Linxr | Part 6 — Building Virtio-9P Storage Sharing & SAF on Android

In Part 5 of our engineering series, we introduced the high-level features of Linxr v3.0.0. Today in Part 6, we take a deep technical dive into how we solved host-to-guest file sharing on modern Android using Virtio-9P and Android’s Storage Access Framework (SAF).

The Challenge: Android Scoped Storage vs Linux Filesystems

Standard Linux virtual machines expect direct POSIX file path access (/home/root/files, /mnt/storage). However, Android 10+ enforces strict Scoped Storage rules and granular URI permissions.

Passing raw host paths to QEMU without root permissions often fails due to Android app sandbox boundaries. We needed a dual-layer solution:

1. Direct high-speed POSIX passthrough for app-managed storage.


2. User-configurable folder selection via Android Storage Access Framework (SAF).

Layer 1: Virtio-9P Driver in QEMU

QEMU provides the 9P network filesystem protocol (virtio-9p-pci), which exposes host directory trees to Linux guests with low CPU overhead.

We configured QEMU command-line parameters in VmManager.kt:

val virtioArgs = listOf(

    "-fsdev", "local,id=fsdev0,path=$sharePath,security_model=none",

    "-device", "virtio-9p-pci,fsdev=fsdev0,mount_tag=LinxrShare"

)

Inside Alpine Linux, an OpenRC startup service automounts this tag on boot:

mount -t 9p -o trans=virtio,version=9p2000.L LinxrShare /mnt/sdcard

Any file saved in /storage/emulated/0/LinxrShare on Android instantly appears inside Alpine Linux under /mnt/sdcard!

Layer 2: Storage Access Framework (SAF) & pickFolder Channel

To allow users to select custom folders (such as Downloads, Documents, or external SD cards), we implemented a native Kotlin MethodChannel handler (pickFolder):

MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "com.ai2th.linxr/vm")

    .setMethodCallHandler { call, result ->

        if (call.method == "pickFolder") {

            val intent = Intent(Intent.ACTION_OPEN_DOCUMENT_TREE).apply {

                addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION or Intent.FLAG_GRANT_WRITE_URI_PERMISSION)

            }

            activity.startActivityForResult(intent, REQUEST_CODE_SAF_PICK_FOLDER)

        }

    }

When a user selects a folder, Android returns a persistent DocumentFile URI. We resolve the underlying POSIX path and pass it to QEMU's Virtio-9P filesystem driver.

What’s Next?

In Part 7, we will explore how we embedded the Docker Engine and built a real-time container management dashboard in Flutter!

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