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/max25-bcpr/src/bcpr_runas.c | 81 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 stacks/max25-bcpr/src/bcpr_runas.c

(limited to 'stacks/max25-bcpr/src/bcpr_runas.c')

diff --git a/stacks/max25-bcpr/src/bcpr_runas.c b/stacks/max25-bcpr/src/bcpr_runas.c
new file mode 100644
index 0000000..8c930bf
--- /dev/null
+++ b/stacks/max25-bcpr/src/bcpr_runas.c
@@ -0,0 +1,81 @@
+/*
+ * Process identity: max25-bcprd runs unprivileged; init drops from root.
+ */
+#define _GNU_SOURCE
+#include "bcpr/bcpr_runas.h"
+
+#include <errno.h>
+#include <grp.h>
+#include <pwd.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+int bcpr_runas_refuse_root(int dry_run)
+{
+    if (geteuid() == 0 && !dry_run) {
+        fprintf(stderr,
+                "max25-bcprd: refusing to run as root — use max25-bcprd-init "
+                "(setuid) for live SER12\n");
+        return -1;
+    }
+    return 0;
+}
+
+int bcpr_runas_drop(const bcpr_config_t *cfg)
+{
+    struct passwd *pw;
+    struct group *gr = NULL;
+    gid_t gid;
+    uid_t uid;
+    const char *name;
+
+    if (!cfg || !cfg->run_user[0]) {
+        fprintf(stderr, "max25-bcpr: [max25-bcpr] user= required for daemon\n");
+        return -1;
+    }
+    if (geteuid() != 0) {
+        fprintf(stderr, "max25-bcpr: privilege drop requires euid=0\n");
+        return -1;
+    }
+
+    pw = getpwnam(cfg->run_user);
+    if (!pw) {
+        fprintf(stderr, "max25-bcpr: unknown user=%s\n", cfg->run_user);
+        return -1;
+    }
+    uid = pw->pw_uid;
+    gid = pw->pw_gid;
+    name = pw->pw_name;
+
+    if (cfg->run_group[0]) {
+        gr = getgrnam(cfg->run_group);
+        if (!gr) {
+            fprintf(stderr, "max25-bcpr: unknown group=%s\n", cfg->run_group);
+            return -1;
+        }
+        gid = gr->gr_gid;
+    }
+
+    if (initgroups(name, gid) != 0) {
+        fprintf(stderr, "max25-bcpr: initgroups(%s) failed: %s\n", name,
+                strerror(errno));
+        return -1;
+    }
+    if (setgid(gid) != 0) {
+        fprintf(stderr, "max25-bcpr: setgid failed: %s\n", strerror(errno));
+        return -1;
+    }
+    if (setuid(uid) != 0) {
+        fprintf(stderr, "max25-bcpr: setuid failed: %s\n", strerror(errno));
+        return -1;
+    }
+    if (setuid(0) == 0) {
+        fprintf(stderr, "max25-bcpr: privilege drop incomplete\n");
+        return -1;
+    }
+
+    fprintf(stderr, "max25-bcpr: running as uid=%u gid=%u (%s)\n",
+            (unsigned)uid, (unsigned)gid, name);
+    return 0;
+}
-- 
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