Android 的系統屬性(SystemProperties)設定分析

Android 的系統屬性包括兩部分：檔案儲存的持久屬性和每次開機匯入的cache屬性。前者主要儲存在下面幾個檔案中：

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"

後者則通過frameworks/base/core/java/android/os/SystemProperties.java的介面定義，

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);
}

該介面類在初始化運行環境中註冊對應的cpp介面android_os_SystemProperties.cpp，實際操作通過JNI調用的是cpp檔案對應的介面：

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);
}
}

設定key的value時，需要作鑒權，根據設定程式所在進程的fd獲知uid值，比如system server進程可以設定net打頭的key，不可以設定gsm打頭的key，相關的定義如下：

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