1  /* Copyright (C) 2012-2023 Free Software Foundation, Inc.
       2  
       3  This file is part of GCC.
       4  
       5  GCC is free software; you can redistribute it and/or modify it under
       6  the terms of the GNU General Public License as published by the Free
       7  Software Foundation; either version 3, or (at your option) any later
       8  version.
       9  
      10  GCC is distributed in the hope that it will be useful, but WITHOUT ANY
      11  WARRANTY; without even the implied warranty of MERCHANTABILITY or
      12  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      13  for more details.
      14  
      15  Under Section 7 of GPL version 3, you are granted additional
      16  permissions described in the GCC Runtime Library Exception, version
      17  3.1, as published by the Free Software Foundation.
      18  
      19  You should have received a copy of the GNU General Public License and
      20  a copy of the GCC Runtime Library Exception along with this program;
      21  see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
      22  <http://www.gnu.org/licenses/>.  */
      23  
      24  /* This file is part of the vtable verification feature (for a
      25     detailed description of the feature, see comments in
      26     vtable-verify.c).  The vtable verification feature creates
      27     certain global symbols that need to be read-write sometimes during
      28     program execution, and read-only at others.  It uses 'mprotect' to
      29     change the memory protections of the pages on which these variables
      30     are stored.  In order to not affect the protections of other
      31     program variables, these variables are put into a special named
      32     section, ".vtable_map_vars", which is page-aligned at the start,
      33     and which is padded with a page-sized amount of zeros at the end.
      34     To make this section page aligned, we create a special symbol,
      35     "_vtable_map_vars_start" which we make the very first thing that
      36     goes into the section.  This file defines that symbol (and only
      37     that symbol).  GCC compiles this file into vtv_start.o, and
      38     inserts vtv_start.o into the link line immediately after
      39     crtbegin.o, if the program is compiled with -fvtable.verify.
      40  
      41     In order to pad the ".vtable_map_vars" section with a page-sized
      42     amount of zeros at the end, there is a second symbol,
      43     _vtable_map_vars_end, which is defined in another file, vtv_end.c.
      44     This second symbol is a page-sized array of chars, zero-filled, and
      45     is the very last thing to go into the section.  When the GCC driver
      46     inserts vtv_start.o into the link line (just after crtbegin.o) it
      47     also inserts vtv_end.o into the link line, just before crtend.o.
      48     This has the desired effect of making our section page-aligned and
      49     page-size paded, ensuring that no other program data lands on our
      50     pages.  */
      51  
      52  #include "vtv-change-permission.h"
      53  
      54  __attribute__ ((constructor(98))) void
      55  __VLTunprotect (void)
      56  {
      57    __VLTChangePermission (__VLTP_READ_WRITE);
      58  }
      59  
      60  /* Page-aligned symbol to mark beginning of .vtable_map_vars section.  */
      61  char _vtable_map_vars_start []
      62  __attribute__ ((__visibility__ ("protected"), used, aligned(VTV_PAGE_SIZE),
      63  		section(".vtable_map_vars")))
      64    = { };