Code: test_function (char * (*my_asnprintf) (char *, size_t *, const char *, ...))
{
  char buf[8];
  int size;

  for (size = 0; size <= 8; size++)
    {
      size_t length = size;
      char *result = my_asnprintf (NULL, &length, "%d", 12345);
      ASSERT (result != NULL);
      ASSERT (strcmp (result, "12345") == 0);
      ASSERT (length == 5);
      free (result);
    }

  for (size = 0; size <= 8; size++)
    {
      size_t length;
      char *result;

      memcpy (buf, "DEADBEEF", 8);
      length = size;
      result = my_asnprintf (buf, &length, "%d", 12345);
       ASSERT (result != NULL);
       ASSERT (strcmp (result, "12345") == 0);
       ASSERT (length == 5);
      if (size < 6)
         ASSERT (result != buf);
       ASSERT (memcmp (buf + size, &"DEADBEEF"[size], 8 - size) == 0);
       if (result != buf)
        free (result);
    }
}

Vulnerability Type: Overflow 
CWE ID: CWE-119
Summary: The convert_to_decimal function in vasnprintf.c in Gnulib before 2018-09-23 has a heap-based buffer overflow because memory is not allocated for a trailing '0' character during %f processing.
Commit Message: vasnprintf: Fix heap memory overrun bug.

Reported by Ben Pfaff <blp@cs.stanford.edu> in
<https://lists.gnu.org/archive/html/bug-gnulib/2018-09/msg00107.html>.

* lib/vasnprintf.c (convert_to_decimal): Allocate one more byte of
memory.
* tests/test-vasnprintf.c (test_function): Add another test.