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

#define PROP_SERVICE_NAME "property_service"
#define PROP_PATH_RAMDISK_DEFAULT "/default.prop"
#define PROP_PATH_SYSTEM_BUILD "/system/build.prop"
#define PROP_PATH_SYSTEM_DEFAULT "/system/default.prop"
#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，

private static native String native_get(String key);
private static native String native_get(String key, String def);
private static native void native_set(String key, String def);
public static void set(String key, String val) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException( " key.length > " + PROP_NAME_MAX);
}
if (val != null && val.length() > PROP_VALUE_MAX) {
throw new IllegalArgumentException( " 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

namespace android {
extern int register_android_os_SystemProperties(JNIEnv * env);
}

frameworks/base/core/jni/android_os_SystemProperties.cpp

static void SystemProperties_set(JNIEnv * env, jobject clazz, jstring keyJ, jstring valJ)
{
int err;
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

#define AID_ROOT 0 /* traditional unix root user */
#define AID_SYSTEM 1000 /* system server */
#define AID_RADIO 1001 /* telephony subsystem, RIL */
#define AID_DHCP 1014 /* dhcp client */
#define AID_SHELL 2000 /* adb and debug shell user */
#define AID_CACHE 2001 /* cache access */
#define AID_APP 10000 /* first app user */

system/core/init/property_service.c

#define PERSISTENT_PROPERTY_DIR "/data/property"
struct {
const char * prefix;
unsigned int uid;
} 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 }
};
int property_set( const char * name, const char * value)
{
property_changed(name, value);
return 0 ;
}
int start_property_service( void )
{
int fd;
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 );
return fd;
}
void handle_property_set_fd( int fd)
{
switch (msg.cmd) {
case PROP_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:%d name:%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

void property_changed( const char * name, const char * value)
{
if (property_triggers_enabled) {
queue_property_triggers(name, value);
drain_action_queue();
}
}
int main( int argc, 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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