mmdebstrap: do not use pwd module

The pwd module started caching /etc/passwd and fails to notice the user
created using useradd.

1 files changed, 13 insertions, 4 deletions

diff --git a/mdbp/mmdebstrap.py b/mdbp/mmdebstrap.py
index 9858b65..cbb52a9 100644
--- a/mdbp/mmdebstrap.py
+++ b/mdbp/mmdebstrap.py
@@ -9,7 +9,6 @@ import ctypes.util
 import functools
 import os
 import pathlib
-import pwd
 import shlex
 import shutil
 import subprocess
@@ -27,6 +26,18 @@ def unshare_network() -> None:
     if libc.unshare(0x40000000) < 0:
         raise OSError("unshare() failed", ctypes.get_errno())
 
+def set_uids(username):
+    """Look up the given user in /etc/passwd (e.g. after chroot) and drop
+    privileges to this user."""
+    with open("/etc/passwd", "r") as f:
+        for line in f:
+            parts = line.strip().split(":")
+            if parts[0] == username:
+                os.setgid(int(parts[3]))
+                os.setuid(int(parts[2]))
+                return
+    raise OSError("user %s not found in /etc/passwd" % username)
+
 def priv_drop(cmd: typing.List[str], *,
               chroot: typing.Optional[pathlib.Path] = None,
               chdir: typing.Union[None, str, pathlib.PurePath] = None,
@@ -51,9 +62,7 @@ def priv_drop(cmd: typing.List[str], *,
         if chroot or chdir:
             os.chdir(chdir or "/")
         if setuid:
-            pwentry = pwd.getpwnam(setuid)
-            os.setgid(pwentry.pw_gid)
-            os.setuid(pwentry.pw_uid)
+            set_uids(setuid)
     subprocess.check_call(cmd, preexec_fn=preexec_fn, env=env)
 
 def native_architecture() -> str: