| /* 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. This file defines that symbol (and only |
| that symbol). GCC compiles this file into vtv_start.o, and |
| inserts vtv_start.o 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, which is defined in another file, vtv_end.c. |
| 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(98))) void |
| __VLTunprotect (void) |
| { |
| __VLTChangePermission (__VLTP_READ_WRITE); |
| } |
| |
| /* Page-aligned symbol to mark beginning of .vtable_map_vars section. */ |
| char _vtable_map_vars_start [] |
| __attribute__ ((__visibility__ ("protected"), used, aligned(VTV_PAGE_SIZE), |
| section(".vtable_map_vars"))) |
| = { }; |