libgcc/vtv_end.c

/* Copyright (C) 2012-2021 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.

You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
<http://www.gnu.org/licenses/>.  */

/* This file is part of the vtable verification feature (for a
   detailed description of the feature, see comments in
   vtable-verify.c).  The vtable verification feature creates
   certain global symbols that need to be read-write sometimes during
   program execution, and read-only at others.  It uses 'mprotect' to
   change the memory protections of the pages on which these variables
   are stored.  In order to not affect the protections of other
   program variables, these variables are put into a special named
   section, ".vtable_map_vars", which is page-aligned at the start,
   and which is padded with a page-sized amount of zeros at the end.
   To make this section page aligned, we create a special symbol,
   "_vtable_map_vars_start" which we make the very first thing that
   goes into the section.  That is defined in vtv_start.c (which
   contains nothing else).  vtv_start.c gest compiled into
   vtv_start.o, and vtv_start.o gets inserted into the link line
   immediately after crtbegin.o, if the program is compiled with
   -fvtable.verify.

   In order to pad the ".vtable_map_vars" section with a page-sized
   amount of zeros at the end, there is a second symbol,
   _vtable_map_vars_end.  This file defines that symbol (and only this
   symbol).  This second symbol is a page-sized array of chars,
   zero-filled, and is the very last thing to go into the section.
   When the GCC driver inserts vtv_start.o into the link line (just
   after crtbegin.o) it also inserts vtv_end.o into the link line,
   just before crtend.o.  This has the desired effect of making our
   section page-aligned and page-size paded, ensuring that no other
   program data lands on our pages.  */


#include "vtv-change-permission.h"

__attribute__ ((constructor(100))) void
__VLTprotect (void)
{
  __VLTChangePermission (__VLTP_READ_ONLY);
}

/* Page-sized variable to mark end of .vtable_map_vars section.  */
char _vtable_map_vars_end[VTV_PAGE_SIZE]
  __attribute__ ((__visibility__ ("protected"), used,
		  section(".vtable_map_vars")));