Init 属性服务

一、初始化property

void property_init() {
    mkdir("/dev/__properties__", S_IRWXU | S_IXGRP | S_IXOTH);
    CreateSerializedPropertyInfo();
    if (__system_property_area_init()) {
        LOG(FATAL) << "Failed to initialize property area";
    }
    if (!property_info_area.LoadDefaultPath()) {
        LOG(FATAL) << "Failed to load serialized property info file";
    }
}

二、序列化property信息

void CreateSerializedPropertyInfo() {
    auto property_infos = std::vector<PropertyInfoEntry>();
    if (access("/system/etc/selinux/plat_property_contexts", R_OK) != -1) {
        if (!LoadPropertyInfoFromFile("/system/etc/selinux/plat_property_contexts",
                                      &property_infos)) {
            return;
        }
        // Don't check for failure here, so we always have a sane list of properties.
        // E.g. In case of recovery, the vendor partition will not have mounted and we
        // still need the system / platform properties to function.
        if (!LoadPropertyInfoFromFile("/vendor/etc/selinux/vendor_property_contexts",
                                      &property_infos)) {
            // Fallback to nonplat_* if vendor_* doesn't exist.
            LoadPropertyInfoFromFile("/vendor/etc/selinux/nonplat_property_contexts",
                                     &property_infos);
        }
    } else {
        if (!LoadPropertyInfoFromFile("/plat_property_contexts", &property_infos)) {
            return;
        }
        if (!LoadPropertyInfoFromFile("/vendor_property_contexts", &property_infos)) {
            // Fallback to nonplat_* if vendor_* doesn't exist.
            LoadPropertyInfoFromFile("/nonplat_property_contexts", &property_infos);
        }
    }
    auto serialized_contexts = std::string();
    auto error = std::string();
    if (!BuildTrie(property_infos, "u:object_r:default_prop:s0", "string", &serialized_contexts,
                   &error)) {
        LOG(ERROR) << "Unable to serialize property contexts: " << error;
        return;
    }
    constexpr static const char kPropertyInfosPath[] = "/dev/__properties__/property_info";
    if (!WriteStringToFile(serialized_contexts, kPropertyInfosPath, 0444, 0, 0, false)) {
        PLOG(ERROR) << "Unable to write serialized property infos to file";
    }
    selinux_android_restorecon(kPropertyInfosPath, 0);
}

三、启动property服务

void start_property_service() {
    selinux_callback cb;
    cb.func_audit = SelinuxAuditCallback;
    selinux_set_callback(SELINUX_CB_AUDIT, cb);
    property_set("ro.property_service.version", "2");
    property_set_fd = CreateSocket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
                                   false, 0666, 0, 0, nullptr);
    if (property_set_fd == -1) {
        PLOG(FATAL) << "start_property_service socket creation failed";
    }
    listen(property_set_fd, 8);
    register_epoll_handler(property_set_fd, handle_property_set_fd);
}

四、设置属性

static void handle_property_set_fd() {
    static constexpr uint32_t kDefaultSocketTimeout = 2000; /* ms */

    int s = accept4(property_set_fd, nullptr, nullptr, SOCK_CLOEXEC);
    if (s == -1) {
        return;
    }

    ucred cr;
    socklen_t cr_size = sizeof(cr);
    if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
        close(s);
        PLOG(ERROR) << "sys_prop: unable to get SO_PEERCRED";
        return;
    }

    SocketConnection socket(s, cr);
    uint32_t timeout_ms = kDefaultSocketTimeout;

    uint32_t cmd = 0;
    if (!socket.RecvUint32(&cmd, &timeout_ms)) {
        PLOG(ERROR) << "sys_prop: error while reading command from the socket";
        socket.SendUint32(PROP_ERROR_READ_CMD);
        return;
    }

    switch (cmd) {
    case PROP_MSG_SETPROP: {
        char prop_name[PROP_NAME_MAX];
        char prop_value[PROP_VALUE_MAX];

        if (!socket.RecvChars(prop_name, PROP_NAME_MAX, &timeout_ms) ||
            !socket.RecvChars(prop_value, PROP_VALUE_MAX, &timeout_ms)) {
          PLOG(ERROR) << "sys_prop(PROP_MSG_SETPROP): error while reading name/value from the socket";
          return;
        }

        prop_name[PROP_NAME_MAX-1] = 0;
        prop_value[PROP_VALUE_MAX-1] = 0;

        const auto& cr = socket.cred();
        std::string error;
        uint32_t result =
            HandlePropertySet(prop_name, prop_value, socket.source_context(), cr, &error);
        if (result != PROP_SUCCESS) {
            LOG(ERROR) << "Unable to set property '" << prop_name << "' to '" << prop_value
                       << "' from uid:" << cr.uid << " gid:" << cr.gid << " pid:" << cr.pid << ": "
                       << error;
        }

        break;
      }

    case PROP_MSG_SETPROP2: {
        std::string name;
        std::string value;
        if (!socket.RecvString(&name, &timeout_ms) ||
            !socket.RecvString(&value, &timeout_ms)) {
          PLOG(ERROR) << "sys_prop(PROP_MSG_SETPROP2): error while reading name/value from the socket";
          socket.SendUint32(PROP_ERROR_READ_DATA);
          return;
        }

        const auto& cr = socket.cred();
        std::string error;
        uint32_t result = HandlePropertySet(name, value, socket.source_context(), cr, &error);
        if (result != PROP_SUCCESS) {
            LOG(ERROR) << "Unable to set property '" << name << "' to '" << value
                       << "' from uid:" << cr.uid << " gid:" << cr.gid << " pid:" << cr.pid << ": "
                       << error;
        }
        socket.SendUint32(result);
        break;
      }

    default:
        LOG(ERROR) << "sys_prop: invalid command " << cmd;
        socket.SendUint32(PROP_ERROR_INVALID_CMD);
        break;
    }
}