WIP: Symlink /usr

Dockerfile

@@ -4,7 +4,7 @@
 
 ARG DEBIAN_IMAGE_DATE=20250113
 
-FROM debian:bookworm-${DEBIAN_IMAGE_DATE}-slim
+FROM debian:bookworm-${DEBIAN_IMAGE_DATE}-slim as dangerzone-image
 
 ARG GVISOR_ARCHIVE_DATE=20250113
 ARG DEBIAN_ARCHIVE_DATE=20250120
@@ -64,18 +64,71 @@ RUN touch /opt/dangerzone/dangerzone/__init__.py
 # Copy only the Python code, and not any produced .pyc files.
 COPY conversion/*.py /opt/dangerzone/dangerzone/conversion/
 
-# Let the entrypoint script write the OCI config for the inner container under
-# /config.json.
-RUN touch /config.json
-RUN chown dangerzone:dangerzone /config.json
-
-# Switch to the dangerzone user for the rest of the script.
-USER dangerzone
-
 # Create a directory that will be used by gVisor as the place where it will
 # store the state of its containers.
 RUN mkdir /home/dangerzone/.containers
 
+# XXX: Create a new root hierarchy, that will be used in the final container
+# image:
+#
+# /bin -> usr/bin
+# /lib -> usr/lib
+# /lib64 -> usr/lib64
+# /root
+# /run
+# /tmp
+# /usr -> /home/dangerzone/dangerzone-image/rootfs/usr/
+#
+# We have to create this hierarchy beforehand because we want to use the same
+# /usr for both the inner and outer container. The problem though is that /usr
+# is very sensitive, and you can't manipulate in a live system. That is, I
+# haven't found a way to do the following, or something equivalent:
+#
+#    rm -r /usr && ln -s /home/dangerzone/dangerzone-image/rootfs/usr/ /usr
+#
+# So, we prefer to create the symlinks here instead, and create the image
+# manually in the next steps.
+RUN mkdir /new_root
+RUN mkdir /new_root/root /new_root/run /new_root/tmp
+RUN chmod 777 /new_root/tmp
+RUN ln -s /home/dangerzone/dangerzone-image/rootfs/usr/ /new_root/usr
+RUN ln -s usr/bin /new_root/bin
+RUN ln -s usr/lib /new_root/lib
+RUN ln -s usr/lib64 /new_root/lib64
+RUN ln -s usr/sbin /new_root/sbin
+
+# Intermediate layer
+
+FROM debian:bookworm-${DEBIAN_IMAGE_DATE}-slim as debian-utils
+
+## Final image
+
+FROM scratch
+
+# Copy the filesystem hierarchy that we created in the previous layer, so that
+# /usr can be a symlink.
+COPY --from=dangerzone-image /new_root/ /
+
+# Copy some files that are necessary to use the outer container image, e.g., in
+# order to run `apt`. We _could_ avoid doing this, but the space cost is very
+# small.
+COPY --from=dangerzone-image /etc/ /etc/
+COPY --from=debian-utils /var/ /var/
+
+# Copy the bare minimum to run Dangerzone in the inner container image.
+COPY --from=dangerzone-image /etc/ /home/dangerzone/dangerzone-image/rootfs/etc/
+COPY --from=dangerzone-image /usr/ /home/dangerzone/dangerzone-image/rootfs/usr/
+COPY --from=dangerzone-image /opt/ /home/dangerzone/dangerzone-image/rootfs/opt/
+RUN ln -s usr/bin /home/dangerzone/dangerzone-image/rootfs/bin
+RUN ln -s usr/lib /home/dangerzone/dangerzone-image/rootfs/lib
+RUN ln -s usr/lib64 /home/dangerzone/dangerzone-image/rootfs/lib64
+
+# Allow our entrypoint script to make changes in the following folders.
+RUN chown dangerzone:dangerzone /home/dangerzone /home/dangerzone/dangerzone-image/
+
+# Switch to the dangerzone user for the rest of the script.
+USER dangerzone
+
 COPY container_helpers/entrypoint.py /
 
 ENTRYPOINT ["/entrypoint.py"]

dangerzone/container_helpers/entrypoint.py

@@ -56,14 +56,14 @@ def log(message: str, *values: typing.Any) -> None:
             {"type": "RLIMIT_NOFILE", "hard": 4096, "soft": 4096},
         ],
     },
-    "root": {"path": "/", "readonly": True},
+    "root": {"path": "rootfs", "readonly": True},
     "hostname": "dangerzone",
     "mounts": [
         # Mask almost every system directory of the outer container, by mounting tmpfs
         # on top of them. This is done to avoid leaking any sensitive information,
         # either mounted by Podman/Docker, or when gVisor runs, since we reuse the same
         # rootfs. We basically mask everything except for `/usr`, `/bin`, `/lib`,
-        # and `/etc`.
+        # `/etc`, and `/opt`.
         #
         # Note that we set `--root /home/dangerzone/.containers` for the directory where
         # gVisor will create files at runtime, which means that in principle, we are
@@ -219,7 +219,7 @@ def log(message: str, *values: typing.Any) -> None:
     json.dump(oci_config, sys.stderr, indent=2, sort_keys=True)
     # json.dump doesn't print a trailing newline, so print one here:
     log("")
-with open("/config.json", "w") as oci_config_out:
+with open("/home/dangerzone/dangerzone-image/config.json", "w") as oci_config_out:
     json.dump(oci_config, oci_config_out, indent=2, sort_keys=True)
 
 # Run gVisor.
@@ -236,7 +236,7 @@ def log(message: str, *values: typing.Any) -> None:
     runsc_argv += ["--debug=true", "--alsologtostderr=true"]
 if os.environ.get("RUNSC_FLAGS"):
     runsc_argv += [x for x in shlex.split(os.environ.get("RUNSC_FLAGS", "")) if x]
-runsc_argv += ["run", "--bundle=/", "dangerzone"]
+runsc_argv += ["run", "--bundle=/home/dangerzone/dangerzone-image", "dangerzone"]
 log(
     "Running gVisor with command line: {}", " ".join(shlex.quote(s) for s in runsc_argv)
 )