malloc/mallocbug.c

   1 /* Reproduce a GNU malloc bug.  */
   2 #include <malloc.h>
   3 #include <stdio.h>
   4 #include <string.h>
   5
   6 #define size_t unsigned int
   7
   8 /* Defined as global variables to avoid warnings about unused variables.  */
   9 char *dummy0;
  10 char *dummy1;
  11 char *fill_info_table1;
  12
  13
  14 int
  15 main (int argc, char *argv[])
  16 {
  17   char *over_top;
  18   size_t over_top_size = 0x3000;
  19   char *over_top_dup;
  20   size_t over_top_dup_size = 0x7000;
  21   char *x;
  22   size_t i;
  23
  24   /* Here's what memory is supposed to look like (hex):
  25         size  contents
  26         3000  original_info_table, later fill_info_table1
  27       3fa000  dummy0
  28       3fa000  dummy1
  29         6000  info_table_2
  30         3000  over_top
  31
  32    */
  33   /* mem: original_info_table */
  34   dummy0 = malloc (0x3fa000);
  35   /* mem: original_info_table, dummy0 */
  36   dummy1 = malloc (0x3fa000);
  37   /* mem: free, dummy0, dummy1, info_table_2 */
  38   fill_info_table1 = malloc (0x3000);
  39   /* mem: fill_info_table1, dummy0, dummy1, info_table_2 */
  40
  41   x = malloc (0x1000);
  42   free (x);
  43   /* mem: fill_info_table1, dummy0, dummy1, info_table_2, freexx */
  44
  45   /* This is what loses; info_table_2 and freexx get combined unbeknownst
  46      to mmalloc, and mmalloc puts over_top in a section of memory which
  47      is on the free list as part of another block (where info_table_2 had
  48      been).  */
  49   over_top = malloc (over_top_size);
  50   over_top_dup = malloc (over_top_dup_size);
  51   memset (over_top, 0, over_top_size);
  52   memset (over_top_dup, 1, over_top_dup_size);
  53
  54   for (i = 0; i < over_top_size; ++i)
  55     if (over_top[i] != 0)
  56       {
  57         printf ("FAIL: malloc expands info table\n");
  58         return 0;
  59       }
  60
  61   for (i = 0; i < over_top_dup_size; ++i)
  62     if (over_top_dup[i] != 1)
  63       {
  64         printf ("FAIL: malloc expands info table\n");
  65         return 0;
  66       }
  67
  68   printf ("PASS: malloc expands info table\n");
  69   return 0;
  70 }