Android(安卓)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

#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"

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

private static nativeString native_get(String key);
private static nativeString native_get(String key, String def);
private static native voidnative_set(String key, String def);
public static voidset(String key, String val) {
if(key.length()>PROP_NAME_MAX) {
throw newIllegalArgumentException( " key.length >" +PROP_NAME_MAX);
}
if(val!= null &&val.length()>PROP_VALUE_MAX) {
throw newIllegalArgumentException( " val.length >" +
PROP_VALUE_MAX);
}
native_set(key, val);
}

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 {
extern intregister_android_os_SystemProperties(JNIEnv* env);
}

frameworks/base/core/jni/android_os_SystemProperties.cpp

static voidSystemProperties_set(JNIEnv* env, jobject clazz, jstring keyJ, jstring valJ)
{
interr;
const char *key;
const char *val;
key=env -> GetStringUTFChars(keyJ, NULL);
if(valJ==NULL) {
val= "" ; /*NULL pointer not allowed here*/
}else{
val=env -> GetStringUTFChars(valJ, NULL);
}
err=property_set(key, val);
env -> ReleaseStringUTFChars(keyJ, key);
if(valJ!=NULL) {
env -> ReleaseStringUTFChars(valJ, val);
}
}

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_ROOT 0/* traditional unix root user */
#defineAID_SYSTEM 1000/* system server */
#defineAID_RADIO 1001/* telephony subsystem, RIL */
#defineAID_DHCP 1014/* dhcp client */
#defineAID_SHELL 2000/* adb and debug shell user */
#defineAID_CACHE 2001/* cache access */
#defineAID_APP 10000 /* first app user */

system/core/init/property_service.c

#definePERSISTENT_PROPERTY_DIR"/data/property"
struct{
const char * 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( const char * name,const char * value)
{
property_changed(name, value);
return 0 ;
}
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);
/*Read persistent properties after all default values have been loaded.*/
load_persistent_properties();
fd=create_socket(PROP_SERVICE_NAME, SOCK_STREAM,0666 ,0 ,0 );
if (fd< 0 )return -;
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 -]= 0 ;
msg.value[PROP_VALUE_MAX -]= 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: Unable to %s service ctl [%s] uid: %d pid:%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: permission denied uid:%dname:%s\n " ,
cr.uid, msg.name);
}
}
break ;
default :
break ;
}
}

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:000
name:gsm.phone.id

system/core/init/init.c

voidproperty_changed( const char * name,const char * 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[].fd=property_set_fd;
ufds[].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[].revents==POLLIN)
handle_property_set_fd(property_set_fd);
if(ufds[ 3 ].revents==POLLIN)
handle_keychord(keychord_fd);
}
return 0 ;
}

来自：http://blog.csdn.net/mars5337/archive/200/07/0/5705840.aspx

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