Android系统属性SystemProperties分析 .
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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
#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"
2.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);}
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
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);}}
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
<SPAN style="FONT-FAMILY: SimSun; FONT-SIZE: 16px">#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 */</SPAN>
system/core/init/property_service.c
<SPAN style="FONT-FAMILY: SimSun; FONT-SIZE: 16px">#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 -1;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-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: 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;}}</SPAN>
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
<SPAN style="FONT-FAMILY: SimSun; FONT-SIZE: 16px">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[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);}return 0;}</SPAN>
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