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

#definePROP_SERVICE_NAME"property_service"
#definePROP_PATH_RAMDISK_DEFAULT"/default.prop"
#definePROP_PATH_SYSTEM_BUILD"/system/build.prop"
#definePROP_PATH_SYSTEM_DEFAULT"/system/default.prop"
#definePROP_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，

privatestaticnativeStringnative_get(Stringkey);
privatestaticnativeStringnative_get(Stringkey,Stringdef);
privatestaticnativevoidnative_set(Stringkey,Stringdef);
publicstaticvoidset(Stringkey,Stringval){
if(key.length()>PROP_NAME_MAX){
thrownewIllegalArgumentException("key.length>"+PROP_NAME_MAX);
}
if(val!=null&&val.length()>PROP_VALUE_MAX){
thrownewIllegalArgumentException("val.length>"+
PROP_VALUE_MAX);
}
native_set(key,val);
}

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

namespaceandroid{
externintregister_android_os_SystemProperties(JNIEnv*env);
}

frameworks/base/core/jni/android_os_SystemProperties.cpp

staticvoidSystemProperties_set(JNIEnv*env,jobjectclazz,jstringkeyJ,jstringvalJ)
{
interr;
constchar*key;
constchar*val;
key=env->GetStringUTFChars(keyJ,NULL);
if(valJ==NULL){
val="";/*NULLpointernotallowedhere*/
}else{
val=env->GetStringUTFChars(valJ,NULL);
}
err=property_set(key,val);
env->ReleaseStringUTFChars(keyJ,key);
if(valJ!=NULL){
env->ReleaseStringUTFChars(valJ,val);
}
}

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

#defineAID_ROOT0/*traditionalunixrootuser*/
#defineAID_SYSTEM1000/*systemserver*/
#defineAID_RADIO1001/*telephonysubsystem,RIL*/
#defineAID_DHCP1014/*dhcpclient*/
#defineAID_SHELL2000/*adbanddebugshelluser*/
#defineAID_CACHE2001/*cacheaccess*/
#defineAID_APP10000/*firstappuser*/

system/core/init/property_service.c

#definePERSISTENT_PROPERTY_DIR"/data/property"
struct{
constchar*prefix;
unsignedintuid;
}property_perms[]={
{"net.rmnet0.",AID_RADIO},
{"net.gprs.",AID_RADIO},
{"ril.",AID_RADIO},
{"gsm.",AID_RADIO},
{"net.dns",AID_RADIO},
{"net.usb0",AID_RADIO},
{"net.",AID_SYSTEM},
{"dev.",AID_SYSTEM},
{"runtime.",AID_SYSTEM},
{"hw.",AID_SYSTEM},
{"sys.",AID_SYSTEM},
{"service.",AID_SYSTEM},
{"wlan.",AID_SYSTEM},
{"dhcp.",AID_SYSTEM},
{"dhcp.",AID_DHCP},
{"debug.",AID_SHELL},
{"log.",AID_SHELL},
{"service.adb.root",AID_SHELL},
{"persist.sys.",AID_SYSTEM},
{"persist.service.",AID_SYSTEM},
{NULL,0}
};
intproperty_set(constchar*name,constchar*value)
{
property_changed(name,value);
return0;
}
intstart_property_service(void)
{
intfd;
load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
load_properties_from_file(PROP_PATH_LOCAL_OVERRIDE);
/*Readpersistentpropertiesafteralldefaultvalueshavebeenloaded.*/
load_persistent_properties();

fd=create_socket(PROP_SERVICE_NAME,SOCK_STREAM,0666,0,0);
if(fd<0)return-1;
fcntl(fd,F_SETFD,FD_CLOEXEC);
fcntl(fd,F_SETFL,O_NONBLOCK);

listen(fd,8);
returnfd;
}
voidhandle_property_set_fd(intfd)
{
switch(msg.cmd){
casePROP_MSG_SETPROP:
msg.name[PROP_NAME_MAX-1]=0;
msg.value[PROP_VALUE_MAX-1]=0;

if(memcmp(msg.name,"ctl.",4)==0){
if(check_control_perms(msg.value,cr.uid)){
handle_control_message((char*)msg.name+4,(char*)msg.value);
}else{
ERROR("sys_prop:Unableto%sservicectl[%s]uid:%dpid:%d\n",
msg.name+4,msg.value,cr.uid,cr.pid);
}
}else{
if(check_perms(msg.name,cr.uid)){
property_set((char*)msg.name,(char*)msg.value);
}else{
ERROR("sys_prop:permissiondenieduid:%dname:%s\n",
cr.uid,msg.name);
}
}
break;

default:
break;
}
}

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

voidproperty_changed(constchar*name,constchar*value)
{
if(property_triggers_enabled){
queue_property_triggers(name,value);
drain_action_queue();
}
}
intmain(intargc,char**argv)
{
parse_config_file("/init.rc");
qemu_init();
device_fd=device_init();
property_init();
fd=open(console_name,O_RDWR);
property_set_fd=start_property_service();
ufds[0].fd=device_fd;
ufds[0].events=POLLIN;
ufds[1].fd=property_set_fd;
ufds[1].events=POLLIN;
ufds[2].fd=signal_recv_fd;
ufds[2].events=POLLIN;
fd_count=3;
for(;;){
if(ufds[0].revents==POLLIN)
handle_device_fd(device_fd);

if(ufds[1].revents==POLLIN)
handle_property_set_fd(property_set_fd);
if(ufds[3].revents==POLLIN)
handle_keychord(keychord_fd);
}
return0;
}