[cmake-developers] [CMake 0015976]: cmSystemTools::RandomSeed() requests excessive entropy per invocation

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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https://cmake.org/Bug/view.php?id=15976 
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Reported By:                Cristian Rodríguez
Assigned To:                
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Project:                    CMake
Issue ID:                   15976
Category:                   CMake
Reproducibility:            always
Severity:                   minor
Priority:                   normal
Status:                     new
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Date Submitted:             2016-02-16 20:26 EST
Last Modified:              2016-02-16 20:26 EST
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Summary:                    cmSystemTools::RandomSeed() requests excessive
entropy per invocation
Description: 
cmSystemTools::RandomSeed() opens a *buffered* ifstream, the final result is
that each invocation requests  _IO_BUFSIZ -1 of entropy to the operating system
instead of just sizeof(seed)

Steps to Reproduce: 
mkdir test_cmake
echo "PROJECT(foo)" > CMakeLists.txt
cd test_cmake
strace -f -ocmake.trace cmake .

open cmake.trace 

10768 26289 open("/dev/urandom", O_RDONLY)    = 4
10769 26289 read(4,
"h\304j\vx\314\26\0042\250g\275\0014,\256\251\257T&\304\212\310\347q\201\263\324\330\273z8"...,
8191) = 8191
10770 26289 close(4)                          = 0
10771 26289 write(3, "cmake_minimum_required(VERSION 3"..., 795) = 795
10772 26289 close(3)                          = 0
10773 26289 stat("/home/crrodriguez/test_cmake/CMakeFiles/CMakeTmp",
{st_mode=S_IFDIR|0755, st_size=58, ...}) = 0
10774 26289 getcwd("/home/crrodriguez/test_cmake", 2048) = 29

Additional Information: 
git diff
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 3ba7287..26bec34 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -2184,8 +2184,10 @@ 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);
+  fin.open("/dev/urandom");
+  if(fin.good() && fin.read(seed.bytes, sizeof(seed)) &&
      fin.gcount() == sizeof(seed))
     {
     return seed.integer;

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Issue History 
Date Modified    Username       Field                    Change               
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2016-02-16 20:26 Cristian RodríguezNew Issue                                   

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