1  /* Memory allocation next to an unmapped page.
       2     Copyright (C) 2017-2023 Free Software Foundation, Inc.
       3     This file is part of the GNU C Library.
       4  
       5     The GNU C Library is free software; you can redistribute it and/or
       6     modify it under the terms of the GNU Lesser General Public
       7     License as published by the Free Software Foundation; either
       8     version 2.1 of the License, or (at your option) any later version.
       9  
      10     The GNU C Library 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 GNU
      13     Lesser General Public License for more details.
      14  
      15     You should have received a copy of the GNU Lesser General Public
      16     License along with the GNU C Library; if not, see
      17     <https://www.gnu.org/licenses/>.  */
      18  
      19  #include <support/check.h>
      20  #include <support/next_to_fault.h>
      21  #include <support/xunistd.h>
      22  #include <sys/mman.h>
      23  #include <sys/param.h>
      24  
      25  struct support_next_to_fault
      26  support_next_to_fault_allocate (size_t size)
      27  {
      28    long page_size = sysconf (_SC_PAGE_SIZE);
      29    TEST_VERIFY_EXIT (page_size > 0);
      30    struct support_next_to_fault result;
      31    result.region_size = roundup (size, page_size) + page_size;
      32    if (size + page_size <= size || result.region_size <= size)
      33      FAIL_EXIT1 ("support_next_to_fault_allocate (%zu): overflow", size);
      34    result.region_start
      35      = xmmap (NULL, result.region_size, PROT_READ | PROT_WRITE,
      36               MAP_PRIVATE | MAP_ANONYMOUS, -1);
      37    /* Unmap the page after the allocation.  */
      38    xmprotect (result.region_start + (result.region_size - page_size),
      39               page_size, PROT_NONE);
      40    /* Align the allocation within the region so that it ends just
      41       before the PROT_NONE page.  */
      42    result.buffer = result.region_start + result.region_size - page_size - size;
      43    result.length = size;
      44    return result;
      45  }
      46  
      47  void
      48  support_next_to_fault_free (struct support_next_to_fault *ntf)
      49  {
      50    xmunmap (ntf->region_start, ntf->region_size);
      51    *ntf = (struct support_next_to_fault) { NULL, };
      52  }