From 04d965d67a7264a1c7c211494aebda1953df7603 Mon Sep 17 00:00:00 2001
From: "info@mode42.com" <info@mode42.com>
Date: Fri, 7 Aug 2026 18:25:13 +0000
Subject: Initial push

---
 stacks/daemon/privilege_drop.py | 110 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 stacks/daemon/privilege_drop.py

(limited to 'stacks/daemon/privilege_drop.py')

diff --git a/stacks/daemon/privilege_drop.py b/stacks/daemon/privilege_drop.py
new file mode 100644
index 0000000..5e68ca8
--- /dev/null
+++ b/stacks/daemon/privilege_drop.py
@@ -0,0 +1,110 @@
+"""Drop max25d privileges after hardware stack is up (Linux)."""
+from __future__ import annotations
+
+import configparser
+import grp
+import os
+import pwd
+import sys
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass
+class RunAsConfig:
+    user: str = ""
+    group: str = ""
+    uid: Optional[int] = None
+    gid: Optional[int] = None
+
+
+def _parse_optional_int(raw: str) -> Optional[int]:
+    text = str(raw).strip()
+    if not text:
+        return None
+    return int(text)
+
+
+def parse_run_as(cp: configparser.ConfigParser) -> RunAsConfig:
+    cfg = RunAsConfig()
+    if not cp.has_section("daemon"):
+        return cfg
+    sec = cp["daemon"]
+    cfg.user = sec.get("user", fallback="").strip()
+    cfg.group = sec.get("group", fallback="").strip()
+    if cp.has_option("daemon", "uid"):
+        cfg.uid = _parse_optional_int(sec.get("uid", fallback=""))
+    if cp.has_option("daemon", "gid"):
+        cfg.gid = _parse_optional_int(sec.get("gid", fallback=""))
+    return cfg
+
+
+def run_as_configured(cfg: RunAsConfig) -> bool:
+    return bool(
+        cfg.user
+        or cfg.group
+        or cfg.uid is not None
+        or cfg.gid is not None
+    )
+
+
+def resolve_run_as(cfg: RunAsConfig) -> tuple[int, int, str]:
+    pw: pwd.struct_passwd | None = None
+    name = cfg.user
+
+    if cfg.user:
+        pw = pwd.getpwnam(cfg.user)
+        uid = pw.pw_uid
+        gid = pw.pw_gid
+    elif cfg.uid is not None:
+        pw = pwd.getpwuid(cfg.uid)
+        uid = pw.pw_uid
+        gid = pw.pw_gid
+        name = pw.pw_name
+    else:
+        raise ValueError("[daemon] user= or uid= required for privilege drop")
+
+    if cfg.uid is not None:
+        uid = cfg.uid
+    if cfg.group:
+        gid = grp.getgrnam(cfg.group).gr_gid
+    elif cfg.gid is not None:
+        gid = cfg.gid
+
+    return uid, gid, name
+
+
+def drop_privileges(cfg: RunAsConfig, *, log) -> None:
+    if os.name != "posix":
+        return
+    if not run_as_configured(cfg):
+        if os.geteuid() == 0:
+            log(
+                "running as root after stack start — set [daemon] user= to drop",
+                area="privilege",
+            )
+        return
+    if os.geteuid() != 0:
+        return
+
+    uid, gid, name = resolve_run_as(cfg)
+
+    os.initgroups(name, gid)
+    os.setgid(gid)
+    os.setuid(uid)
+
+    if os.geteuid() == 0:
+        raise OSError("privilege drop failed — still root")
+
+    log(
+        f"dropped privileges to uid={uid} gid={gid} ({name})",
+        area="privilege",
+    )
+
+
+def drop_privileges_or_exit(cfg: RunAsConfig, *, log) -> None:
+    try:
+        drop_privileges(cfg, log=log)
+    except (KeyError, ValueError, OSError) as exc:
+        log(f"privilege drop failed: {exc}", area="privilege")
+        raise SystemExit(1) from exc
-- 
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