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

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"

2.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. }

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

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. }

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
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 */</span>

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. load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
37. load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
38. load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
39. /* Read persistent properties after all default values have been loaded. */
40. load_persistent_properties();
41.
42. fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666, 0, 0);
43. if(fd < 0) return -1;
44. fcntl(fd, F_SETFD, FD_CLOEXEC);
45. fcntl(fd, F_SETFL, O_NONBLOCK);
46.
47. listen(fd, 8 );
48. return fd;
49. }
50. void handle_property_set_fd(int fd)
51. {
52. switch(msg.cmd) {
53. case PROP_MSG_SETPROP:
54. msg.name[PROP_NAME_MAX-1] = 0;
55. msg.value[PROP_VALUE_MAX-1] = 0;
56.
57. if(memcmp(msg.name,"ctl.",4) == 0) {
58. if (check_control_perms(msg.value, cr.uid)) {
59. handle_control_message((char*) msg.name + 4, (char*) msg.value);
60. } else {
61. ERROR("sys_prop: Unable to %s service ctl [%s] uid: %d pid:%d\n",
62. msg.name + 4, msg.value, cr.uid, cr.pid);
63. }
64. } else {
65. if (check_perms(msg.name, cr.uid)) {
66. property_set((char*) msg.name, (char*) msg.value);
67. } else {
68. ERROR("sys_prop: permission denied uid:%d name:%s\n",
69. cr.uid, msg.name);
70. }
71. }
72. break;
73.
74. default:
75. break;
76. }
77. }

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
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. }

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