1 ] = 0 ;
57
58 if (memcmp(msg.name, " ctl. " , 4 ) == 0 ) {
59 if (check_control_perms(msg.value, cr.uid)) {
60 handle_control_message(( char * ) msg.name + 4 , ( char * ) msg.value);
61 } else {
62 ERROR( " sys_prop: Unable to %s service ctl [%s] uid: %d pid:%d/n " ,
63 msg.name + 4 , msg.value, cr.uid, cr.pid);
64 }
65 } else {
66 if (check_perms(msg.name, cr.uid)) {
67 property_set(( char * ) msg.name, ( char * ) msg.value);
68 } else {
69 ERROR( " sys_prop: permission denied uid:%d name:%s/n " ,
70 cr.uid, msg.name);
71 }
72 }
73 break ;
74
75 default :
76 break ;
77 }
78 }

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

1 void property_changed( const char * name, const char * value)
2 {
3 if (property_triggers_enabled) {
4 queue_property_triggers(name, value);
5 drain_action_queue();
6 }
7 }
8 int main( int argc, char ** argv)
9 {
10 parse_config_file( " /init.rc " );
11 qemu_init();
12 device_fd = device_init();
13 property_init();
14 fd = open(console_name, O_RDWR);
15 property_set_fd = start_property_service();
16 ufds[ 0 ].fd = device_fd;
17 ufds[ 0 ].events = POLLIN;
18 ufds[ 1 ].fd = property_set_fd;
19 ufds[ 1 ].events = POLLIN;
20 ufds[ 2 ].fd = signal_recv_fd;
21 ufds[ 2 ].events = POLLIN;
22 fd_count = 3 ;
23 for (;;) {
24 if (ufds[ 0 ].revents == POLLIN)
25 handle_device_fd(device_fd);
26
27 if (ufds[ 1 ].revents == POLLIN)
28 handle_property_set_fd(property_set_fd);
29 if (ufds[ 3 ].revents == POLLIN)
30 handle_keychord(keychord_fd);
31 }
32 return 0 ;
33 }