From 05db341ef5454ba5bbf613f7f84890fd16c9d532 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Wed, 18 Apr 2018 17:25:52 +0900
Subject: `mrb_dup()` should fail if `fd` is a negative integer; ref #4005

---
 mrbgems/mruby-io/src/io.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index 6ace9e167..e11aa3300 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -542,12 +542,12 @@ mrb_dup(mrb_state *mrb, int fd, mrb_bool *failed)
 {
   int new_fd;
 
-  *failed = FALSE;
+  *failed = TRUE;
   if (fd < 0)
     return fd;
 
   new_fd = dup(fd);
-  if (new_fd == -1) *failed = TRUE;
+  if (new_fd > 0) *failed = FALSE;
   return new_fd;
 }
 
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