passwd: add UPN validation support

Add User Principal Name (UPN) validation to allow passwd command to
accept usernames in user@domain.com format. Currently, passwd will
accept both traditional usernames and UPN format.

Fixes: 326889c (2024-10-22; "Fix coverity unbound buffer issues")
Closes: <#1626>
Reported-by: @nooreldeenmansour
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

Author: ikerexxe

src/passwd.c

@@ -993,7 +993,7 @@ main(int argc, char **argv)
 	}
 	myname = xstrdup (pw->pw_name);
 	if (optind < argc) {
-		if (!is_valid_user_name (argv[optind])) {
+		if (!is_valid_user_name (argv[optind]) && !is_valid_upn (argv[optind])) {
 			fprintf (stderr, _("%s: Provided user name is not a valid name\n"), Prog);
 			fail_exit (E_NOPERM, process_selinux);
 		}