1  /* Some auxiliary stuff for defining an alternate stack.
       2     Copyright (C) 2010-2023 Free Software Foundation, Inc.
       3  
       4     This program is free software: you can redistribute it and/or modify
       5     it under the terms of the GNU General Public License as published by
       6     the Free Software Foundation; either version 2 of the License, or
       7     (at your option) any later version.
       8  
       9     This program is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      12     GNU General Public License for more details.
      13  
      14     You should have received a copy of the GNU General Public License
      15     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      16  
      17  /* Written by Eric Blake and Bruno Haible.  */
      18  
      19  #include <stdint.h> /* uintptr_t */
      20  #include <string.h> /* for memset */
      21  
      22  #define MYSTACK_SIZE (1 << 24)
      23  
      24  /* glibc says: Users should use SIGSTKSZ as the size of user-supplied
      25     buffers.  We want to detect stack overflow of the alternate stack
      26     in a nicer manner than just crashing, so we overallocate in
      27     comparison to what we hand libsigsegv.  Also, we intentionally hand
      28     an unaligned pointer, to ensure the alternate stack still ends up
      29     aligned.  */
      30  #define MYSTACK_CRUMPLE_ZONE 8192
      31  static char mystack_storage[MYSTACK_SIZE + 2 * MYSTACK_CRUMPLE_ZONE + 31];
      32  static char *mystack; /* MYSTACK_SIZE bytes in the middle of storage. */
      33  
      34  static void
      35  prepare_alternate_stack (void)
      36  {
      37  #ifdef SIGSTKSZ
      38    if (MYSTACK_SIZE < SIGSTKSZ)
      39      {
      40        size_t size = SIGSTKSZ;
      41        printf ("SIGSTKSZ=%zu exceeds MYSTACK_SIZE=%d\n", size, MYSTACK_SIZE);
      42        exit (1);
      43      }
      44  #endif
      45    memset (mystack_storage, 's', sizeof mystack_storage);
      46    mystack = (char *) ((uintptr_t) (mystack_storage + MYSTACK_CRUMPLE_ZONE) | 31);
      47  }
      48  
      49  static void
      50  check_alternate_stack_no_overflow (void)
      51  {
      52    unsigned int i;
      53  
      54    for (i = MYSTACK_CRUMPLE_ZONE; i > 0; i--)
      55      if (*(mystack - i) != 's')
      56        {
      57          printf ("Alternate stack was exceeded by %u bytes!!\n", i);
      58          exit (1);
      59        }
      60    for (i = MYSTACK_CRUMPLE_ZONE; i > 0; i--)
      61      if (*(mystack + MYSTACK_SIZE - 1 + i) != 's')
      62        {
      63          printf ("Alternate stack was exceeded by %u bytes!!\n", i);
      64          exit (1);
      65        }
      66  }