Android系统启动流程（三）——属性服务（基于Android13）

weixin_49274713

已于 2023-07-09 18:09:34 修改

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分类专栏： # 系统启动流程文章标签： android java 开发语言

于 2023-05-06 19:16:53 首次发布

本文链接：https://blog.youkuaiyun.com/weixin_49274713/article/details/129348743

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文章目录

1 属性服务
2 启动属性服务
3 启动属性服务
4 设置属性
5 总结

1 属性服务

属性服务的启动在init进程中，init进程初始化的第二阶段初始化中启动了属性服务。
system/core/init/init.cpp

int SecondStageMain(int argc, char** argv) {
   
	...
	PropertyInit();
	...
	StartPropertyService(&property_fd);
	...

2 启动属性服务

system/core/init/property_service.cpp

void PropertyInit() {
   
    selinux_callback cb;
    cb.func_audit = PropertyAuditCallback;
    selinux_set_callback(SELINUX_CB_AUDIT, cb);

    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";
    }

    // If arguments are passed both on the command line and in DT,
    // properties set in DT always have priority over the command-line ones.
    ProcessKernelDt();
    ProcessKernelCmdline();
    ProcessBootconfig();

    // Propagate the kernel variables to internal variables
    // used by init as well as the current required properties.
    ExportKernelBootProps();

    PropertyLoadBootDefaults();
}

接下来分析一下这个函数

mkdir("/dev/__properties__", S_IRWXU | S_IXGRP | S_IXOTH);

首先创建属性服务的存储目录，位于/dev/properties，权限为711，即rwx–x–x

2.1 写入序列化属性上下文信息

system/core/init/property_service.cpp

void CreateSerializedPropertyInfo() {
   
    //创建一个vector用于存储selinux中属性相关
    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",   //这个函数的具体作用就是从文件中解析属性相关的，写到vector中
                                      &property_infos)) {
   
            return;
        }
        // Don't check for failure here, since we don't always have all of these partitions.
        // E.g. In case of recovery, the vendor partition will not have mounted and we
        // still need the system / platform properties to function.
        if (access("/dev/selinux/apex_property_contexts", R_OK) != -1) {
   
            LoadPropertyInfoFromFile("/dev/selinux/apex_property_contexts", &property_infos);
        }
        if (access("/system_ext/etc/selinux/system_ext_property_contexts", R_OK) != -1) {
   
            LoadPropertyInfoFromFile("/system_ext/etc/selinux/system_ext_property_contexts",
                                     &property_infos);
        }
        if (access("/vendor/etc/selinux/vendor_property_contexts", R_OK) != -1) {
   
            LoadPropertyInfoFromFile("/vendor/etc/selinux/vendor_property_contexts",
                                     &property_infos);
        }
        if (access("/product/etc/selinux/product_property_contexts", R_OK) != -1) {
   
            LoadPropertyInfoFromFile("/product/etc/selinux/product_property_contexts",
                                     &property_infos);
        }
        if (access("/odm/etc/selinux/odm_property_contexts", R_OK) != -1) {
   
            LoadPropertyInfoFromFile("/odm/etc/selinux/odm_property_contexts", &property_infos);
        }
    } else {
   
        if (!LoadPropertyInfoFromFile("/plat_property_contexts", &property_infos)) {
   
            return;
        }
        LoadPropertyInfoFromFile("/system_ext_property_contexts", &property_infos);
        LoadPropertyInfoFromFile("/vendor_property_contexts", &property_infos);
        LoadPropertyInfoFromFile("/product_property_contexts", &property_infos);
        LoadPropertyInfoFromFile("/odm_property_contexts", &property_infos);
        LoadPropertyInfoFromFile("/dev/selinux/apex_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;
    }

    //上面解析的数据都写到了property_info中
    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);
}

创建一个存储属性服务的vector——property_infos，存储的是PropertyInfoEntry，是一个结构体

struct PropertyInfoEntry {
   
  PropertyInfoEntry() {
   }
  template <typename T, typename U, typename V>
  PropertyInfoEntry(T&& name, U&& context, V&& type, bool exact_match)
      : name(std::forward<T>(name)),
        context(std::forward<U>(context)),
        type(std::forward<V>(type)),
        exact_match(exact_match) {
   }
  std::string name;
  std::string context;
  std::string type;
  bool exact_match;
};

包含一些函数和一些字段。

if (!LoadPropertyInfoFromFile("/system/etc/selinux/plat_property_contexts", &property_infos)) {
   
    return;
}

然后就是调用LoadPropertyInfoFromFile加载各个镜像中能够的属性上下文到property_infos这个结构体中。

bool LoadPropertyInfoFromFile(const std::string& filename,
                              std::vector<PropertyInfoEntry>* property_infos) {
   
    auto file_contents = std::string();
    if (!ReadFileToString(filename, &file_contents)) {
   
        PLOG(ERROR) << "Could not read properties from '" << filename << "'";
        return false;
    }

    auto errors = std::vector<std::string>{
   };
    bool require_prefix_or_exact = SelinuxGetVendorAndroidVersion() >= __ANDROID_API_R__;
    ParsePropertyInfoFile(file_contents, require_prefix_or_exact, property_infos, &errors);
    // Individual parsing errors are reported but do not cause a failed boot, which is what
    // returning false would do here.
    for (const auto& error : errors) {
   
        LOG(ERROR) << "Could not read line from '" << filename << "': " << error;
    }

    return true;
}

读取文件中的属性上下文，并生成string字符串file_contents。require_prefix_or_exact用于判断属性是不是分离存放在各个镜像中，如果版本大于R，安卓11，则为true。
system/core/property_service/libpropertyinfoserializer/property_info_file.cpp

void ParsePropertyInfoFile(const std::string& file_contents, bool require_prefix_or_exact,
                           std::vector<PropertyInfoEntry>* property_infos,
                           std::vector<std::string>* errors) {
   
  // Do not clear property_infos to allow this function to be called on multiple files, with
  // their results concatenated.
  errors->clear();

  for (const auto& line : Split(file_contents, "\n")) {
   
    auto trimmed_line = Trim(line);
    if (trimmed_line.empty() || StartsWith(trimmed_line, "#")) {
   
      continue;
    }

    auto property_info_entry = PropertyInfoEntry{
   };
    auto parse_error = std::string{
   };
    if (!ParsePropertyInfoLine(trimmed_line, require_prefix_or_exact, &property_info_entry,
                               &parse_error)) {
   
      errors->emplace_back(parse_error);
      continue;
    }

    property_infos->emplace_back(property_info_entry);
  }
}

然后解析文件中的属性，以行\n为分割，然后在ParsePropertyInfoLine中解析。

bool ParsePropertyInfoLine(const std::string& line, bool require_prefix_or_exact,
                           PropertyInfoEntry* out, std::string* error)<

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