WSL/doc/docs/technical-documentation/drvfs.md

Accessing Windows drives from Linux

WSL offers mountpoints to access Windows drives from Linux. These mountpoints are mounted under /mnt by default, and point to the root of Windows drives.

Elevated vs non-elevated mountpoints

Within a distribution, WSL separates between Linux processes that have been created from an elevated (as in administrator level) and from a non-elevated (user level) context.

This is done by having two separate mount namespaces within the distribution. One of them offers an elevated access to Windows drives, and the other offers a non-elevated access to Windows drives.

When a Linux process is created, wslservice.exe determines its elevation status, and then tell init to create the process in the appropriate mount namespace.

Mounting a Windows drive

*Note: This section only applies to WSL2 distributions. *

When a session leader is created, wslservice.exe starts a plan9 file server. This file server can be connected to from the WSL2 virtual machine to mount Windows drives.

When the WSL distribution is created, wslservice.exe uses the LX_INIT_CONFIGURATION_INFORMATION message to indicate whether the process that created the distribution is elevated or not. Based on this, init will mount either or the elevated, or un-elevated version of the plan9 server.

Later when the first command is created in the namespace that hasn't been mounted yet, (either elevated, or non-elevated), wslservice.exe sends a LxInitMessageRemountDrvfs to init, which tell init to mount the other namespace.

See: src/windows/service/exe/WslCoreInstance.cpp and src/linux/drvfs.cpp.

Mounting a drive from Linux

As long as the Windows plan9 server is running, drives can be mounted simply by calling mount. For instance mounting the C: drive manually can be done via:

mount -t drvfs C: /tmp/my-mount-point

Internally, this handled by /usr/sbin/mount.drvfs, which is a symlink to /init. When /init starts, it looks at argv[0] to determine which entrypoint to run. If argv[0] is mount.drvfs, then /init runs the mount.drvfs entrypoint (see MountDrvfsEntry() in src/linux/init/drvfs.cpp).

Depending on the distribution configuration, mount.drvfs will either mount the drive as drvfs (WSL1), or plan9, virtio-plan9 or virtiofs (WSL), depending on .wslconfig.