1 files changed, 147 insertions, 0 deletions
diff --git a/stdlib/tst-arc4random-stats.c b/stdlib/tst-arc4random-stats.c
new file mode 100644
index 0000000000..f6fc1f10c4
--- /dev/null
+++ b/stdlib/tst-arc4random-stats.c
@@ -0,0 +1,147 @@
+/* Statistical tests for arc4random-related functions.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include <support/check.h>
+
+enum
+{
+  arc4random_key_size = 32
+};
+
+struct key
+{
+  unsigned char data[arc4random_key_size];
+};
+
+/* With 12,000 keys, the probability that a byte in a predetermined
+   position does not have a predetermined value in all generated keys
+   is about 4e-21.  The probability that this happens with any of the
+   16 * 256 possible byte position/values is 1.6e-17.  This results in
+   an acceptably low false-positive rate.  */
+enum { key_count = 12000 };
+
+static struct key keys[key_count];
+
+/* Used to perform the distribution check.  */
+static int byte_counts[arc4random_key_size][256];
+
+/* Bail out after this many failures.  */
+enum { failure_limit = 100 };
+
+static void
+find_stuck_bytes (bool (*func) (unsigned char *key))
+{
+  memset (&keys, 0xcc, sizeof (keys));
+
+  int failures = 0;
+  for (int key = 0; key < key_count; ++key)
+    {
+      while (true)
+        {
+          if (func (keys[key].data))
+            break;
+          ++failures;
+          if (failures >= failure_limit)
+            {
+              printf ("warning: bailing out after %d failures\n", failures);
+              return;
+            }
+        }
+    }
+  printf ("info: key generation finished with %d failures\n", failures);
+
+  memset (&byte_counts, 0, sizeof (byte_counts));
+  for (int key = 0; key < key_count; ++key)
+    for (int pos = 0; pos < arc4random_key_size; ++pos)
+      ++byte_counts[pos][keys[key].data[pos]];
+
+  for (int pos = 0; pos < arc4random_key_size; ++pos)
+    for (int byte = 0; byte < 256; ++byte)
+      if (byte_counts[pos][byte] == 0)
+        {
+          support_record_failure ();
+          printf ("error: byte %d never appeared at position %d\n", byte, pos);
+        }
+}
+
+/* Test adapter for arc4random.  */
+static bool
+generate_arc4random (unsigned char *key)
+{
+  uint32_t words[arc4random_key_size / 4];
+  _Static_assert (sizeof (words) == arc4random_key_size, "sizeof (words)");
+
+  for (int i = 0; i < array_length (words); ++i)
+    words[i] = arc4random ();
+  memcpy (key, &words, arc4random_key_size);
+  return true;
+}
+
+/* Test adapter for arc4random_buf.  */
+static bool
+generate_arc4random_buf (unsigned char *key)
+{
+  arc4random_buf (key, arc4random_key_size);
+  return true;
+}
+
+/* Test adapter for arc4random_uniform.  */
+static bool
+generate_arc4random_uniform (unsigned char *key)
+{
+  for (int i = 0; i < arc4random_key_size; ++i)
+    key[i] = arc4random_uniform (256);
+  return true;
+}
+
+/* Test adapter for arc4random_uniform with argument 257.  This means
+   that byte 0 happens more often, but we do not perform such a
+   statistcal check, so the test will still pass */
+static bool
+generate_arc4random_uniform_257 (unsigned char *key)
+{
+  for (int i = 0; i < arc4random_key_size; ++i)
+    key[i] = arc4random_uniform (257);
+  return true;
+}
+
+static int
+do_test (void)
+{
+  puts ("info: arc4random implementation test");
+  find_stuck_bytes (generate_arc4random);
+
+  puts ("info: arc4random_buf implementation test");
+  find_stuck_bytes (generate_arc4random_buf);
+
+  puts ("info: arc4random_uniform implementation test");
+  find_stuck_bytes (generate_arc4random_uniform);
+
+  puts ("info: arc4random_uniform implementation test (257 variant)");
+  find_stuck_bytes (generate_arc4random_uniform_257);
+
+  return 0;
+}
+
+#include <support/test-driver.c>