Do not call va_arg when open O_DIRECTORY flag is set.

Also, fix build on systems without O_TMPFILE.

userhosts.c

@@ -29,6 +29,14 @@
 
 #define ORIGINAL_HOSTS_PATH "/etc/hosts"
 
+/* Standard O_TMPFILE includes O_DIRECTORY bit, but we do not want to call
+ * va_arg if O_DIRECTORY is set alone. Hence, define a "purer" O_TMPFILE. */
+#define PURE_O_TMPFILE (020000000)
+/* And sanity-check when O_TMPFILE is defined. */
+#if defined(O_TMPFILE) && (O_TMPFILE & PURE_O_TMPFILE) == 0
+#error PURE_O_TMPFILE is incorrectly defined
+#endif
+
 static int (*original_open)(const char *, int, ...);
 static FILE *(*original_fopen)(const char *, const char *);
 static const char *replacement_hosts;
@@ -59,7 +67,7 @@ static void __attribute__ ((constructor)) init(void) {
 int open(const char *__file, int __oflag, ...) {
   if(strcmp(__file, ORIGINAL_HOSTS_PATH) == 0)
     __file = replacement_hosts;
-  if (__oflag & (O_CREAT | O_TMPFILE)) {
+  if (__oflag & (O_CREAT | PURE_O_TMPFILE)) {
     va_list ap;
     mode_t mode;