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     }