1  /* Some auxiliary stuff for defining an alternate stack.
       2     Copyright (C) 2010  Eric Blake <eblake@redhat.com>
       3     Copyright (C) 2010-2021  Bruno Haible <bruno@clisp.org>
       4  
       5     This program is free software: you can redistribute it and/or modify
       6     it under the terms of the GNU General Public License as published by
       7     the Free Software Foundation; either version 2 of the License, or
       8     (at your option) any later version.
       9  
      10     This program is distributed in the hope that it will be useful,
      11     but WITHOUT ANY WARRANTY; without even the implied warranty of
      12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      13     GNU General Public License for more details.
      14  
      15     You should have received a copy of the GNU General Public License
      16     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      17  
      18  #include <stdint.h> /* uintptr_t */
      19  #include <string.h> /* for memset */
      20  
      21  #define MYSTACK_SIZE (1 << 24)
      22  
      23  /* glibc says: Users should use SIGSTKSZ as the size of user-supplied
      24     buffers.  We want to detect stack overflow of the alternate stack
      25     in a nicer manner than just crashing, so we overallocate in
      26     comparison to what we hand libsigsegv.  Also, we intentionally hand
      27     an unaligned pointer, to ensure the alternate stack still ends up
      28     aligned.  */
      29  #define MYSTACK_CRUMPLE_ZONE 8192
      30  static char mystack_storage[MYSTACK_SIZE + 2 * MYSTACK_CRUMPLE_ZONE + 31];
      31  static char *mystack; /* MYSTACK_SIZE bytes in the middle of storage. */
      32  
      33  static void
      34  prepare_alternate_stack (void)
      35  {
      36  #ifdef SIGSTKSZ
      37    if (MYSTACK_SIZE < SIGSTKSZ)
      38      {
      39        size_t size = SIGSTKSZ;
      40        printf ("SIGSTKSZ=%zu exceeds MYSTACK_SIZE=%d\n", size, MYSTACK_SIZE);
      41        exit (1);
      42      }
      43  #endif
      44    memset (mystack_storage, 's', sizeof mystack_storage);
      45    mystack = (char *) ((uintptr_t) (mystack_storage + MYSTACK_CRUMPLE_ZONE) | 31);
      46  }
      47  
      48  static void
      49  check_alternate_stack_no_overflow (void)
      50  {
      51    unsigned int i;
      52  
      53    for (i = MYSTACK_CRUMPLE_ZONE; i > 0; i--)
      54      if (*(mystack - i) != 's')
      55        {
      56          printf ("Alternate stack was exceeded by %u bytes!!\n", i);
      57          exit (1);
      58        }
      59    for (i = MYSTACK_CRUMPLE_ZONE; i > 0; i--)
      60      if (*(mystack + MYSTACK_SIZE - 1 + i) != 's')
      61        {
      62          printf ("Alternate stack was exceeded by %u bytes!!\n", i);
      63          exit (1);
      64        }
      65  }