...and actually adding new file now...

libbb/die_if_bad_username.c

@@ -0,0 +1,34 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Ckeck user and group names for illegal characters
+ *
+ * Copyright (C) 2008 Tito Ragusa <farmatito@tiscali.it>
+ *
+ * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
+ */
+
+#include "libbb.h"
+
+/* To avoid problems, the username should consist only of
+ * letters, digits, underscores, periods, at signs and dashes,
+ * and not start with a dash (as defined by IEEE Std 1003.1-2001).
+ * For compatibility with Samba machine accounts $ is also supported
+ * at the end of the username.
+ */
+
+void die_if_bad_username(const char *name)
+{
+	int i = 0;
+
+	do {
+		if (!isalnum(*name)
+		 && !(*name == '_')
+		 && !(*name == '.')
+		 && !(*name == '@')
+		 && !(*name == '-' && i)
+		 && !(*name == '$' && !*(name + 1))
+		)
+			bb_error_msg_and_die("illegal character '%c'", *name);
+		i++;
+	} while (*++name);
+}