android12-release

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1、SystemProperties属性加载

查看 SystemProperties属性加载 属性映射区域LoadPath("/dev/__properties__/property_info")；在PropertyLoadBootDefaults()加载如下配置文件等属性，最终设置PropertySet()

• /system/build.prop
• /system_ext/etc/build.prop
  /system_ext/default.prop
  /system_ext/build.prop
• /vendor/default.prop
• /vendor/build.prop
• /vendor_dlkm/etc/build.prop
• /odm_dlkm/etc/build.prop
• /odm/etc/build.prop
  /odm/default.prop
  /odm/build.prop
• /product/etc/build.prop
  /product/default.prop
  /product/build.prop

2、PropertySet条件限制

查看 SystemProperties属性加载 条件限制IsLegalPropertyName、IsLegalPropertyValue()

static uint32_t PropertySet(const std::string& name, const std::string& value, std::string* error) {size_t valuelen = value.size();if (!IsLegalPropertyName(name)) {*error = "Illegal property name";return PROP_ERROR_INVALID_NAME;}if (auto result = IsLegalPropertyValue(name, value); !result.ok()) {*error = result.error().message();return PROP_ERROR_INVALID_VALUE;}prop_info* pi = (prop_info*) __system_property_find(name.c_str());if (pi != nullptr) {// ro.* properties are actually "write-once".if (StartsWith(name, "ro.")) {*error = "Read-only property was already set";return PROP_ERROR_READ_ONLY_PROPERTY;}__system_property_update(pi, value.c_str(), valuelen);} else {int rc = __system_property_add(name.c_str(), name.size(), value.c_str(), valuelen);if (rc < 0) {*error = "__system_property_add failed";return PROP_ERROR_SET_FAILED;}}// Don't write properties to disk until after we have read all default// properties to prevent them from being overwritten by default values.if (persistent_properties_loaded && StartsWith(name, "persist.")) {WritePersistentProperty(name, value);}// If init hasn't started its main loop, then it won't be handling property changed messages// anyway, so there's no need to try to send them.auto lock = std::lock_guard{accept_messages_lock};if (accept_messages) {PropertyChanged(name, value);}return PROP_SUCCESS;
}

3、SystemProperties属性变化

PropertySet条件限制 关注"ro."、"persist."开头属性：

• "ro."属性实际上是“一次写入”，只能设置一次
• "persist."在我们读取所有默认属性之前，不要将persist开头属性写入磁盘，以防止它们被默认值覆盖。，所以只有persist开头的系统属性在重启后保持不变
• persistent_properties_loaded表示StartPropertyService(&property_fd)服务启动完成，即ro.persistent_properties.ready=true。PropertyService在PropertyInit()之后启动。
• WritePersistentProperty(name, value)其中persist开头属性位置std::string persistent_property_filename = "/data/property/persistent_properties"
  system/core/init/persistent_properties.cpp