feat(left4me-overlay): idmap bind mounts for l4d2-sandbox-owned lowerdirs
Insert an idmapped bind mount in front of each lowerdir whose top-level uid matches l4d2-sandbox at overlay-mount time, so that overlayfs copy-up produces left4me-owned upperdir entries instead of EACCES. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
mwiegand
parent
3a2c379b71
commit
2f6a9cfba0
2 changed files with 261 additions and 8 deletions
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@ -29,6 +29,7 @@ shell-quoted) and exit 0 instead of execv. Used by tests.
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"""
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import os
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import pwd
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import re
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import shlex
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import shutil
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@ -54,6 +55,45 @@ def die(msg: str) -> None:
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sys.exit(1)
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def _lookup_uid(username: str) -> tuple[int, int]:
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"""Return (uid, gid) for *username*, dying with a clear message if missing."""
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try:
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entry = pwd.getpwnam(username)
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except KeyError:
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die(
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f"required system user {username!r} does not exist; "
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"this is a deploy misconfiguration"
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)
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return entry.pw_uid, entry.pw_gid
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def _get_user_ids() -> tuple[int, int, int, int]:
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"""Return (sandbox_uid, sandbox_gid, left4me_uid, left4me_gid).
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In normal operation, looks up the real system users. When the test-only
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env vars _L4D2_SANDBOX_UID/_L4D2_SANDBOX_GID/_LEFT4ME_UID/_LEFT4ME_GID
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are set, those values are used directly so tests can run without root
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and without real system users being present.
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"""
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sandbox_uid_env = os.environ.get("_L4D2_SANDBOX_UID")
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sandbox_gid_env = os.environ.get("_L4D2_SANDBOX_GID")
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left4me_uid_env = os.environ.get("_LEFT4ME_UID")
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left4me_gid_env = os.environ.get("_LEFT4ME_GID")
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if all(v is not None for v in (sandbox_uid_env, sandbox_gid_env,
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left4me_uid_env, left4me_gid_env)):
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return (
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int(sandbox_uid_env), # type: ignore[arg-type]
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int(sandbox_gid_env), # type: ignore[arg-type]
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int(left4me_uid_env), # type: ignore[arg-type]
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int(left4me_gid_env), # type: ignore[arg-type]
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)
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sandbox_uid, sandbox_gid = _lookup_uid("l4d2-sandbox")
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left4me_uid, left4me_gid = _lookup_uid("left4me")
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return sandbox_uid, sandbox_gid, left4me_uid, left4me_gid
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def root() -> Path:
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return Path(os.environ.get("LEFT4ME_ROOT") or DEFAULT_ROOT)
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@ -128,9 +168,14 @@ def assert_no_fuse_xattrs(upper: Path) -> None:
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)
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def _print_argv(argv: list[str]) -> None:
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"""Emit one shell-quoted argv line to stdout (PRINT_ONLY helper, no exit)."""
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print(" ".join(shlex.quote(a) for a in argv))
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def exec_or_print(argv: list[str]) -> None:
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if os.environ.get("LEFT4ME_OVERLAY_PRINT_ONLY") == "1":
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print(" ".join(shlex.quote(a) for a in argv))
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_print_argv(argv)
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sys.exit(0)
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os.execv(argv[0], argv)
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@ -168,7 +213,62 @@ def cmd_mount(name: str) -> None:
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assert_no_fuse_xattrs(upper)
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options = f"lowerdir={':'.join(canonical_lowerdirs)},upperdir={upper},workdir={work}"
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# Resolve user ids now (fails fast on deploy misconfiguration).
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sandbox_uid, sandbox_gid, left4me_uid, left4me_gid = _get_user_ids()
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# Build the final lowerdir list, substituting idmap bind-mount paths for
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# any lowerdir owned by l4d2-sandbox. An idmap bind mount makes the kernel
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# see the l4d2-sandbox-owned tree as if it were owned by left4me, so that
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# overlayfs copy-up produces left4me-owned upperdir entries.
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idmap_dir = runtime_name_dir / "idmap"
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final_lowerdirs: list[str] = []
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bind_argvs: list[list[str]] = []
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for lowerdir in canonical_lowerdirs:
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st = os.stat(lowerdir)
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if st.st_uid == sandbox_uid:
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# This lowerdir needs idmap remapping.
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overlay_id = Path(lowerdir).name
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idmap_target = idmap_dir / overlay_id
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if os.environ.get("LEFT4ME_OVERLAY_PRINT_ONLY") != "1":
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idmap_dir.mkdir(mode=0o700, exist_ok=True)
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idmap_target.mkdir(mode=0o700, exist_ok=True)
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if not os.path.ismount(idmap_target) or \
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os.environ.get("LEFT4ME_OVERLAY_PRINT_ONLY") == "1":
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# --map-users / --map-groups argument format:
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# <on-disk-uid>:<in-mount-uid>:<count>
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# The util-linux man page calls these <inner>:<outer>, which is
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# misleading. Empirically (verified on left4.me, kernel 6.12,
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# ext4) the FIRST number is the on-disk uid and the SECOND is
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# the uid exposed inside the mount. Don't swap them.
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bind_argv = [
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MOUNT_BIN,
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"--bind",
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f"--map-users={sandbox_uid}:{left4me_uid}:1",
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f"--map-groups={sandbox_gid}:{left4me_gid}:1",
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lowerdir,
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str(idmap_target),
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]
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bind_argvs.append(bind_argv)
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final_lowerdirs.append(str(idmap_target))
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else:
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final_lowerdirs.append(lowerdir)
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print_only = os.environ.get("LEFT4ME_OVERLAY_PRINT_ONLY") == "1"
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if print_only:
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# Emit each bind-mount argv first, then fall through to the overlay argv.
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for bind_argv in bind_argvs:
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_print_argv(bind_argv)
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else:
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# Actually exec each bind mount before the overlay mount.
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for bind_argv in bind_argvs:
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subprocess.run(bind_argv, check=True)
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options = f"lowerdir={':'.join(final_lowerdirs)},upperdir={upper},workdir={work}"
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argv = [
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MOUNT_BIN,
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"-t", "overlay",
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@ -186,17 +286,26 @@ def cmd_umount(name: str) -> None:
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merged_path = runtime_name_dir / "merged"
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work_inner = runtime_name_dir / "work" / "work"
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argv = [
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overlay_umount_argv = [
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UMOUNT_BIN,
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# Resolve only if it exists; PRINT_ONLY tests always pre-create it.
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str(merged_path.resolve(strict=True) if merged_path.exists() else merged_path),
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]
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# PRINT_ONLY: emit the umount argv and exit. Tests assert exact shape
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# of this dry-run; the post-umount cleanup of work_inner is a runtime
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# behaviour exercised on the host, not in unit tests.
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# Collect idmap bind-umount argvs: one per direct subdir of runtime/<name>/idmap/.
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idmap_dir = runtime_name_dir / "idmap"
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bind_umount_argvs: list[list[str]] = []
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if idmap_dir.is_dir():
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for entry in sorted(idmap_dir.iterdir()):
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if entry.is_dir():
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bind_umount_argvs.append([UMOUNT_BIN, str(entry)])
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# PRINT_ONLY: emit the overlay umount argv, then each bind-umount argv, then exit.
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# Order matches real execution (overlay first, then idmap binds underneath).
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if os.environ.get("LEFT4ME_OVERLAY_PRINT_ONLY") == "1":
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print(" ".join(shlex.quote(a) for a in argv))
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_print_argv(overlay_umount_argv)
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for bind_umount_argv in bind_umount_argvs:
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_print_argv(bind_umount_argv)
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sys.exit(0)
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if merged_path.exists():
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@ -214,7 +323,7 @@ def cmd_umount(name: str) -> None:
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# reaped → umount-clears sequence happens without any race
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# window for us to ride out. EBUSY here is a real error.
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if os.path.ismount(merged):
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subprocess.run(argv, check=True)
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subprocess.run(overlay_umount_argv, check=True)
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# Kernel-overlayfs creates work_inner during mount with root:root mode
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# 0/0. After unmount it's an orphan that the unit's User= (left4me)
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@ -227,6 +336,15 @@ def cmd_umount(name: str) -> None:
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if work_inner.exists():
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shutil.rmtree(work_inner)
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# Unwind idmap bind mounts, then remove the idmap directory. Each bind
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# is only umounted if it is still a mountpoint (idempotent across partial
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# teardowns).
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for bind_umount_argv in bind_umount_argvs:
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target = Path(bind_umount_argv[-1])
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if os.path.ismount(target):
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subprocess.run(bind_umount_argv, check=True)
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shutil.rmtree(idmap_dir, ignore_errors=True)
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def main(argv: list[str]) -> None:
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if len(argv) != 3 or argv[1] not in ("mount", "umount"):
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@ -41,6 +41,11 @@ def _run(args: list[str], root: Path, extra_env: dict[str, str] | None = None) -
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**os.environ,
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"LEFT4ME_ROOT": str(root),
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"LEFT4ME_OVERLAY_PRINT_ONLY": "1",
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# Inject synthetic user ids so tests work without real system users.
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"_L4D2_SANDBOX_UID": str(FAKE_SANDBOX_UID),
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"_L4D2_SANDBOX_GID": str(FAKE_SANDBOX_GID),
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"_LEFT4ME_UID": str(FAKE_LEFT4ME_UID),
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"_LEFT4ME_GID": str(FAKE_LEFT4ME_GID),
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}
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if extra_env:
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env.update(extra_env)
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@ -156,6 +161,136 @@ def test_rejects_empty_lowerdir_entry(tmp_path: Path) -> None:
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assert "empty entry" in result.stderr
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FAKE_SANDBOX_UID = 7001
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FAKE_SANDBOX_GID = 7001
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FAKE_LEFT4ME_UID = 7002
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FAKE_LEFT4ME_GID = 7002
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def _setup_instance_with_uid(
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root: Path,
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name: str = "alpha",
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lowerdir_uid: int = FAKE_LEFT4ME_UID,
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lowerdir_gid: int = FAKE_LEFT4ME_GID,
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) -> Path:
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"""Like _setup_instance but chowns the lowerdir to a specific uid/gid."""
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overlay_dir = root / "overlays" / "workshop"
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overlay_dir.mkdir(parents=True, exist_ok=True)
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try:
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os.chown(overlay_dir, lowerdir_uid, lowerdir_gid)
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except PermissionError:
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pass # tests not running as root — uid won't match; that's fine for the "skips idmap" test
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(root / "installation").mkdir(parents=True, exist_ok=True)
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lowerdirs = [str(overlay_dir), str(root / "installation")]
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inst_dir = root / "instances" / name
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inst_dir.mkdir(parents=True, exist_ok=True)
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(inst_dir / "instance.env").write_text(
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f"L4D2_LOWERDIRS={':'.join(lowerdirs)}\n"
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)
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runtime = root / "runtime" / name
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(runtime / "upper").mkdir(parents=True, exist_ok=True)
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(runtime / "work").mkdir(parents=True, exist_ok=True)
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(runtime / "merged").mkdir(parents=True, exist_ok=True)
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return overlay_dir
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def test_mount_idmaps_sandbox_owned_lowerdir(tmp_path: Path) -> None:
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"""A lowerdir owned by l4d2-sandbox uid triggers an idmap bind mount.
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The overlay lowerdir= string must reference the idmap path, not the raw
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overlay path. A mount --bind --map-users/--map-groups argv must be emitted
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before the overlay mount argv.
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"""
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overlay_dir = _setup_instance_with_uid(
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tmp_path, lowerdir_uid=FAKE_SANDBOX_UID, lowerdir_gid=FAKE_SANDBOX_GID
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)
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try:
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os.chown(overlay_dir, FAKE_SANDBOX_UID, FAKE_SANDBOX_GID)
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except PermissionError:
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pytest.skip("chown requires root — skip on unprivileged runner")
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result = _run(["mount", "alpha"], tmp_path)
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assert result.returncode == 0, result.stderr
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lines = [l for l in result.stdout.splitlines() if l.strip()]
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assert len(lines) == 2, f"expected 2 argv lines, got: {result.stdout!r}"
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bind_parts = shlex.split(lines[0])
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assert bind_parts[0] == "/bin/mount"
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assert "--bind" in bind_parts
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assert f"--map-users={FAKE_SANDBOX_UID}:{FAKE_LEFT4ME_UID}:1" in bind_parts
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assert f"--map-groups={FAKE_SANDBOX_GID}:{FAKE_LEFT4ME_GID}:1" in bind_parts
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assert bind_parts[-2] == str(overlay_dir)
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idmap_target = str(tmp_path / "runtime" / "alpha" / "idmap" / "workshop")
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assert bind_parts[-1] == idmap_target
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overlay_parts = shlex.split(lines[1])
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assert overlay_parts[0] == "/bin/mount"
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assert overlay_parts[1:3] == ["-t", "overlay"]
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options = overlay_parts[5]
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assert f"lowerdir={idmap_target}:" in options, \
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f"lowerdir should start with idmap path; got: {options!r}"
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assert str(overlay_dir) not in options, \
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f"raw overlay path should not appear in lowerdir; got: {options!r}"
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def test_mount_skips_idmap_for_left4me_owned_lowerdir(tmp_path: Path) -> None:
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"""A lowerdir already owned by the left4me uid needs no idmap bind mount."""
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overlay_dir = _setup_instance_with_uid(
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tmp_path, lowerdir_uid=FAKE_LEFT4ME_UID, lowerdir_gid=FAKE_LEFT4ME_GID
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)
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# Best-effort chown to the left4me uid — skip if not root.
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try:
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os.chown(overlay_dir, FAKE_LEFT4ME_UID, FAKE_LEFT4ME_GID)
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except PermissionError:
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# Without root, st_uid is 0 or our own uid; neither matches FAKE_SANDBOX_UID,
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# so the helper will correctly skip the idmap bind either way.
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pass
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result = _run(["mount", "alpha"], tmp_path)
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assert result.returncode == 0, result.stderr
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lines = [l for l in result.stdout.splitlines() if l.strip()]
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assert len(lines) == 1, f"expected 1 argv line (no bind mount), got: {result.stdout!r}"
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overlay_parts = shlex.split(lines[0])
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assert overlay_parts[0] == "/bin/mount"
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assert "--bind" not in overlay_parts
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options = overlay_parts[5]
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idmap_subdir = str(tmp_path / "runtime" / "alpha" / "idmap")
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assert idmap_subdir not in options, f"idmap path should not appear; got: {options!r}"
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assert str(overlay_dir) in options
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def test_umount_unwinds_idmap_binds(tmp_path: Path) -> None:
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"""umount emits bind-umount lines for each idmap subdir, after the overlay umount."""
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_setup_instance(tmp_path)
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# Pre-seed an idmap subdir as if a previous mount had set it up.
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idmap_dir = tmp_path / "runtime" / "alpha" / "idmap"
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idmap_dir.mkdir(parents=True)
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idmap_sub = idmap_dir / "workshop"
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idmap_sub.mkdir()
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result = _run(["umount", "alpha"], tmp_path)
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assert result.returncode == 0, result.stderr
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lines = [l for l in result.stdout.splitlines() if l.strip()]
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assert len(lines) >= 2, f"expected at least 2 argv lines, got: {result.stdout!r}"
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# First line: overlay umount
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overlay_umount_parts = shlex.split(lines[0])
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assert overlay_umount_parts == [
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"/bin/umount",
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str(tmp_path / "runtime" / "alpha" / "merged"),
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]
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# Subsequent lines: bind umounts for each idmap subdir
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bind_umount_parts = shlex.split(lines[1])
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assert bind_umount_parts[0] == "/bin/umount"
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assert bind_umount_parts[-1] == str(idmap_sub)
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@pytest.mark.skipif(sys.platform != "linux", reason="user.* xattrs are Linux-only")
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def test_rejects_upperdir_with_fuseoverlayfs_xattr(tmp_path: Path) -> None:
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_setup_instance(tmp_path)
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