Android P: U盘挂载过程简述

                                                       图 vold模块简要的层次结构图

 

                                                                                           图 U盘挂载事件的函数调用时序图

其中，在PublicVolume接收到StorageManagerService中的mount方法执行sdcard命令时，会传入相应的参数，如下代码所示：

            if (execl(kFusePath, kFusePath,
                    "-u", "1023", // AID_MEDIA_RW
                    "-g", "1023", // AID_MEDIA_RW
                    "-U", std::to_string(getMountUserId()).c_str(),
                    mRawPath.c_str(),
                    stableName.c_str(),
                    NULL)) {
                PLOG(ERROR) << "Failed to exec";
            }

其中u, g, U等最终会传递给系统调用mount，因此在这条调用中加入w参数，最终挂载的U盘便对上层应用开放了完整的权限，如下：

        if (execl(kFusePath, kFusePath,
                    "-u", "1023", // AID_MEDIA_RW
                    "-g", "1023", // AID_MEDIA_RW
                    "-U", std::to_string(getMountUserId()).c_str(),
                    "-w", // WRITE permission of USB storage is needed
                    mRawPath.c_str(),
                    stableName.c_str(),
                    NULL)) {
                PLOG(ERROR) << "Failed to exec";
            }