OpenShell/crates/openshell-driver-podman

openshell-driver-podman

The Podman compute driver runs inside the gateway and uses the native libpod REST API over a Unix socket. Each sandbox has two independent containers:

• openshell-sandbox owns the agent process in the workload container.
• openshell-supervisor evaluates policy, holds gateway credentials, and proxies approved egress in a separate companion container.

The driver provisions placement, identity, credentials, transport, and lifecycle. The shared isolation interface supplies exec, attach, signal, terminate, binary identity, DNS, TCP, and loopback-forwarding semantics.

Runtime posture

Caller driver config is disabled by default. Existing volumes require administrator-controlled approval labels; bind and supplemental image mounts are denied under enforcement. Private-volume names alone do not prove ownership. GPU devices are temporarily exempt. Admission runs before launch, restart, and periodically for running workloads. See resource admission configuration.

Property	Workload	Supervisor
UID/GID	Pinned non-root workload identity	Same mapped identity
Capabilities	Drop all; add none	Drop all; add none
Seccomp	Runtime default plus sandbox-installed filters	Runtime default
Network	none; loopback only	Podman host network
Gateway JWT and upstream credentials	Never mounted	Podman secrets
User volumes and CDI devices	Workload only	Never mounted
Channel	Private named volume, writable	Same volume, read-only

Podman creates the namespaces and volume ownership before the workload runs. Rootless operation uses the operator's Podman service and subordinate-ID configuration; it does not require adding capabilities to either container. The supervisor joins the workload's user namespace only to preserve UID/GID mapping for shared-volume access. PID, mount, and network namespaces remain separate. The channel volume uses shared SELinux relabeling (:z).

Before starting either container, the driver uploads volume-relative archives directly to the channel and workspace volume destinations. A rootfs upload on a stopped Podman container does not populate nested named volumes. Restart restores only the channel bootstrap into the existing channel volume, preserving the workspace. The workload starts before the supervisor so its user namespace exists when the supervisor joins it; a stopped supervisor resolves that namespace again on its next start. The driver creates the managed workspace volume owned by the workload's final UID and GID, so the workload never starts as root. Custom image workspaces have no workspace volume or upload.

The runtime must pass the sandbox's unprivileged enforcement probe, including nested seccomp notification and Landlock. Unsupported runtime defaults fail closed; do not switch to an unconfined profile or add capabilities.

Protected channel and network enforcement

agent -> openshell-sandbox === authenticated gRPC / private UDS === supervisor -> network
         network=none           TCP, DNS, control streams         policy + gateway JWT

The workload has no external interface or published port. Seccomp socket mediation carries TCP and DNS through one authenticated gRPC connection. DNS remains supervisor-mediated; general UDP is unsupported. The driver sets net.ipv4.ip_unprivileged_port_start=0 in the isolated workload network namespace so the sandbox's loopback DNS relay can bind port 53 without a capability. No nftables or nested network namespace setup runs in the sandbox.

The channel contains the sandbox bootstrap and sandbox-side TLS identity only. Supervisor private keys and the runtime descriptor stay in the supervisor's private filesystem. Landlock denies agent access to the top-level /.openshell control hierarchy. The driver verifies Podman's reported network=none fence before launch and restart. Host networking applies to the supervisor, not the agent.

Gateway sessions use the sandbox JWT and optional server-authenticated TLS. Only the gateway CA is delivered to the supervisor; user client certificates and private keys are not mounted into either container. The sandbox/supervisor channel always uses its separate, per-sandbox mutual TLS material. These are distinct authentication relationships.

Identity and trusted binaries

Both workload and supervisor images are pinned by immutable image ID. The driver reads account files from a stopped workload-image container; it never executes the image to resolve an account. Policy identity fields override OCI USER independently. Named users/groups resolve against that image, including supplementary groups. Root and unresolved identities fail before provisioning. Images must not prepopulate the reserved /.openshell hierarchy; this prevents image-controlled symlinks from aliasing private control state into user mounts.

sandbox_runtime_image supplies the statically linked musl /openshell-sandbox binary. Podman's read-only image volume delivers it to the workload; user-namespace modes that cannot use image volumes retain the trusted binary extraction path. supervisor_image supplies the dynamically linked glibc /openshell-supervisor binary outside the workload. Image and request environment belong to agent children, never the supervisor process.

OCI working directory

OpenShell reads WORKDIR from the workload image. If it is unset, /, or /sandbox, OpenShell uses its managed /sandbox workspace volume. A custom path must be absolute and normalized, and cannot overlap /proc, /sys, /dev, OpenShell-reserved paths, or the workload's private control and CA mounts. Image volumes and driver mounts cannot cover it; mounts nested below it remain valid.

A custom path stays in the image's container filesystem with its ownership and permissions. The workload starts as the final non-root user, which must be able to reach and write the directory. Agent commands use the path as their working directory.

Custom workspace contents survive stop/start of the same workload container, but are removed with that container. They do not live in a managed named volume and are not copied to a replacement container.

Lifecycle and readiness

Create builds both stopped containers and stages the private archives before starting either container. The sandbox does not execute the agent until the supervisor authenticates and confirms the common boundary contract. Failed creation removes only containers created by that attempt, then cleans up driver-owned volumes and secrets.

Stop retains both containers, workspace, channel, and secrets. Start restores the consumed sandbox bootstrap from a copy in the supervisor's private filesystem, verifies the fence, and starts the same pair. A failed supervisor start stops the workload. Delete removes the companion first, then the workload, channel, workspace, and driver-owned secrets. User-owned volumes are retained.

Only workload containers appear in sandbox list/watch results. Readiness uses the supervisor's private health socket; there is no shell, legacy marker, or TCP-listener shortcut. Watch reconciliation and supervisor exit/removal events stop a running workload whose companion is unavailable. The gateway also requires the authenticated supervisor session before publishing Ready.

Mounts, GPUs, and configuration

User bind, volume, tmpfs, and image mounts and CDI GPU selection remain native Podman features and apply only to the workload. Bind mounts require the operator's enable_bind_mounts opt-in and disabled label admission. Supplemental image mounts also require disabled admission. Driver JSON requires allow_driver_config = true. Reserved control paths and the workspace root cannot be replaced. User-owned volumes are never created or deleted.

See gateway configuration for operator settings and NETWORKING.md for supervisor networking. The supervisor uses Podman's host network and owns the upstream proxy settings. Omit health_check_interval_secs to disable Podman's periodic health command. Explicit zero is invalid. OpenShell still gates readiness on the supervisor's authenticated health signal.

Gateway OTLP configuration continues to export compute-driver spans under the openshell-driver-podman service, preserving gateway trace context.