Fix permission errors for mounts in rootless docker

matthewhughes934 · Kurt-von-Laven · themr0c · matthewhughes934 · commit 7180cdaf5507 · 2025-04-13T19:15:13.000+01:00
By running containers in a rootless docker context as root. This is because user and group IDs are remapped in the user namespaces uses by rootless docker, and it's unlikely that the current user ID will map to the same ID under this remap (see docs[1] for some more details). Specifically, it means ownership of mounted volumes will not be for the current user and trying to write can result in permission errors. This change borrows heavily from an existing PR[2]. The output format of `docker system info` I don't think is documented/guaranteed anywhere, but it should corresponding to the format of a `/info` API request to Docker[3] The added test _hopes_ to avoid regressions in this behaviour, but since tests aren't run in a rootless docker context on the PR checks (and I couldn't find an easy way to make it the case) there's still a risk of regressions sneaking in. Link: https://docs.docker.com/engine/security/rootless/ [1] Link: #1484 [2] Link: https://docs.docker.com/reference/api/engine/version/v1.48/#tag/System/operation/SystemAuth [3] Co-authored-by: Kurt von Laven <Kurt-von-Laven@users.noreply.github.com> Co-authored-by: Fabrice Flore-Thébault <ffloreth@redhat.com>
diff --git a/pre_commit/languages/docker.py b/pre_commit/languages/docker.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import functools
 import hashlib
 import json
 import os
@@ -101,7 +102,23 @@ def install_environment(
     os.mkdir(directory)
 
 
+@functools.lru_cache(maxsize=1)
+def _is_rootless_docker() -> bool:  # pragma: win32 no cover
+    retcode, out, _ = cmd_output_b(
+        'docker', 'system', 'info', '--format', '{{ json .SecurityOptions }}',
+    )
+    # some failures are to be expected, e.g. for 'podman' aliased as 'docker'
+    if retcode != 0:
+        return False
+
+    info = json.loads(out)
+    return any(opt == 'name=rootless' for opt in info)
+
+
 def get_docker_user() -> tuple[str, ...]:  # pragma: win32 no cover
+    if _is_rootless_docker():
+        return ()
+
     try:
         return ('-u', f'{os.getuid()}:{os.getgid()}')
     except AttributeError:
diff --git a/tests/languages/docker_test.py b/tests/languages/docker_test.py
@@ -62,6 +62,25 @@ def invalid_attribute():
         assert docker.get_docker_user() == ()
 
 
+@pytest.mark.parametrize(
+    ('info_ret', 'expect_root'),
+    (
+        ((0, b'["name=rootless","name=cgroupns"]', b''), True),
+        ((0, b'["name=cgroupns"]', b''), False),
+        ((1, b'', b''), False),
+    ),
+)
+def test_docker_user_rootless_docker(info_ret, expect_root):
+    with mock.patch.object(
+            docker, 'cmd_output_b', return_value=info_ret,
+    ):
+        if expect_root:
+            assert docker.get_docker_user() == ()
+        else:
+            assert docker.get_docker_user() != ()
+    docker._is_rootless_docker.cache_clear()
+
+
 def test_in_docker_no_file():
     with mock.patch.object(builtins, 'open', side_effect=FileNotFoundError):
         assert docker._is_in_docker() is False
@@ -195,3 +214,14 @@ def test_docker_hook(tmp_path):
 
     ret = run_language(tmp_path, docker, 'echo hello hello world')
     assert ret == (0, b'hello hello world\n')
+
+
+@xfailif_windows  # pragma: win32 no cover
+def test_docker_hook_mount_permissions(tmp_path):
+    dockerfile = '''\
+FROM ubuntu:22.04
+'''
+    tmp_path.joinpath('Dockerfile').write_text(dockerfile)
+
+    retcode, _ = run_language(tmp_path, docker, 'touch', ('README.md',))
+    assert retcode == 0
