Android SystemProperties系统属性分析

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 }