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>