[Bug libc/16996] New: get_nprocs caching breaks when 'time (NULL) == 0'

[Bug libc/16996] New: get_nprocs caching breaks when 'time (NULL) == 0'

[meadori at codesourcery dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16996

            Bug ID: 16996
           Summary: get_nprocs caching breaks when 'time (NULL) == 0'
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: meadori at codesourcery dot com
                CC: drepper.fsp at gmail dot com

Currently the implementation of __get_nprocs uses a stactic variable to cache
the value of the current number of processors.  The caching breaks when
'time (NULL) == 0':

  $ cat nproc.c 
  #include <stdio.h>
  #include <time.h>
  #include <sys/time.h>

  int main(int argc, char *argv[])
  {
    time_t t;
    struct timeval tv = {0, 0};
    printf("settimeofday({0, 0}, NULL) = %d\n", settimeofday(&tv, NULL));
    t = time(NULL);
    printf("Time: %d, CPUs: %d\n", (unsigned int)t, get_nprocs());
    return 0;
  }
  $ gcc -O3 nproc.c 
  $ ./a.out 
  settimeofday({0, 0}, NULL) = -1
  Time: 1401311578, CPUs: 4
  $ sudo ./a.out 
  settimeofday({0, 0}, NULL) = 0
  Time: 0, CPUs: 0

I know the circumstances of setting the time of day to (0, 0) are odd,
but it still shouldn't be so trivial to break the caching.

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[Bug libc/16996] get_nprocs caching breaks when 'time (NULL) == 0'

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16996

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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=995a46bbfba9964e328e3947130919d8bd3cd62a

commit 995a46bbfba9964e328e3947130919d8bd3cd62a
Author: Meador Inge <meadori@codesourcery.com>
Date:   Fri Jun 13 14:02:04 2014 +0530

    get_nprocs: Only return explictly set cache values (BZ #16996)

    The implementation of __get_nprocs uses a stactic variable to cache
    the value of the current number of processors.  The caching breaks when
    'time (NULL) == 0':

      $ cat nproc.c
      #include <stdio.h>
      #include <time.h>
      #include <sys/time.h>

      int main(int argc, char *argv[])
      {
        time_t t;
        struct timeval tv = {0, 0};
        printf("settimeofday({0, 0}, NULL) = %d\n", settimeofday(&tv, NULL));
        t = time(NULL);
        printf("Time: %d, CPUs: %d\n", (unsigned int)t, get_nprocs());
        return 0;
      }
      $ gcc -O3 nproc.c
      $ ./a.out
      settimeofday({0, 0}, NULL) = -1
      Time: 1401311578, CPUs: 4
      $ sudo ./a.out
      settimeofday({0, 0}, NULL) = 0
      Time: 0, CPUs: 0

    The problem is with the condition used to check whether a cached
    value should be returned or not:

      static int cached_result;
      static time_t timestamp;

      time_t now = time (NULL);
      time_t prev = timestamp;
      atomic_read_barrier ();
      if (now == prev)
        return cached_result;

    This patch fixes the problem by ensuring that 'cached_result' has
    been set at least once before returning it.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                             |    6 ++++++
 NEWS                                  |    2 +-
 sysdeps/unix/sysv/linux/getsysstats.c |    4 ++--
 3 files changed, 9 insertions(+), 3 deletions(-)

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[Bug libc/16996] get_nprocs caching breaks when 'time (NULL) == 0'

[siddhesh at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16996

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |siddhesh at redhat dot com
         Resolution|---                         |FIXED

--- Comment #2 from Siddhesh Poyarekar <siddhesh at redhat dot com> ---
Fixed in master.

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[Bug libc/16996] get_nprocs caching breaks when 'time (NULL) == 0'

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16996

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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