[release/0.1] forward ro bind-mount option to virtio-fs share

[dmcgowan]

The ext4 mount path in mount.go already parses ro from the OCI mount options; the bind-mount path silently dropped it. Plumb the flag through bindMounter and vmInstance.AddFS so the virtio-fs share itself is marked read-only at the host edge via krun_add_virtiofs3 (libkrun >= 1.18), giving us defense-in-depth on top of the guest-side ro bind mount and opening the door to VMM-side optimizations on immutable shares. When the loaded libkrun is older and does not export krun_add_virtiofs3, the binding falls back to krun_add_virtiofs with an info log and enforcement stays on the guest mount options exactly as today.

(cherry picked from commit 6db5a00)

[Copilot]

Pull request overview

This PR extends the bind-mount (virtio-fs) path to honor the OCI ro mount option by plumbing a read-only flag through the shim’s bind-mount transform into the libkrun virtio-fs attachment call, adding defense-in-depth by enforcing read-only at the host-side virtio-fs share.

Changes:

• Parse ro/rw from OCI bind-mount options and forward the resulting read-only intent into sandbox.WithFS(...).
• Extend the libkrun virtio-fs attachment API to call krun_add_virtiofs3(..., readonly) and plumb the flag through vmInstance.AddFS.
• Add unit tests covering both bind-mount option parsing and libkrun readonly forwarding; bump documented/built libkrun version to >= 1.18.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Documents the libkrun >= 1.18 requirement for krun_add_virtiofs3 readonly support.
internal/vm/libkrun/krun.go	Switches virtio-fs attachment to krun_add_virtiofs3 and adds a readonly parameter.
internal/vm/libkrun/krun_test.go	Adds unit tests validating readonly propagation and error behavior for virtio-fs attach.
internal/vm/libkrun/instance.go	Plumbs vm.MountOpt readonly config into the virtio-fs attachment call.
internal/shim/task/mount.go	Parses ro/rw for bind mounts and forwards to sandbox.WithFS(..., readonly).
internal/shim/task/mount_test.go	Adds assertions that bind-mount readonly flows into sandbox filesystem opts.
Dockerfile	Bumps the libkrun build ARG version to v1.18.1.

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