Android P: U盤掛載過程簡述

                                                       圖 vold模塊簡要的層次結構圖

 

                                                                                           圖 U盤掛載事件的函數調用時序圖

其中，在PublicVolume接收到StorageManagerService中的mount方法執行sdcard命令時，會傳入相應的參數，如下代碼所示：

            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等最終會傳遞給系統調用mount，因此在這條調用中加入w參數，最終掛載的U盤便對上層應用開放了完整的權限，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";
            }