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#definePROP_SERVICE_NAME"property_service"
2#definePROP_PATH_RAMDISK_DEFAULT"/default.prop"
3#definePROP_PATH_SYSTEM_BUILD"/system/build.prop"
4#definePROP_PATH_SYSTEM_DEFAULT"/system/default.prop"
5#definePROP_PATH_LOCAL_OVERRIDE"/data/local.prop"

后者则通过frameworks/base/core/java/android/os/SystemProperties.java的接口定义，

1privatestaticnativeStringnative_get(Stringkey);
2privatestaticnativeStringnative_get(Stringkey,Stringdef);
3privatestaticnativevoidnative_set(Stringkey,Stringdef);
4publicstaticvoidset(Stringkey,Stringval){
5if(key.length()>PROP_NAME_MAX){
6thrownewIllegalArgumentException("key.length>"+PROP_NAME_MAX);
7}
8if(val!=null&&val.length()>PROP_VALUE_MAX){
9thrownewIllegalArgumentException("val.length>"+
10PROP_VALUE_MAX);
11}
12native_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

1namespaceandroid{
2externintregister_android_os_SystemProperties(JNIEnv*env);
3}

frameworks/base/core/jni/android_os_SystemProperties.cpp

1staticvoidSystemProperties_set(JNIEnv*env,jobjectclazz,jstringkeyJ,jstringvalJ)
2{
3interr;
4constchar*key;
5constchar*val;
6key=env->GetStringUTFChars(keyJ,NULL);
7if(valJ==NULL){
8val="";/*NULLpointernotallowedhere*/
9}else{
10val=env->GetStringUTFChars(valJ,NULL);
11}
12err=property_set(key,val);
13env->ReleaseStringUTFChars(keyJ,key);
14if(valJ!=NULL){
15env->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#defineAID_ROOT0/*traditionalunixrootuser*/
2#defineAID_SYSTEM1000/*systemserver*/
3#defineAID_RADIO1001/*telephonysubsystem,RIL*/
4#defineAID_DHCP1014/*dhcpclient*/
5#defineAID_SHELL2000/*adbanddebugshelluser*/
6#defineAID_CACHE2001/*cacheaccess*/
7#defineAID_APP10000/*firstappuser*/

system/core/init/property_service.c

1#definePERSISTENT_PROPERTY_DIR"/data/property"
2struct{
3constchar*prefix;
4unsignedintuid;
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};
28intproperty_set(constchar*name,constchar*value)
29{
30property_changed(name,value);
31return0;
32}
33intstart_property_service(void)
34{
35intfd;
36
37load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
38load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
39load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
40/*Readpersistentpropertiesafteralldefaultvalueshavebeenloaded.*/
41load_persistent_properties();
42
43fd=create_socket(PROP_SERVICE_NAME,SOCK_STREAM,0666,0,0);
44if(fd<0)return-1;
45fcntl(fd,F_SETFD,FD_CLOEXEC);
46fcntl(fd,F_SETFL,O_NONBLOCK);
47
48listen(fd,8);
49returnfd;
50}
51voidhandle_property_set_fd(intfd)
52{
53switch(msg.cmd){
54casePROP_MSG_SETPROP:
55msg.name[PROP_NAME_MAX-1]=0;
56msg.value[PROP_VALUE_MAX-1]=0;
57
58if(memcmp(msg.name,"ctl.",4)==0){
59if(check_control_perms(msg.value,cr.uid)){
60handle_control_message((char*)msg.name+4,(char*)msg.value);
61}else{
62ERROR("sys_prop:Unableto%sservicectl[%s]uid:%dpid:%d/n",
63msg.name+4,msg.value,cr.uid,cr.pid);
64}
65}else{
66if(check_perms(msg.name,cr.uid)){
67property_set((char*)msg.name,(char*)msg.value);
68}else{
69ERROR("sys_prop:permissiondenieduid:%dname:%s/n",
70cr.uid,msg.name);
71}
72}
73break;
74
75default:
76break;
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

1voidproperty_changed(constchar*name,constchar*value)
2{
3if(property_triggers_enabled){
4queue_property_triggers(name,value);
5drain_action_queue();
6}
7}
8intmain(intargc,char**argv)
9{
10parse_config_file("/init.rc");
11qemu_init();
12device_fd=device_init();
13property_init();
14fd=open(console_name,O_RDWR);
15property_set_fd=start_property_service();
16ufds[0].fd=device_fd;
17ufds[0].events=POLLIN;
18ufds[1].fd=property_set_fd;
19ufds[1].events=POLLIN;
20ufds[2].fd=signal_recv_fd;
21ufds[2].events=POLLIN;
22fd_count=3;
23for(;;){
24if(ufds[0].revents==POLLIN)
25handle_device_fd(device_fd);
26
27if(ufds[1].revents==POLLIN)
28handle_property_set_fd(property_set_fd);
29if(ufds[3].revents==POLLIN)
30handle_keychord(keychord_fd);
31}
32return0;
33}