1 files changed, 7 insertions, 2 deletions

diff --git a/src/lib/libc/string/strnlen.c b/src/lib/libc/string/strnlen.c
index 26e9743f18..33c3b6e2ca 100644
--- a/src/lib/libc/string/strnlen.c
+++ b/src/lib/libc/string/strnlen.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: strnlen.c,v 1.6 2015/08/31 02:53:57 guenther Exp $    */
+/*      $OpenBSD: strnlen.c,v 1.7 2016/10/14 18:19:04 dtucker Exp $     */
 
 /*
  * Copyright (c) 2010 Todd C. Miller <Todd.Miller@courtesan.com>
@@ -19,6 +19,7 @@
 #include <sys/types.h>
 
 #include <string.h>
+#include <stdint.h>
 
 size_t
 strnlen(const char *str, size_t maxlen)
@@ -28,6 +29,10 @@ strnlen(const char *str, size_t maxlen)
         for (cp = str; maxlen != 0 && *cp != '\0'; cp++, maxlen--)
                 ;
 
-        return (size_t)(cp - str);
+        /*
+         * Cast pointers to unsigned type before calculation, to avoid signed
+         * overflow when the string ends where the MSB has changed.
+         */
+        return (size_t)((uintptr_t)cp - (uintptr_t)str);
 }
 DEF_WEAK(strnlen);