From f23f18ab686faa0ce91486143469bb58753b0843 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cristian=20Rodr=C3=ADguez?= <crrodriguez@opensuse.org>
Date: Wed, 17 Feb 2016 09:02:04 -0500
Subject: [PATCH] cmSystemTools: Avoid excess entropy consumption by RandomSeed
 (#15976)

Read `/dev/urandom` without buffering to avoid taking more than we need.
---
 Source/cmSystemTools.cxx | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index c9670faf0..9af54bffc 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -2183,8 +2183,10 @@ unsigned int cmSystemTools::RandomSeed()
   } seed;
 
   // Try using a real random source.
-  cmsys::ifstream fin("/dev/urandom");
-  if(fin && fin.read(seed.bytes, sizeof(seed)) &&
+  cmsys::ifstream fin;
+  fin.rdbuf()->pubsetbuf(0, 0); // Unbuffered read.
+  fin.open("/dev/urandom");
+  if(fin.good() && fin.read(seed.bytes, sizeof(seed)) &&
      fin.gcount() == sizeof(seed))
     {
     return seed.integer;