feat(macos): replace rootless Docker spike with rootless podman

Podman avoids the CAP_SYS_ADMIN requirement that makes rootless Docker
impossible here: with no subordinate UID range configured it falls back
to a single-UID self-mapping, which an unprivileged process may write
itself, so newuidmap is never invoked. The image build therefore strips
/etc/subuid and /etc/subgid entries rather than adding them.

The agent-facing surface is unchanged — docker and docker compose talk
to podman's Docker-compatible API socket.

Two device nodes need relaxing as root inside the bottle (0666 on
/dev/fuse and /dev/net/tun); both already exist and neither needs a
capability the bottle lacks, unlike CAP_SYS_ADMIN.

Verified live on macOS 26 / Apple Container 1.0.0: service starts with
zero added capabilities, compose pulls and serves from the workspace on
a published port, and a nested container cannot reach the network
outside the egress path.

Known limitation, not yet addressed: the agent base image is Debian
bookworm, whose podman 4.3.1 swallows container exit codes through the
compat API (docker run returns 0 regardless). podman 5.4.2 on trixie
fixes it; the base bump is a separate PR. The acceptance test asserts
on an in-band marker rather than an exit code so it cannot false-pass
in the meantime.

Nested containers give no isolation from the agent itself — root inside
one is the agent user outside it. They are a build/test convenience;
the bottle remains the security boundary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GnMp8SUGH57hZX7192Rv2F

bot_bottle/backend/macos_container/launch.py

@@ -64,7 +64,7 @@ from .gateway_hosts import (
     refresh_gateway_host,
     set_gateway_host,
 )
-from . import rootless_docker
+from . import rootless_podman
 from .bottle_plan import MacosContainerBottlePlan
 from ...orchestrator.config_store import resolve_teardown_timeout
 from .consolidated_launch import (
@@ -174,7 +174,7 @@ def launch(
 
         exec_env = {
             **_identity_proxy_env(endpoint, ctx.identity_token),
-            **rootless_docker.guest_env(plan.docker_access),
+            **rootless_podman.guest_env(plan.docker_access),
         }
         bottle = MacosContainerBottle(
             plan.container_name,
@@ -194,7 +194,10 @@ def launch(
         bottle.prompt_path = provision(plan, bottle)
 
         if plan.docker_access:
-            rootless_docker.start(bottle)
+            rootless_podman.prepare_guest_devices(
+                plan.container_name, container_mod.exec_container_as_root,
+            )
+            rootless_podman.start(bottle)
 
         yield bottle
     finally:
@@ -218,7 +221,7 @@ def _build_images(plan: MacosContainerBottlePlan) -> MacosContainerBottlePlan:
             plan.image, _REPO_DIR, dockerfile=plan.dockerfile_path,
         )
     if plan.docker_access:
-        image = rootless_docker.build_image(plan.image, container_mod.build_image)
+        image = rootless_podman.build_image(plan.image, container_mod.build_image)
         plan = dataclasses.replace(
             plan,
             agent_provision=dataclasses.replace(plan.agent_provision, image=image),

bot_bottle/backend/macos_container/rootless-docker-init.sh

@@ -1,57 +0,0 @@
-#!/bin/sh
-set -eu
-
-uid="$(id -u)"
-if [ "$uid" -eq 0 ]; then
-    echo "refusing to run rootless Docker as root" >&2
-    exit 1
-fi
-
-for command in dockerd-rootless.sh rootlesskit slirp4netns newuidmap newgidmap docker; do
-    command -v "$command" >/dev/null 2>&1 || {
-        echo "missing rootless Docker prerequisite: $command" >&2
-        exit 1
-    }
-done
-
-grep -q "^$(id -un):.*:65536$" /etc/subuid || {
-    echo "missing 65536-entry subordinate UID range for $(id -un)" >&2
-    exit 1
-}
-grep -q "^$(id -gn):.*:65536$" /etc/subgid || {
-    echo "missing 65536-entry subordinate GID range for $(id -gn)" >&2
-    exit 1
-}
-
-export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/tmp/bot-bottle-docker-run}"
-mkdir -p "$XDG_RUNTIME_DIR" "$HOME/.docker"
-chmod 700 "$XDG_RUNTIME_DIR"
-
-# Docker uses this config for build and child-container proxy injection.  The
-# token-bearing proxy URL is already available to the agent; persisting it
-# inside this disposable VM does not broaden its authority.
-python3 - <<'PY'
-import json
-import os
-from pathlib import Path
-
-proxy = os.environ.get("HTTPS_PROXY") or os.environ.get("https_proxy", "")
-no_proxy = os.environ.get("NO_PROXY") or os.environ.get("no_proxy", "")
-config = {"proxies": {"default": {
-    "httpProxy": proxy,
-    "httpsProxy": proxy,
-    "noProxy": no_proxy,
-}}}
-path = Path.home() / ".docker" / "config.json"
-path.write_text(json.dumps(config), encoding="utf-8")
-path.chmod(0o600)
-PY
-
-if docker info >/dev/null 2>&1; then
-    exit 0
-fi
-
-log=/tmp/bot-bottle-rootless-docker.log
-nohup dockerd-rootless.sh \
-    --storage-driver=fuse-overlayfs \
-    >"$log" 2>&1 </dev/null &

bot_bottle/backend/macos_container/rootless-podman-init.sh

@@ -0,0 +1,89 @@
+#!/bin/sh
+set -eu
+
+uid="$(id -u)"
+if [ "$uid" -eq 0 ]; then
+    echo "refusing to run rootless podman as root" >&2
+    exit 1
+fi
+
+for command in podman docker fuse-overlayfs slirp4netns; do
+    command -v "$command" >/dev/null 2>&1 || {
+        echo "missing rootless podman prerequisite: $command" >&2
+        exit 1
+    }
+done
+
+# The inverse of the rootless-Docker check, and the whole point of the podman
+# variant: a subordinate range would push podman onto newuidmap, which cannot
+# write a multi-range uid_map without CAP_SYS_ADMIN in this guest.  An empty
+# range keeps it on the single-UID self-mapping an unprivileged process may
+# write itself.
+if grep -q "^$(id -un):" /etc/subuid 2>/dev/null; then
+    echo "unexpected subordinate UID range for $(id -un): podman would" >&2
+    echo "require CAP_SYS_ADMIN via newuidmap in this guest" >&2
+    exit 1
+fi
+
+for device in /dev/fuse /dev/net/tun; do
+    [ -r "$device" ] && [ -w "$device" ] || {
+        echo "device $device is not readable/writable by $(id -un)" >&2
+        exit 1
+    }
+done
+
+export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/tmp/bot-bottle-podman-run}"
+config="$HOME/.config/containers"
+mkdir -p "$XDG_RUNTIME_DIR" "$config"
+chmod 700 "$XDG_RUNTIME_DIR"
+
+# ignore_chown_errors is required, not incidental: with a single-UID mapping
+# there is no second UID for image layers to be chowned to, so layers that
+# record other owners would otherwise fail to extract.
+cat > "$config/storage.conf" <<'CONF'
+[storage]
+driver="overlay"
+[storage.options.overlay]
+mount_program="/usr/bin/fuse-overlayfs"
+ignore_chown_errors="true"
+CONF
+
+# No cgroup delegation reaches this guest, so asking podman to manage cgroups
+# fails; events_logger=file avoids the journald socket that is equally absent.
+cat > "$config/containers.conf" <<'CONF'
+[containers]
+cgroups="disabled"
+[engine]
+cgroup_manager="cgroupfs"
+events_logger="file"
+CONF
+
+# Registry pulls egress through the bottle's proxy like everything else. The
+# token-bearing proxy URL is already in the agent's environment; persisting it
+# inside this disposable VM does not broaden its authority.
+python3 - <<'PY'
+import json
+import os
+from pathlib import Path
+
+proxy = os.environ.get("HTTPS_PROXY") or os.environ.get("https_proxy", "")
+no_proxy = os.environ.get("NO_PROXY") or os.environ.get("no_proxy", "")
+config = {"proxies": {"default": {
+    "httpProxy": proxy,
+    "httpsProxy": proxy,
+    "noProxy": no_proxy,
+}}}
+path = Path.home() / ".docker" / "config.json"
+path.parent.mkdir(parents=True, exist_ok=True)
+path.write_text(json.dumps(config), encoding="utf-8")
+path.chmod(0o600)
+PY
+
+if docker info >/dev/null 2>&1; then
+    exit 0
+fi
+
+log=/tmp/bot-bottle-rootless-podman.log
+nohup podman system service --time=0 \
+    "unix://$XDG_RUNTIME_DIR/podman.sock" \
+    >"$log" 2>&1 </dev/null &

bot_bottle/backend/macos_container/rootless_docker.py

@@ -1,94 +0,0 @@
-"""Experimental rootless Docker bootstrap for Apple-container bottles.
-
-The daemon and every child remain inside the existing per-bottle VM.  This
-module deliberately refuses to compensate for missing prerequisites with
-outer capabilities, a privileged container, or a host Docker socket.
-"""
-
-from __future__ import annotations
-
-import shlex
-import shutil
-import tempfile
-import time
-from pathlib import Path
-from typing import Callable
-
-from ...log import die, info
-
-_INIT = "/usr/local/libexec/bot-bottle/rootless-docker-init"
-_RUNTIME_DIR = "/tmp/bot-bottle-docker-run"
-_SOCKET = f"{_RUNTIME_DIR}/docker.sock"
-READY_RETRIES = 30
-
-
-def build_image(
-    base_image: str,
-    build: Callable[..., None],
-) -> str:
-    """Layer spike-only tooling on an already-built provider image."""
-    image = f"{base_image}-rootless-docker"
-    init_script = Path(__file__).with_name("rootless-docker-init.sh")
-    with tempfile.TemporaryDirectory(prefix="bot-bottle-rootless-docker.") as tmp:
-        context = Path(tmp)
-        shutil.copy2(init_script, context / "rootless-docker-init.sh")
-        (context / "Dockerfile").write_text(
-            "FROM docker:28-cli AS docker_cli\n"
-            f"FROM {base_image}\n"
-            "USER root\n"
-            "COPY --from=docker_cli /usr/local/libexec/docker/cli-plugins/"
-            "docker-compose /usr/local/libexec/docker/cli-plugins/docker-compose\n"
-            "RUN apt-get update \\\n"
-            " && apt-get install -y --no-install-recommends docker.io uidmap "
-            "rootlesskit slirp4netns fuse-overlayfs \\\n"
-            " && rm -rf /var/lib/apt/lists/* \\\n"
-            " && sed -i '/^node:/d' /etc/subuid /etc/subgid \\\n"
-            " && printf 'node:100000:65536\\n' >> /etc/subuid \\\n"
-            " && printf 'node:100000:65536\\n' >> /etc/subgid \\\n"
-            " && ln -s /usr/share/docker.io/contrib/dockerd-rootless.sh "
-            "/usr/local/bin/dockerd-rootless.sh\n"
-            "COPY rootless-docker-init.sh "
-            "/usr/local/libexec/bot-bottle/rootless-docker-init\n"
-            "RUN chmod 0755 /usr/local/libexec/bot-bottle/rootless-docker-init\n"
-            "USER node\n",
-            encoding="utf-8",
-        )
-        build(image, str(context), dockerfile=str(context / "Dockerfile"))
-    return image
-
-
-def guest_env(enabled: bool) -> dict[str, str]:
-    """Environment consumed by the Docker CLI inside an enabled bottle."""
-    if not enabled:
-        return {}
-    return {
-        "DOCKER_HOST": f"unix://{_SOCKET}",
-        "XDG_RUNTIME_DIR": _RUNTIME_DIR,
-    }
-
-
-def start(bottle: object) -> None:
-    """Start and verify the unprivileged daemon through the bottle exec API."""
-    info("starting experimental rootless Docker daemon")
-    result = bottle.exec(shlex.quote(_INIT))  # type: ignore[attr-defined]
-    if result.returncode != 0:
-        detail = (result.stderr or result.stdout or "").strip()
-        die(f"rootless Docker bootstrap failed: {detail or '<no output>'}")
-
-    for _ in range(READY_RETRIES):
-        result = bottle.exec("docker info >/dev/null 2>&1")  # type: ignore[attr-defined]
-        if result.returncode == 0:
-            info("rootless Docker daemon is ready")
-            return
-        time.sleep(0.2)
-
-    logs = bottle.exec(  # type: ignore[attr-defined]
-        "tail -n 80 /tmp/bot-bottle-rootless-docker.log 2>/dev/null || true"
-    )
-    die(
-        "rootless Docker did not become ready without additional outer "
-        f"privileges:\n{(logs.stdout or logs.stderr or '<no log>').strip()}"
-    )
-
-
-__all__ = ["build_image", "guest_env", "start"]

bot_bottle/backend/macos_container/rootless_podman.py

@@ -0,0 +1,132 @@
+"""Experimental rootless podman bootstrap for Apple-container bottles.
+
+The service and every nested container remain inside the existing per-bottle
+VM.  This module refuses to compensate for missing prerequisites with outer
+capabilities, a privileged container, or a host Docker socket.
+
+Podman is used rather than rootless Docker for one specific reason: Apple
+Container's capability bounding set omits `CAP_SYS_ADMIN`, which the kernel
+requires to write a multi-range `uid_map` via `newuidmap`.  Rootless Docker
+has no path that avoids that write.  Podman does — with no subordinate UID
+range configured it falls back to a single-UID self-mapping, which an
+unprivileged process may write itself.  See
+`docs/research/rootless-docker-in-apple-container-spike.md`.
+
+That fallback is why `build_image` *removes* the agent user's `/etc/subuid`
+and `/etc/subgid` entries instead of adding them: their presence is precisely
+what would send podman down the `newuidmap` path that cannot work here.
+
+The agent still talks to `docker` and `docker compose`; those speak to
+podman's Docker-compatible API socket, so nothing in the agent's habits
+changes.
+"""
+
+from __future__ import annotations
+
+import shlex
+import shutil
+import tempfile
+import time
+from pathlib import Path
+from typing import Callable
+
+from ...log import die, info
+
+_INIT = "/usr/local/libexec/bot-bottle/rootless-podman-init"
+_RUNTIME_DIR = "/tmp/bot-bottle-podman-run"
+_SOCKET = f"{_RUNTIME_DIR}/podman.sock"
+_LOG = "/tmp/bot-bottle-rootless-podman.log"
+READY_RETRIES = 30
+
+# Apple Container creates both device nodes 0600 root:root, so the agent user
+# cannot open them: /dev/fuse blocks the fuse-overlayfs storage driver and
+# /dev/net/tun blocks slirp4netns, which rootless podman uses for the default
+# bridge network that stock compose files expect.  Relaxing the modes needs no
+# capability the bottle does not already hold — unlike CAP_SYS_ADMIN, which is
+# what killed the rootless-Docker approach.
+_GUEST_DEVICES = ("/dev/fuse", "/dev/net/tun")
+
+
+def build_image(
+    base_image: str,
+    build: Callable[..., None],
+) -> str:
+    """Layer spike-only tooling on an already-built provider image."""
+    image = f"{base_image}-rootless-podman"
+    init_script = Path(__file__).with_name("rootless-podman-init.sh")
+    with tempfile.TemporaryDirectory(prefix="bot-bottle-rootless-podman.") as tmp:
+        context = Path(tmp)
+        shutil.copy2(init_script, context / "rootless-podman-init.sh")
+        (context / "Dockerfile").write_text(
+            "FROM docker:28-cli AS docker_cli\n"
+            f"FROM {base_image}\n"
+            "USER root\n"
+            "COPY --from=docker_cli /usr/local/bin/docker /usr/local/bin/docker\n"
+            "COPY --from=docker_cli /usr/local/libexec/docker/cli-plugins/"
+            "docker-compose /usr/local/libexec/docker/cli-plugins/docker-compose\n"
+            "RUN apt-get update \\\n"
+            " && apt-get install -y --no-install-recommends podman "
+            "fuse-overlayfs slirp4netns uidmap \\\n"
+            " && rm -rf /var/lib/apt/lists/* \\\n"
+            # Deliberate: an empty subordinate range keeps podman on the
+            # single-UID mapping that needs no CAP_SYS_ADMIN.  Adding ranges
+            # here would reintroduce the newuidmap failure this spike exists
+            # to route around.
+            " && sed -i '/^node:/d' /etc/subuid /etc/subgid\n"
+            "COPY rootless-podman-init.sh "
+            "/usr/local/libexec/bot-bottle/rootless-podman-init\n"
+            "RUN chmod 0755 /usr/local/libexec/bot-bottle/rootless-podman-init\n"
+            "USER node\n",
+            encoding="utf-8",
+        )
+        build(image, str(context), dockerfile=str(context / "Dockerfile"))
+    return image
+
+
+def guest_env(enabled: bool) -> dict[str, str]:
+    """Environment consumed by the Docker CLI inside an enabled bottle."""
+    if not enabled:
+        return {}
+    return {
+        "DOCKER_HOST": f"unix://{_SOCKET}",
+        "XDG_RUNTIME_DIR": _RUNTIME_DIR,
+    }
+
+
+def prepare_guest_devices(container_name: str, exec_as_root: Callable[..., None]) -> None:
+    """Make /dev/fuse and /dev/net/tun openable by the agent user.
+
+    Runs as root inside the bottle because the agent must not be able to
+    re-mode device nodes itself.  No outer capability is involved.
+    """
+    exec_as_root(
+        container_name,
+        ["sh", "-c", f"chmod 0666 {' '.join(_GUEST_DEVICES)}"],
+    )
+
+
+def start(bottle: object) -> None:
+    """Start and verify the unprivileged service through the bottle exec API."""
+    info("starting experimental rootless podman service")
+    result = bottle.exec(shlex.quote(_INIT))  # type: ignore[attr-defined]
+    if result.returncode != 0:
+        detail = (result.stderr or result.stdout or "").strip()
+        die(f"rootless podman bootstrap failed: {detail or '<no output>'}")
+
+    for _ in range(READY_RETRIES):
+        result = bottle.exec("docker info >/dev/null 2>&1")  # type: ignore[attr-defined]
+        if result.returncode == 0:
+            info("rootless podman service is ready")
+            return
+        time.sleep(0.2)
+
+    logs = bottle.exec(  # type: ignore[attr-defined]
+        f"tail -n 80 {_LOG} 2>/dev/null || true"
+    )
+    die(
+        "rootless podman did not become ready without additional outer "
+        f"privileges:\n{(logs.stdout or logs.stderr or '<no log>').strip()}"
+    )
+
+
+__all__ = ["build_image", "guest_env", "prepare_guest_devices", "start"]