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rootless.md

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Rootless mode

Rootless mode allows running BuildKit daemon as a non-root user.

Known limitations

  • Using the overlayfs snapshotter requires kernel >= 5.11 or Ubuntu kernel. On kernel >= 4.18, the fuse-overlayfs snapshotter is used instead of overlayfs. On kernel < 4.18, the native snapshotter is used.
  • Network mode is always set to network.host.

Running BuildKit in Rootless mode (OCI worker)

RootlessKit needs to be installed.

rootlesskit buildkitd
buildctl --addr unix:///run/user/$UID/buildkit/buildkitd.sock build ...

Tip

To isolate BuildKit daemon's network namespace from the host (recommended):

rootlesskit --net=slirp4netns --copy-up=/etc --disable-host-loopback buildkitd

Running BuildKit in Rootless mode (containerd worker)

RootlessKit needs to be installed.

Run containerd in rootless mode using rootlesskit following containerd's document.

containerd-rootless.sh

CONTAINERD_NAMESPACE=default containerd-rootless-setuptool.sh install-buildkit-containerd
Advanced guide

Alternatively, you can specify the full command line flags as follows:

containerd-rootless.sh --config /path/to/config.toml

containerd-rootless-setuptool.sh nsenter -- buildkitd --oci-worker=false --containerd-worker=true

Containerized deployment

Kubernetes

See ../examples/kubernetes.

Docker

docker run \
  --name buildkitd \
  -d \
  --security-opt seccomp=unconfined \
  --security-opt apparmor=unconfined \
  --security-opt systempaths=unconfined \
  moby/buildkit:rootless

buildctl --addr docker-container://buildkitd build ...

Tip

If you don't mind using --privileged (almost safe for rootless), the docker run flags can be shorten as follows:

docker run --name buildkitd -d --privileged moby/buildkit:rootless

Justification of the --security-opt flags:

  • seccomp=unconfined: For allowing several syscalls such as unshare (used by runc) and mount (used by snapshotters, etc).

  • apparmor=unconfined: For allowing mounting filesystems, etc. This flag is not needed when the host operating system does not use AppArmor.

  • systempaths=unconfined: For disabling the masks for the /proc mount in the container, so that each of ExecOp (corresponds to a RUN instruction in Dockerfile) can have a dedicated /proc filesystem. systempaths=unconfined potentially allows reading and writing dangerous kernel files from a container, but it is safe when you are running buildkitd as non-root.

Tip

Instead of --security-opt systempaths=unconfined, buildkitd can be also executed with --oci-worker-no-process-sandbox (flag of buildkitd, not docker) to avoid creating a new PID namespace and mounting a new /proc for it.

Using --oci-worker-no-process-sandbox is discouraged, as it cannot terminate processes that did not exit during an ExecOp. Also, --oci-worker-no-process-sandbox allows ExecOp containers to kill (and potentially ptrace depending on the seccomp configuration) an arbitrary process in the BuildKit daemon container.

Despite these caveats, the Kubernetes examples uses --oci-worker-no-process-sandbox, as Kubernetes lacks the equivalent of systempaths=unconfined. (securityContext.procMount=Unmasked is similar, but different in the sense that it depends on hostUsers: false)

Change UID/GID

The moby/buildkit:rootless image has the following UID/GID configuration:

Actual ID (shown in the host and the BuildKit daemon container) Mapped ID (shown in build executor containers)
1000 0
100000 1
... ...
165535 65536 
$ docker exec buildkitd id
uid=1000(user) gid=1000(user)
$ docker exec buildkitd ps aux
PID   USER     TIME   COMMAND
    1 user       0:00 rootlesskit buildkitd --addr tcp://0.0.0.0:1234
   13 user       0:00 /proc/self/exe buildkitd --addr tcp://0.0.0.0:1234
   21 user       0:00 buildkitd --addr tcp://0.0.0.0:1234
   29 user       0:00 ps aux

$ docker exec cat /etc/subuid
user:100000:65536

To change the UID/GID configuration, you need to modify and build the BuildKit image manually.

vi Dockerfile
make images
docker run ... moby/buildkit:local-rootless ...

Troubleshooting

Error related to overlayfs

Try running buildkitd with --oci-worker-snapshotter=fuse-overlayfs:

$ rootlesskit buildkitd --oci-worker-snapshotter=fuse-overlayfs

Error related to fuse-overlayfs

Run docker run with --device /dev/fuse.

Also try running buildkitd with --oci-worker-snapshotter=native:

$ rootlesskit buildkitd --oci-worker-snapshotter=native

Error related to newuidmap or /etc/subuid

See https://rootlesscontaine.rs/getting-started/common/subuid/

Error Options:[rbind ro]}]: operation not permitted

Make sure to mount an emptyDir volume on /home/user/.local/share/buildkit .

Error fork/exec /proc/self/exe: no space left on device with level=warning msg="/proc/sys/user/max_user_namespaces needs to be set to non-zero."

Run sysctl -w user.max_user_namespaces=N (N=positive integer, like 63359) on the host nodes.

See ../examples/kubernetes/sysctl-userns.privileged.yaml.

Error fork/exec /proc/self/exe: permission denied with This error might have happened because /proc/sys/kernel/apparmor_restrict_unprivileged_userns is set to 1

Add kernel.apparmor_restrict_unprivileged_userns=0 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl -p.

Error mount proc:/proc (via /proc/self/fd/6), flags: 0xe: operation not permitted

This error is known to happen when BuildKit is executed in a container without the --security-opt systempaths=unconfined flag. Make sure to specify it (See above).

Distribution-specific hint

Using Ubuntu kernel is recommended.

Ubuntu, 24.04 or later

Add kernel.apparmor_restrict_unprivileged_userns=0 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl -p.

Container-Optimized OS from Google

Make sure to have an emptyDir volume below:

spec:
  containers:
    - name: buildkitd
      volumeMounts:
        # Dockerfile has `VOLUME /home/user/.local/share/buildkit` by default too,
        # but the default VOLUME does not work with rootless on Google's Container-Optimized OS
        # as it is mounted with `nosuid,nodev`.
        # https://github.com/moby/buildkit/issues/879#issuecomment-1240347038
        - mountPath: /home/user/.local/share/buildkit
          name: buildkitd
  volumes:
    - name: buildkitd
      emptyDir: {}

See also the example manifests.

Bottlerocket OS

Needs to set the max user namespaces to a positive integer, through the API settings:

[settings.kernel.sysctl]
"user.max_user_namespaces" = "16384"

See ../examples/eksctl/bottlerocket.yaml for an example to configure a Node Group in EKS.

Old distributions

Debian GNU/Linux 10

Add kernel.unprivileged_userns_clone=1 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl -p. This step is not needed for Debian GNU/Linux 11 and later.

RHEL/CentOS 7

Add user.max_user_namespaces=28633 to /etc/sysctl.conf (or /etc/sysctl.d) and run sudo sysctl -p. This step is not needed for RHEL/CentOS 8 and later.

Fedora, before kernel 5.13

You may have to disable SELinux, or run BuildKit with --oci-worker-snapshotter=fuse-overlayfs.