From bb83f3fe1bec0730788607aab7ca874441a74100 Mon Sep 17 00:00:00 2001
From: Helmut Grohne <helmut@subdivi.de>
Date: Thu, 1 Feb 2024 19:46:46 +0100
Subject: add examples/cgroup.py: writeable cgroup hierarchy

---
 examples/cgroup.py | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 pyproject.toml     |   2 +-
 2 files changed, 140 insertions(+), 1 deletion(-)
 create mode 100755 examples/cgroup.py

diff --git a/examples/cgroup.py b/examples/cgroup.py
new file mode 100755
index 0000000..06eb3b3
--- /dev/null
+++ b/examples/cgroup.py
@@ -0,0 +1,139 @@
+#!/usr/bin/python3
+# Copyright 2024 Helmut Grohne <helmut@subdivi.de>
+# SPDX-License-Identifier: GPL-3
+
+"""Unshare a cgroup (and user) namespace such that the entire cgroup hierarchy
+(inside the namespace) becomes writeable to the user.
+"""
+
+import os
+import pathlib
+import sys
+import time
+
+try:
+    import ravel
+except ImportError:
+    ravel = None
+
+if __file__.split("/")[-2:-1] == ["examples"]:
+    sys.path.insert(0, "/".join(__file__.split("/")[:-2]))
+
+import linuxnamespaces
+
+
+def get_cgroup(pid: int = -1) -> pathlib.PurePath:
+    """Look up the cgroup that the given pid or the running process belongs
+    to.
+    """
+    return pathlib.PurePath(
+        pathlib.Path(
+            f"/proc/{pid}/cgroup" if pid > 0 else "/proc/self/cgroup"
+        ).read_text().split(":", 2)[2].strip()
+    )
+
+
+def start_transient_unit_with_ravel(pid: int) -> None:
+    """Call the StartTransientUnit dbus method on the user manager for the
+    given pid.
+    """
+    bus = ravel.session_bus()
+    jobs_removed = {}
+    systemd_path = "/org/freedesktop/systemd1"
+    systemd_iface = "org.freedesktop.systemd1.Manager"
+
+    @ravel.signal(name="JobRemoved", in_signature="uoss")
+    def handle_job_removed(_1, path, _2, result):
+        nonlocal jobs_removed
+        jobs_removed[path] = result
+
+    bus.listen_signal(
+        systemd_path, False, systemd_iface, "JobRemoved", handle_job_removed
+    )
+    unitpath = (
+        bus["org.freedesktop.systemd1"][systemd_path]
+        .get_interface(systemd_iface)
+        .StartTransientUnit(
+            f"cgroup-{pid}.scope", "fail", [("PIDs", ("au", [pid]))], []
+        )
+    )[0]
+    now = time.monotonic()
+    deadline = now + 60
+    while unitpath not in jobs_removed:
+        if not bus.connection.read_write_dispatch(timeout=deadline - now):
+            break
+        now = time.monotonic()
+        if now > deadline:
+            break
+    bus.unlisten_signal(
+        systemd_path, False, systemd_iface, "JobRemoved", handle_job_removed
+    )
+    if unitpath not in jobs_removed:
+        print("Error: timed out waiting for StartTransientUnit")
+        sys.exit(1)
+    if jobs_removed[unitpath] != "done":
+        print("Error: StartTransientUnit failed: " + jobs_removed[unitpath])
+        sys.exit(1)
+
+
+def main() -> None:
+    mycgroup = get_cgroup()
+    mycgroupdir = pathlib.Path("/sys/fs/cgroup") / mycgroup.relative_to("/")
+    if not os.access(mycgroupdir, os.W_OK):
+        # For some shells - notably from graphical desktop environments, the
+        # hiearchy is immediatly writeable. For others, we may create a scope
+        # unit.
+        if ravel is not None:
+            start_transient_unit_with_ravel(os.getpid())
+            mycgroup = get_cgroup()
+            mycgroupdir = pathlib.Path(
+                "/sys/fs/cgroup"
+            ) / mycgroup.relative_to("/")
+        else:
+            # Re-execute ourselves via systemd-run.
+            if (
+                mycgroup.name.startswith("run-")
+                and mycgroup.name.endswith(".scope")
+            ):
+                print(
+                    "Error: We're running in a .scope cgroup, but it is not writeable. Giving up."
+                )
+                sys.exit(1)
+            os.execvp(
+                "systemd-run", ["systemd-run", "--user", "--scope"] + sys.argv,
+            )
+            print("Error: Failed to re-execute myself inside systemd-run.")
+            sys.exit(1)
+    linuxnamespaces.unshare_user_idmap(
+        [linuxnamespaces.IDMapping(os.getuid(), os.getuid(), 1)],
+        [linuxnamespaces.IDMapping(os.getgid(), os.getgid(), 1)],
+        linuxnamespaces.CloneFlags.NEWUSER
+        | linuxnamespaces.CloneFlags.NEWNS
+        | linuxnamespaces.CloneFlags.NEWCGROUP,
+    )
+    cgroupfd = linuxnamespaces.open_tree(
+        mycgroupdir,
+        linuxnamespaces.OpenTreeFlags.OPEN_TREE_CLONE
+        | linuxnamespaces.OpenTreeFlags.AT_RECURSIVE,
+    )
+    linuxnamespaces.mount("tmpfs", "/sys", "tmpfs", data="mode=0755")
+    os.mkdir("/sys/fs")
+    os.mkdir("/sys/fs/cgroup")
+    linuxnamespaces.mount(
+        "tmpfs",
+        "/sys",
+        "tmpfs", 
+        linuxnamespaces.MountFlags.REMOUNT
+        | linuxnamespaces.MountFlags.RDONLY
+        | linuxnamespaces.MountFlags.NOEXEC
+        | linuxnamespaces.MountFlags.NOSUID
+        | linuxnamespaces.MountFlags.NODEV,
+        "mode=0755",
+    )
+    linuxnamespaces.move_mount(cgroupfd, "/sys/fs/cgroup")
+    cgroupfd.close()
+    os.execlp(os.environ["SHELL"], os.environ["SHELL"])
+
+
+if __name__ == "__main__":
+    main()
diff --git a/pyproject.toml b/pyproject.toml
index a19700e..d9bb0e1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -12,7 +12,7 @@ classifiers = [
 
 [project.optional-dependencies]
 test = ["pytest"]
-examples = ["zstandard"]
+examples = ["dbussy", "zstandard"]
 
 [tool.black]
 line-length = 79
-- 
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