Allow setting the user which we drop permissions to using suid.

aiosmtpd/docs/NEWS.rst

@@ -9,6 +9,7 @@
 Added
 -----
 * Unthreaded Controllers (Closes #160)
+* Ability to drop permissions to a user other than ``nobody`` using ``-S``
 
 Fixed/Improved
 --------------

aiosmtpd/main.py

@@ -48,9 +48,18 @@ def _parser() -> ArgumentParser:
         default=True,
         action="store_false",
         help=(
-            "This program generally tries to setuid ``nobody``, unless this "
-            "flag is set.  The setuid call will fail if this program is not "
-            "run as root (in which case, use this flag)."
+            "This program uses setuid to drop permissions unless this flag "
+            "is set.  The setuid call will fail if this program is not run "
+            "as root (in which case, use this flag)."
+        ),
+    )
+    parser.add_argument(
+        "-S",
+        "--suid-user",
+        dest="suid_user",
+        default="nobody",
+        help=(
+            "The user to change to using suid; defaults to ``nobody``."
         ),
     )
     parser.add_argument(
@@ -224,12 +233,13 @@ def main(args: Optional[Sequence[str]] = None) -> None:
                 file=sys.stderr,
             )
             sys.exit(1)
-        nobody = pwd.getpwnam("nobody").pw_uid
+        suid_user_id = pwd.getpwnam(args.suid_user).pw_uid
         try:
-            os.setuid(nobody)
+            os.setuid(suid_user_id)
         except PermissionError:
             print(
-                'Cannot setuid "nobody"; try running with -n option.', file=sys.stderr
+                f'Cannot setuid to "{args.suid_user}"; try running with -n option.',
+                file=sys.stderr,
             )
             sys.exit(1)
 

aiosmtpd/tests/test_main.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 import asyncio
+from collections import namedtuple
 import logging
 import multiprocessing as MP
 import os
@@ -141,6 +142,17 @@ def test_setuid(self, nobody_uid, mocker):
         main(args=())
         mock.assert_called_with(nobody_uid)
 
+    def test_setuid_other(self, nobody_uid, mocker):
+        other_user = namedtuple(
+            "pwnam",
+            ["pw_uid", "pw_dir", "pw_shell"],
+        )(42, "/", "/bin/sh")
+        mock_getpwnam = mocker.patch("pwd.getpwnam", return_value=other_user)
+        mock_suid = mocker.patch("os.setuid")
+        main(args=("-S", "other"))
+        mock_getpwnam.assert_called_with("other")
+        mock_suid.assert_called_with(42)
+
     def test_setuid_permission_error(self, nobody_uid, mocker, capsys):
         mock = mocker.patch("os.setuid", side_effect=PermissionError)
         with pytest.raises(SystemExit) as excinfo:
@@ -149,7 +161,7 @@ def test_setuid_permission_error(self, nobody_uid, mocker, capsys):
         mock.assert_called_with(nobody_uid)
         assert (
             capsys.readouterr().err
-            == 'Cannot setuid "nobody"; try running with -n option.\n'
+            == 'Cannot setuid to "nobody"; try running with -n option.\n'
         )
 
     def test_setuid_no_pwd_module(self, nobody_uid, mocker, capsys):