Android 的系统属性---SystemProperties分析

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最近在公司搞一个驱动未适配的问题，焦头烂额呀。从网上罗来一些资料。这个转载自从博客园的JAVA社区

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，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.cppframeworks/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.csystem/core/init/property_service.c#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 -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;    }}

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

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;}

还有如下一些资源

http://blog.csdn.net/xujianqun/article/details/6363318 property_get/property_set

http://blog.csdn.net/jackyu613/article/details/6136620 Android Property System | Android属性系统