random32: add test cases for taus113 implementation

We generated a battery of 100 test cases from GSL taus113 implemention
and compare the results from a particular seed and a particular
iteration with our implementation in the kernel. We have verified on
32 and 64 bit machines that our taus113 kernel implementation gives
same results as GSL taus113 implementation:

  [    0.147370] prandom: seed boundary self test passed
  [    0.148078] prandom: 100 self tests passed

This is a Kconfig option that is disabled on default, just like the
crc32 init selftests in order to not unnecessary slow down boot process.
We also refactored out prandom_seed_very_weak() as it's now used in
multiple places in order to reduce redundant code.

GSL code we used for generating test cases:

  int i, j;
  srand(time(NULL));
  for (i = 0; i < 100; ++i) {
    int iteration = 500 + (rand() % 500);
    gsl_rng_default_seed = rand() + 1;
    gsl_rng *r = gsl_rng_alloc(gsl_rng_taus113);
    printf("\t{ %lu, ", gsl_rng_default_seed);
    for (j = 0; j < iteration - 1; ++j)
      gsl_rng_get(r);
    printf("%u, %lu },\n", iteration, gsl_rng_get(r));
    gsl_rng_free(r);
  }

Joint work with Hannes Frederic Sowa.

Cc: Florian Weimer <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

lib/Kconfig

@@ -189,6 +189,13 @@ config AUDIT_GENERIC
 	depends on AUDIT && !AUDIT_ARCH
 	default y
 
+config RANDOM32_SELFTEST
+	bool "PRNG perform self test on init"
+	default n
+	help
+	  This option enables the 32 bit PRNG library functions to perform a
+	  self test on initialization.
+
 #
 # compression support is select'ed if needed
 #